Clinical manifestations

Presentation and course

Viral hemorrhagic fevers begin as a febrile prodrome with myalgia, occasionally accompanied by gastrointestinal symptoms. Sore throats occur in cases of Lassa fever and retroorbital pain associates with dengue fever. By the time medical attention is sought, patients may have a severe acute illness with evidence of abnormal vascular regulation and vascular damage. Endothelial cells can be infected or activated by viral proteins, and they reduce endothelial barrier function. Systemically, the abnormal vascular regulation is shown by mild hypotension in the early stages of the disease and by shock in more severe and advanced infections. Local vascular abnormalities manifest as conjunctival suffusion, flushing over the face and trunk, and various exanthems, including petechiae, maculopapular rash, or ecchymoses. Vascular damage can be seen as capillary leakage in nondependent areas, such as periorbital edema, the development of pulmonary edema after fluid infusions, effusions in serous cavities, or proteinuria (435).

Thrombocytopenia is characteristic and frequently accompanied by hemorrhage, which also signals underlying vascular damage. The hemorrhage is usually petechial and common in skin and mucous membranes. There may be remission of fever before recovery or progression to a more severe stage (435).

Clinically apparent jaundice is not a universal feature. Jaundice is seen in yellow fever and may occur in severe Crimean-Congo hemorrhagic fever, Rift Valley fever, and filovirus hemorrhagic fever. Renal insufficiency, usually proportional to the degree of shock, is dominant in hemorrhagic fever with renal syndrome where it is caused by primary renal lesions (435).

Severe disease is accompanied by bleeding, shock, and CNS disturbances often arising from cerebral edema, anoxia, hemorrhage, microvascular injury, hyponatremia, hepatic failure, renal failure, or soluble toxin release. However, some South American hemorrhagic fevers, mainly Argentine hemorrhagic fever, have been associated with pure neurologic syndromes: ataxia, dysarthria, and intention tremor (435). Lassa causes neurologic syndromes (seizures, movement disorders, tremor, or ataxia) that temporally overlap or follow the hemorrhagic fever (367), frequently causes convalescent stage deafness (83), and can have neuropsychiatric sequelae (367). Lassa fever virus has been associated with a pure neurologic syndrome, aseptic meningitis (278), a delayed (7 week) paraparesis (102), and encephalopathy as bradyphrenia and personality changes, accompanied by focal irregular low frequency EEG abnormalities, subcortical (insula, external capsule, lateral putamen) MRI signal changes on day 11 of a resolving febrile illness, CSF lymphocytic pleocytosis on day 16, and sensorineural hearing loss on day 22 (128).

The lymphocytic choriomeningitis virus and a “lymphocytic choriomeningitis virus-like” arenavirus infection, transmitted through solid organ transplantation, have produced fatal febrile illnesses with hemorrhage and encephalopathy. In 2003 and 2005 in the United States, transplant recipients became ill with fever, thrombocytopenia, elevated transaminases, coagulopathy, encephalopathy, and seizures after receiving organs infected with lymphocytic choriomeningitis virus (111; 24). Corneal transplant recipients from the same donors did not develop disease (385). A transplant-associated arenavirus in Australia, considered a new “lymphocytic choriomeningitis virus-like” arenavirus, produced intraabdominal hematomas (in 1 patient) and encephalopathy (in all 3 patients) (289). Presumably, the lack of effective T-cell response permitted uncontrolled virus replication and prevented the development of the characteristic immunopathological, self-limited, “aseptic” meningitis. Collectively, these cases indicate the importance of transplant-transmitted pathogens as an underrecognized cause of encephalitis.

Classic hemorrhagic fever with renal syndrome is a syndrome characterized by sequential periods of fever, hypotension, oliguria, and diuresis. Headache may be associated with aseptic meningitis, volume overload, hypertension, cerebral edema, or CNS hemorrhage (366). Brainstem or posterior fossa subarachnoid hemorrhages were found in autopsy cases (212). Both Korean hemorrhagic fever and the milder Puumala cause pituitary dysfunction with pituitary apoplexy or panhypopituitarism (378; 144; 299). Crimean-Congo hemorrhagic fever has been associated with encephalopathy characterized by decreased consciousness, agitation, and myoclonic movements. Severe fever with Thrombocytopenia syndrome (SFTS) encephalopathy/encephalitis may be accompanied by reversible corpus callosum splenial lesions and/or brainstem or thalamic lesions (173; 405).

Acute nephropathia epidemic (NE) patients frequently have headache, vertigo, insomnia, nausea, vomiting, dizziness, light sensitivity, and blurred vision. Ophthalmic findings are reduced vision, myopic shift, shallowing of anterior chamber, or changes in intraocular pressure (143). Encephalitis is rare. When found, encephalitis may be evidence of genetic susceptibility, such as toll-like receptor 3 (TLR3). TLR3.p.L742F variant appears enriched in Finnish populations (296).

Rift Valley fever causes encephalitis and retinal vasculitis without overlap with the hemorrhagic fever syndrome (06).

Marburg causes a late sequelae uveitis with virus isolated from aspirate from the anterior chamber of the eye 2 months after the primary illness (120).

In the 2013 to 2016 West Africa Ebola virus epidemic, headache (53%), confusion (13.3%), eye pain (7.7%), and coma or unconsciousness (5.9%) were clinical features in summarized cases of the first 9 months of the epidemic (426). Detailed case reporting described encephalopathy or coma accompanying critical illness, respiratory failure, and gram-negative sepsis (188). Encephalitis or meningitis have been reported in several published case reports and newspaper releases. These are reported as: (1) encephalopathy/encephalitis with positive CSF EBOV PCR in a critically-ill stuporous patient with nuchal rigidity and seizures who expired a day later (341); (2) encephalitis in 3 patients with headache, frontal behavioral disorders, slowed responses, aggression, frontal release or cerebellar signs during acute Ebola, positive CSF EBOV PCR (day 9 to 12 of hospitalization), and survival and behavioral symptom improvement by discharge in all (86); (3) delayed encephalitis developing on day 16 of illness, characterized by precipitous neurologic decline and eventual recovery with supportive care. CSF EBOV PCR was positive when serum was PCR negative on day 40 of illness. CT scan showed generalized brain atrophy (149); (4) meningoencephalitis in a patient with delirium, myoclonus, eye movement disorders, hyperreflexia, frontal release, and meningeal signs, followed by mania, insomnia, and amnestic periods during recovery with supportive care. MRI findings in this case included multifocal punctate lesions in white matter and peri-fourth ventricular locations interpreted as microvascular occlusion and ischemia (72); (5) meningoencephalitis and radiculitis caused by late Ebola virus relapse, first reported in United Kingdom newspapers. An acute Ebola patient initially treated with oral brincidofovir, convalescent plasma, and ZMAb recovered from Ebola but returned to the hospital 9 months later with meningitis, positive CSF EBOV PCR and culture, and MRI evidence of meningeal enhancement of brain, lower spinal cord, and within 1 cerebellar hemisphere (159). Virus was eradicated from the CSF by Gilead’s GS-5734 (Remdesivir) (159).

In summary, Ebola virus can infect the CNS during acute infection, causing encephalitis, which is shown in 1 fatal case with positive CSF EBOV PCR, 3 cases with behavioral changes plus CSF EBOV PCR who recovered, and 1 case with delirium, myoclonus, eye movement changes, pathological reflexes, and abnormal MRI. Ebola virus can also cause late stage encephalitis (from day 16), with positive CSF PCR but negative serum PCR (on day 40). Finally, Ebola virus can relapse as meningoencephalitis and radiculitis 9 months after initial treatment and full recovery. During relapse, Ebola virus RNA was identified by RT-PCR at higher level in CSF than plasma, and infectious virus was recovered from CNS. Sequence comparison of virus and initial illness and relapse showed no changes in coding regions, excluding immune escape mutants as cause of relapse. The authors conclude the CNS was probably infected at the time of original illness and persisted in this immune-privileged site (159). The specific site or cells of virus persistence are not known, nor whether the virus continuously replicates or reactivates to produce relapse. Candidate cells, based on immunohistochemical staining pattern of brain of experimentally-infected nonhuman primates, would be microglia, possibly also astrocytes. At the time of the report, similar CNS relapsing illness had not been seen in thousands of survivors in West Africa, and the authors suggested that aspects of initial viral dynamics, immune response, or particular treatment of this individual increased the risk of CNS relapse (159).

Since then, a proportion of Ebola survivors in Sierra Leone have been shown to have a surge in antibody levels 200 to 300 days after recovery, consistent with de novo antigenic stimulation at immune-privileged sites of the eyes, testes, and/or CNS (02). The antibody surge is common. Previous early flare-ups in West Africa have been thought due to persistence of Ebola virus in sanctuary tissue sites and subclinical infections of survivors, but the possibility of hidden reservoirs of virus persisting a very long time after recovery with the potential for subsequent viral transmission was not fully appreciated. The possibility became reality on February 14, 2021. A new Ebola epidemic started in Guinea triggered by someone who was infected 5 to 7 years prior (194; 311).

Another relapse case in Democratic Republic of Congo led to a transmission chain resulting in 91 cases. The index case had been vaccinated with rVSV-ZEBOV, but still got Ebola. He was treated with monoclonal antibody mAb114 and recovered; however, 6 months later he relapsed with Ebola (230).

The tick-borne hemorrhagic flaviviruses are usually localized in small areas. Kyasanur Forest disease and Omsk hemorrhagic fever are biphasic diseases with a febrile or hemorrhagic period followed by encephalitic syndromes (435). Alkhurma virus patients have hemorrhage, hepatitis, and encephalitis. The frequency of developing CNS disease was 20%, compared to 55% for hemorrhagic signs, in a study (217).

Neurologic complications of dengue, estimated in 4% to 13% of all cases (30), cover all categories of virus-associated disease: direct viral effects, neurologic complications of systemic disease, and immune-mediated neurologic complications.

Dengue accounts for 4% to 47% of hospital admissions with encephalitis-like illness in endemic areas. Dengue, as pure dengue fever or dengue hemorrhagic fever, causes meningitis, meningoencephalitis, bitemporal encephalitis, thalamic or basal ganglia lesions, parkinsonism, stereotypy, dystonia, rhombencephalitis, hemorrhagic encephalitis, SIADH, postencephalitic hydrocephalus, opsoclonus-myoclonus, cerebellitis, acute disseminated encephalomyelitis, subdural hematoma, ischemic or hemorrhagic stroke, encephalopathy, cerebral edema, cortical laminar necrosis, or reversible splenial lesions, cauda equina syndrome due to spontaneous spinal hematoma, myelitis, acute polyradiculoneuritis, mononeuritis multiplex, brachial plexitis retinochoroiditis, retinal vasculopathy (174; 369; 432; 207; 407; 324; 400; 126; 221; 380; 418; 264; 399; 112; 117; 177; 284; 248; 115; 252; 271). In Asia, neurologic complications have been reported as more frequent with serotypes 2 and 3 (369).

Encephalopathy includes depressed sensitivity, convulsion, and behavioral disorders, as mood and personality disorders, acute mania, emotional lability, anxiety, psychosis (20; 375). Meningeal involvement, focal evidence of the virus in the central nervous system, and virus isolation from CSF have been recorded in a number of cases (180; 213; 388; 273; 381). Intracranial hemorrhage due to microcapillary bleed (192), punctate hemorrhages of deep white matter in edematous brains, subarachnoid hemorrhage, and postmortem diencephalic, brainstem, or cerebellar hemorrhages have been found (166). An ischemic stroke patient who received TPA died of intracerebral hemorrhage after developing acute dengue and thrombocytopenia (291). Bilateral thalamic lesions with central hemorrhage associated with dengue are named the “double doughnut” sign (191). Selective pontine tract involvement has a brainstem “jack-o’-lantern” pattern on diffusion-weighted imaging and apparent diffusion coefficient images (356). Additionally, dengue fever may present as encephalopathy with EEG burst suppression or as structural brain lesion (171; 249; 207; 208). Headaches can accompany illness or appear later as post-dengue new daily persistent headache (37).

Although autoimmune encephalitis is possibly underdiagnosed in resource-limited settings, links of dengue to peripheral, immune-mediated, and specific antibody syndromes are well documented. Peripheral nervous system complications include: Guillain-Barré, acute motor sensory axonal neuropathy or acute motor axonal neuropathy types, hypokalemic quadriparesis or plegia, diaphragmatic paralysis, and myositis (161; 352; 401; 136; 330; 357; 131). Immune-mediated syndromes associated with dengue include: Miller-Fisher variant Guillain-Barré syndrome (56; 322), cranial and optic neuritis, limbic encephalitis with NMDA receptor antibodies (30), myoclonus-ataxia with mGluR1 antibodies (70), and NMO-like syndromes. There have been 2 reports of convalescent or recurrent aquaporin-4 negative neuromyelitis optica spectrum disorders. A steroid-responsive optic-spinal syndrome developed in a child of Japanese ancestry 1 week after having a benign form of dengue fever (87). In a 17-year-old, an acute brainstem and transverse myelitis syndrome with longitudinally extensive spinal lesions and persistent DENV-1 infection (shown by + RT-PCR in blood) improved with steroids. In this patient’s second myelitis episode, the patient had DENV-1 RNA in blood along with evidence of acute or recent CMV infection (shown by development of CMV IgM antibodies since the first attack). The myelitis improved with steroids (321).

These immunopathologic cases, examples of ongoing immune activation due to previous viral infection or persistent infection, are now joined by 2 progressive dementia cases related to dengue. Rapidly progressive dementia and seizures in a 64-year-old were reported as post-dengue complications (250) and progressive dementia with extrapyramidal features in a 45-year-old with chronic panencephalitis was discovered to have persistent dengue infection of brain at autopsy (165).

Based on 2012 data, 48.8% of dengue cases with fatal outcomes in Brazil had laboratory evidence of DENV in CSF by viral isolation, PCR, NS1 Ag, or IgM detection (14). From this data point, severe or complicated CNS cases may be increasing due to emergence of other arboviruses associated with neurologic disease, CHIK and Zika, in the same geographic areas (323; 108), potentially through immunologic interactions with co-circulating arboviruses or viral genetic evolution for adaptation (258).

Dengue hemorrhagic fever is not equated to dengue shock syndrome, the most severe of the dengue syndromes due to acute vascular leak and disseminated intravascular coagulation (Srichaikul and Nimmannitya, 2000). Dengue hemorrhagic fever in Asia is generally considered a disease of childhood. The disease in infants is associated with primary infection in the presence of maternal antibody against a different strain. Most cases in older children are due to secondary or a sequence of multiple serotype infections (138; 137). However, evidence suggests that the incidence of severe dengue disease may occur at similar rates between primary and secondary dengue virus infection in the adult population (245). Importantly, although CNS involvement in dengue virus infection was often considered an encephalopathy secondary to a leaky capillary syndrome, reports show increasing prevalence of dengue encephalitis in dengue-prone regions (229). Dengue has been identified in 4% to 47% of admissions with encephalitis-like illness in endemic areas, and patients may have encephalitis without typical signs of systemic or symptomatic dengue (56).

Classic hemorrhagic fever with renal syndrome is a syndrome characterized by sequential periods of fever, hypotension, oliguria, and diuresis. Headache may be associated with aseptic meningitis, volume overload, hypertension, cerebral edema, or CNS hemorrhage (366). Brainstem or posterior fossa subarachnoid hemorrhages were found in autopsy cases (212). Both Korean hemorrhagic fever and the milder Puumala cause pituitary dysfunction with pituitary apoplexy or panhypopituitarism (378; 144; 299). CCHF has been associated with encephalopathy characterized by decreased consciousness, agitation, and myoclonic movements. Severe fever with thrombocytopenia syndrome encephalopathy/encephalitis may be accompanied by reversible corpus callosum splenial lesions or brainstem or thalamic lesions (173; 405).

The lymphocytic choriomeningitis virus and a “lymphocytic choriomeningitis virus-like” arenavirus infection, transmitted through solid organ transplantation, have produced fatal febrile illnesses with hemorrhage and encephalopathy. In 2003 and 2005 in the United States, transplant recipients became ill with fever, thrombocytopenia, elevated transaminases, coagulopathy, encephalopathy, and seizures after receiving organs infected with lymphocytic choriomeningitis virus (111; 24). Corneal transplant recipients from the same donors did not develop disease (385). A transplant-associated arenavirus in Australia, considered a new “lymphocytic choriomeningitis virus-like” arenavirus, produced intraabdominal hematomas (in 1 patient) and encephalopathy (in all 3 patients) (289). Presumably, the lack of effective T-cell response permitted uncontrolled virus replication and prevented the development of the characteristic immunopathological, self-limited, “aseptic” meningitis. Collectively, these cases indicate the importance of transplant-transmitted pathogens as an underrecognized cause of encephalitis.

Prognosis and complications

Case fatality rates have been estimated for many of these organisms (435):

Major reported sequelae of infection by these viruses include the following:

Arenaviridae. Thirty percent of recovered Lassa patients have partial or total hearing loss, an incidence that considerably exceeds all other viral causes of postnatal deafness (83). Other sequelae reported in Sierra Leone have been ataxia and cognitive, mood, psychotic, and behavior disorders (367). Similar proportions of acute meningitic, encephalitic, and encephalopathic syndromes were found during hospitalization for Lassa virus in 1 study in Nigeria (279). Differing from the earlier study and excluding hearing loss patients from this group, they reported survivors had no long-term neurologic sequelae. The different results may reflect advantages of earlier diagnosis (by RT-PCR rather than IgM and IgG detection), early hospitalization and treatment, more advanced care capacity, or strain differences. Twenty-six hundred kilometers separates the Sierra Leone and Nigeria treatment centers.

Filoviridae. Patients who acquire Ebola in the latter stages of an epidemic may have a better chance of recovery.

Patients who recovered from Marburg in 1967 had neuropathy, “restless legs,” psychosis, amnesia, depression, or fatigue (227). Marburg virus was cultured from the anterior chamber of the eye from a uveitis patient 2 months after recovery (120), and the recent Ebola epidemic has presented examples of viral persistence: in the eye, semen, urine, and CSF.

With an estimated 17,000 survivors in West Africa, encounters with "post-Ebola syndrome" spectrum of diseases is increasing in this large survivor cohort. The first reports of signs and symptoms after recovery include eye pain and pressure, vision problems or blindness, eye movement abnormalities, sensorineural hearing loss, headache, musculoskeletal pain, affective and cognitive disorders, memory loss, anxiety attacks, insomnia, tremors, hair loss, abdominal or chest pain, and rash or pruritis (310; 31; 346).

Neurologic, psychiatric, and ophthalmologic evaluations and, in some cases, CT scans have been completed for participants triaged for neurologic care in a survivors’ clinic in Sierra Leone (150). The study confirmed long-term neurologic sequelae and disability and inability to perform activities of daily living (ADLs) in some, and the study confirmed that a substantial portion of patients had ongoing mental health problems. In addition, the study clearly presented the need for services integrated across several specialties for Ebola virus survivors.

Of 35 survivors, 30 had headaches, 13 of which were diagnosed as migraine. Other diagnoses were stroke (2), sensory neuropathy (2), and peripheral nerve or plexus lesions (2). Three CT scans showed cerebral and/or cerebellar atrophy, 2 were from patients known to have had Ebola virus encephalitis. One recovered with mild cognitive impairment and hippocampal atrophy, another was completely disabled, requiring 24-hour care. Two other CTs showed remote infarcts. Among the 35 patients, anxiety, depression, PTSD, psychosis, confusion, psychosocial issues, insomnia, tinnitus, and altered sensations were recognized, and mental health followup was required in several cases. Eye pain, redness, photophobia, and visual loss were reported; uveitis and unique peripapillary lesions were found (150).

Earlier studies of Ebola virus survivors have estimated uveitis at 13% to 34%, and unilateral white cataracts at 7%, which are frequently accompanied by hypotony (low intraocular pressure) (350; 374). A novel retinal lesion consisting of pale peripapillary lesions and pigmentation of large lesion areas was first reported in 14.6% of the Sierra Leonean Ebola virus survivors with ocular symptoms seen in 2016 (374). The pathologic changes were curvilinear lesions distinct from retinal vasculature and retinal nerve fiber wedge defects on the photoreceptor matrix. Lesions followed the retinal ganglion cell nerve fiber distribution in the absence of major retinal vessels, which was interpreted to represent the potential for neurotrophic spread into the eye from the optic nerve and along the retinal ganglion cell axons (374). In the laboratory, researchers have shown human pigment epithelial cells could be infected and were permissive for Ebola (363).

Uveitis, neuropsychiatric syndromes, arthritis, or other conditions may involve persistent infection, treatment effect, and autoimmune or exaggerated convalescent immune responses. Research was undertaken to characterize the post-Ebola virus syndrome clinical spectrum, establish diagnostic criteria and surrogate markers for early diagnosis, predictors, and biomarkers of organ-specific disease (55; 31; 349), and establish specialized or multidisciplinary services for clinical care. Although there had been reports of similar long-term sequelae in survivors of EBOV-Zaire in Kikwit 1995 and EBOV-Bundibugyo in Uganda in 2007, the large numbers of patients are unprecedented. Whether different Ebola virus species cause different post-Ebola syndromes remains unknown.

To evaluate the possibility of persistence of Ebola virus in CNS after recovery and Ebola Treatment Unit discharge, investigators in Liberia performed neurologic and CSF evaluations. Seven men and women were selected at a survivors clinic. They reported symptoms of paresthesias, headache, memory loss, tinnitus, and confusion. On examination, eye movement abnormalities were noted, as well as frontal release or extrapyramidal signs, tremor, sensory loss, or hearing loss. CSFs were normal and negative for Ebola virus by RT-PCR in all cases (31).

At least 8 flare-ups in West Africa have been linked to persistence of Ebola virus in other tissues and subclinical infections of survivors. The virus, detected in semen up to 18 months after recovery and persisting in other body fluid such as breast milk, urine, and aqueous humor, could be found as viral nucleic acid at 40 months, which at the time was the longest time from acute EBOV infection to detection of viral RNA in semen (377; 307). For the first time, investigators found that a woman can harbor virus for more than 1 year and then infect others (97). The virus re-emerged in fatal form in the woman shortly after giving birth, raising the question of the contribution of immunosuppression or immunotolerance during pregnancy.

Recommendations that survivors’ body fluids be monitored for at least 18 months after recovery or until the fluids test negative at least twice were made in 2018 (377). However, intermittent viral RNA detection in semen and the fact that 2 negative tests could be followed by a positive test was noted in the NIAID NEI PREVAIL study (PREVAIL III Study Group 2019). Ebola virus persistence in semen, urine, and aqueous humor is associated with ongoing viral replication and, therefore, is not quiescent or latent infection (412). Guinea’s newest Ebola epidemic declared February 14, 2021 started with sexual transmission by someone who was probably infected 5 to 7 years prior (194; 311). Cumulatively, there were 18 cases (14 confirmed; 4 probable), with 9 deaths and 6 recoveries reported (178; 319).

The Partnership for Research on Ebola in Liberia (PREVAIL) III longitudinal study of Ebola sequelae in Liberia involving survivors of EBOV as well as their seronegative close contacts found the incidence of most symptoms and neurologic findings greater among survivors. Headache, muscle pain, memory loss, and joint pain were more frequent in this group, and neurologic abnormalities among survivors and controls were abnormal reflexes (1.4% vs. 0.7% respectively), tremor (0.9% vs. 0.2%), gait or balance difficulty (0.7% vs. 0.9%), speech abnormality (0.7% vs. 0.2%), and cranial nerve findings (0.7% vs. 0.1%). Prevalence of findings declined over time except for uveitis. There was a positive correlation between viral persistence in semen and detection of uveitis on baseline examination, and the incidence of new uveitis was higher among survivors than controls at 12 months (307).

A simultaneous study, the Ebola Virus Persistence in Ocular Tissues and Fluids (EVICT) II study in Sierra Leone, reported cataract surgery to be safe and effective in Ebola virus disease survivors, resulting in restoration of vision (351).

In Guinea, an estimated three fourths of survivors of the West Africa epidemic experienced symptoms including migraine-like headaches, visual problems, joint and muscle pain, and fatigue a year after initial infection and sometimes for much longer. Sequelae could decrease over time but after 48 months, neurologic (30%), musculoskeletal (6%), and ocular (4%) conditions remained in survivors in Guinea (91). No association between cardinal post-Ebola symptoms and serologic markers of inflammation and immune activation has been found in the longitudinal Liberian EBOV survivor study that followed participants for up to 3 to 4 years after acute infection (391).

Survivor studies from the world’s second largest Ebola epidemic, August 2018 to June 2020 (North Kivu and Ituri Provinces, Democratic Republic of Congo), which infected 3481 individuals and took 2290 lives, showed PTSD, depression, or anxiety in an estimated 25% to 33% of study participants (175). As in West African studies, psychic trauma and mental health issues were not a side show but a significant, ongoing public health issue. Persistent headache or short-term memory impairment accompanied anxiety and depression.

Bunyaviridae. Hemorrhagic fever with renal syndrome has produced hallucinations, delusions, paranoid reactions and affective disorders, myelopathy, fatigue, endocrine disorders, and convalescent headache associated with hypertension. Two convalescent-phase deaths from CNS complications were recorded between 1961 and 1963 in Korea. One was associated with a meningitic syndrome because of severe nausea, vomiting, and CSF pleocytosis. The other was from pituitary dysfunction (183). A complication of Puumala virus, the agent of nephropathia epidemica, is also hypophyseal hemorrhage and panhypopituitarism (144).

Rift Valley fever is associated with major ocular morbidity. There is a characteristic retinitis with white macular lesions accompanied by extensive vasculitis. Retinal hemorrhages, vitreous reactions, and optic disc edema may be present. Due to involvement of the uvea and posterior chorioretinal area, permanent visual loss resulting from macular and paramacular scarring, vascular occlusion, and optic atrophy occurs (89; 05). Cerebral vasculitis may have been the cause of quadriparesis and other poor neurologic outcomes (06).

Severe fever with thrombocytopenia syndrome (SFTSV) virus, a tickborne phlebovirus, is an emerging zoonotic infection and a recognized cause of encephalitis in China (82; 405; 428), Korea (294), Taiwan (301), Japan (173; 187). Haemaphysalis longicornis ticks, the vector of severe fever with thrombocytopenia syndrome, are indigenous Asian ticks that have been detected in the United States in several eastern and southern states on domestic animals, wildlife, and 2 humans during 2017 to 2018 (25).

Flaviviridae. Neurologic complications of dengue virus infection include encephalitis, acute disseminated encephalitis, transverse myelitis, Guillain-Barre syndrome, mononeuropathies, encephalopathy without encephalitis, and subdural hematoma. Patients with encephalopathy have more severe illness and worse outcome compared to patients with acute motor weakness (249). CNS invasion by dengue was an important fatal complication in a postmortem study from Brazil (14). After bilateral hippocampal involvement by dengue, recovery of memory to average performance on the Wechsler Adult Intelligence Scale required 70 days (432). Recovered Omsk hemorrhagic fever patients may have hearing loss or neuropsychiatric syndromes. Complications of yellow fever vaccine include encephalitis, acute disseminated encephalomyelitis, and Guillain-Barre syndrome (239).

Togaviridae. Chikungunya virus is a rapidly emerging virus causing febrile rash and arthritis. The name is from the Makonde (East African) language for “that which bends up”. Virus may affect the CNS, with meningitis, encephalitis, or encephalomyeloradiculitis, generally in pediatric groups or individuals with advanced age and underlying diseases (59; 116; 335; 84; 76; 268; 21; 122).

Clinical vignette

A 16-year-old girl with 4 days of fever, headache, body aches, 2 days of vomiting, and 1 generalized seizure was admitted to a hospital in rural Sierra Leone, West Africa in February. It was the region’s dry season, which is also Lassa season. The patient had a fever of 38.7°C, subconjunctival hemorrhages, and abdominal tenderness. She was drowsy but arousable with a symmetric motor exam and normal tone and reflexes. She was immediately transferred to the Lassa Ward, a building separate from the general hospital with its own supplies, segregated laboratory, and strict barrier nursing practices. Laboratory investigations showed leucopenia, thrombocytopenia (white blood count 2500/mm3; hematocrit 35, platelets 59,000/mm3), elevated liver function tests (serum glutamic pyruvic transaminase 362 IU) (serum aspartate transaminase) (normal range 3 to 35) and no malaria parasites on several peripheral blood smears. There were 8 white blood cells/mm3 in CSF with no gram-stained organisms. Reagents for measuring CSF protein and glucose were not available. Intravenous ribavirin, penicillin, and chloramphenicol were started for possible Lassa fever, bacterial meningitis, or typhoid fever. Involuntary movements, grimacing or stretching of the lower face, and pursing of the lips were noted the next hospital day. She had difficulty maintaining a normal head posture and speech was harsh. Her jaw tightened, her lips retracted, and her head arched back when attempting to drink from a bottle. Diazepam suppressed the lower face movements. The patient’s overall condition improved within days of intravenous ribavirin treatment (2 g loading dose, followed by 1 g every 6 hours for 4 days, then 0.5 g every 8 hours for another 6 days). By hospital day 4, the involuntary movements were gone. She recovered but had no memories of recent events or the circumstances of her illness. She seemed subdued, less fluent, inattentive, and was unable to retrieve some general cognitive information such as the name of her village chief. Antibodies to Lassa virus were detected on day 12 of illness (IgM=4, IgG=64), increasing to IgM greater than 16; IgG was greater than 1024 2 days later. The patient was discharged after 10 days of intravenous ribavirin. When the CSF examination was repeated, there were no cells.

Comment. This case demonstrates typical systemic disease plus a clinical encephalitic syndrome with seizure, focal dystonia, and amnesia associated with Lassa fever. This clinical presentation illustrates overlap with malaria, bacterial meningitis, or enteric pathogens. Because February, the dry Hamartan season in West Africa is, also the region’s N meningitides season, the patient received meningitic doses of antibiotics on the first hospital day. The diagnosis of Lassa fever on site relied on demonstration of antibodies to Lassa virus and exclusion of other bacterial and parasitic infections. Criteria for Lassa fever diagnosis are IgM titers greater than 16 and IgG titers greater than 128 or a 4-fold rise in IgG antibodies to Lassa virus by indirect immunofluorescent antibodies. Indirect fluorescent-antibody tests were performed on slides of infected cells prepared at the U.S. Centers for Disease Control and Prevention. Confirmation was by virus isolation at a biosafety level 4 lab at the U.S. Centers for Disease Control and Prevention.

The patient recovered, highlighting what can be accomplished in a resource-limited setting. Published guidelines for the optimal type and duration of intravenous antiviral treatment for Lassa fever were followed (233). The decision to treat was based on clinically compatible syndrome and elevated liver function tests, which signified significant systemic disease. The patient recovered and there were no secondary cases among patient’s contacts or staff, a result of isolation in the Lassa ward and the work of trained staff.

Biological basis

Etiology and pathogenesis

Viral hemorrhagic fevers are a group of illnesses that are mainly caused by members of 4 different families of viruses: Arenaviridae, Filoviridae, Bunyaviridae, and Flaviviridae.

To date, 5 Old World arenavirus species and 17 New World arenaviruses have been recognized by the International Committee for Taxonomy of Viruses, but the presence of additional arenaviruses in Africa is suspected.

Arenaviruses causing hemorrhagic syndromes are Lassa fever virus (Lassa fever) in West Africa and Lujo virus in South Africa (40; 297) as well as agents of the South American hemorrhagic fevers including: Junin virus (Argentine hemorrhagic fever), Chapare and Machupo (Bolivian hemorrhagic fever), Guanarito (Venezuelan hemorrhagic fever), Sabia virus in Brazil, Ocozocoautla de Espinosa virus in southern Mexico (Chiapas state) (47), and the White Water Arroyo virus in the states of New Mexico and California in the United States. Lymphocytic choriomeningitis virus transmitted by solid-organ transplantation caused hemorrhagic syndromes in the United States (111), and a new lymphocytic choriomeningitis virus-related arenavirus in Australia, Dandenong virus transmitted by solid-organ transplantation, caused a similar hemorrhagic syndrome (289). There have been several reported clusters of lymphocytic choriomeningitis virus donor-derived infections in the United States and worldwide. In the transplant groups, CNS abnormalities were found, including meningoencephalitis, dural effusions and thickening, intracerebral and subarachnoid hemorrhages, and EEG slowing. Often, the CNS disease was overshadowed by the systemic disease looking like Lassa hemorrhagic fever (111; 228). Where a high prevalence of infected mice and seropositive humans are found, there may be more congenital lymphocytic choriomeningitis virus (LCMV) infection than previously thought (36). The congenital syndrome is microcephaly, periventricular calcifications, hydrocephalus, chorioretinitis, and cataracts (36; 88).

Ebola and Marburg viruses, previously the only members of the Filoviridae family, are both hemorrhagic fevers with high mortality rates. To better reflect high-resolution phylogeny now available, a revised taxonomy of the family Filoviridae with species named Reston ebolavirus, Sudan ebolavirus, Zaire ebolavirus, Tai Forest ebolavirus, Bundibugyo ebolavirus, Marburg marburgvirus, and Ravn marburgvirus has been proposed along with a new family member, Lloviu cuenavirus, which is found in Schreiber’s long-fingered bats in Spain (190).

Bunyaviruses causing hemorrhagic fevers are Rift Valley fever virus (Rift Valley fever), Crimean-Congo hemorrhagic fever virus, Hantaan, Seoul, Dobrava/Saaremaa (hemorrhagic fever with renal syndrome), Puumala (nephropathia epidemica), and Ilesha virus (288).

Flaviviruses associated with hemorrhagic fevers are yellow fever virus (yellow fever), dengue virus (dengue hemorrhagic fever/dengue shock syndrome), Kyasanur Forest disease virus, Omsk hemorrhagic fever virus, and Alkhurma virus. Although flaviviral hemorrhagic fevers affect all ages, incidences can be significantly higher in some age groups in certain virus infections, such as dengue virus, for which infection of school-aged children dominates. The reasons thus far are still unclear.

Recent epidemics of Chikungunya virus have focused attention on this member of the Togaviridae family, genus Alphavirus, as a cause of hemorrhagic and encephalitic illness. Neurologic manifestations of Chikungunya fever are influenced by patient age, with greatest frequency in pediatric groups. Clinical signs of meningeal, retinal, or CNS involvement include nuchal rigidity, retinitis, ophthalmoplegia, and slurred speech (265). Neurologic syndromes in children include febrile seizures, aseptic meningitis, seizures, and altered mental status (335). Stimulus-sensitive myoclonus and cerebellar ataxia following chikungunya meningoencephalitis (170) and post chikungunya brain stem encephalitis (118) have been reported. Cases of confirmed meningoencephalitis, though rare, are significant for their association with severe or fatal disease (389; 355). Virus has been recovered from brain and CSF. A steroid-responsive, rapidly progressive flaccid paralysis with cranial nerve sparing was described in the Andaman and Nicobar Island epidemic of 2005-6 (358).

Vascular leakage is common to all the viral hemorrhagic fevers, but no single model for vascular damage or vascular endothelial permeability fully accounts for all syndromes. For example, vascular leak may be due to direct endothelial infection (Ebola/Marburg, hantaviruses), localized hypoxia and release of vasoactive neuropeptides (arenaviruses), release of cytokines and other soluble inflammatory mediators (arenaviruses, Ebola/Marburg, dengue), immune-mediated or immune-complex injury (hantaviruses, dengue), elaboration of matrix metalloproteinases, alterations in the antithrombotic and protein C pathways and vascular endothelial growth factor (dengue), or endothelial injury triggering disseminated intravascular coagulation (Rift Valley fever, Crimean-Congo hemorrhagic fever, Ebola). The transient or reversible vascular permeability and coagulopathy, found in dengue hemorrhagic fever, would be consistent with multiple short-lived and soluble immune effectors as mechanisms of effect. When better understood, each step in the process of endothelial or transendothelial injury and progression of vascular leakage represents a potential checkpoint for infection.

A second pathogenic feature common to many viral hemorrhagic fevers is immune system dysregulation or immunosuppression. Although lymphoid tissues are thought to be the primary targets for many viral infections, there is also evidence of direct viral attack of the bone marrow compartment, with this tissue tropism playing a critical role in outcomes (77; 101; 275). The ability to disable the host immune response by attacking and manipulating cells that initiate antiviral responses has been demonstrated, for example, in Ebola and Lassa fever, by downregulation of dendritic cell function (109; 121). Filoviruses encode proteins with regions similar to the immunosuppressive domain of oncogenic retroviruses (403; 44; 429) and arenavirus protein segments can inhibit interferon pathways (410; 41). Dengue is an exception, instead producing significant pathologic changes in the reticuloendothelial system, liver, and the vascular system (28). Bone marrow suppression at early infection and hypercellularity at late infection in conjunction with circulating immune complexes reaching the liver or spleen may induce further organ damage through positive feedback loops that amplify inflammatory responses (433). These secondary insults could account, in part, for why patients rapidly worsen soon after the first clinical symptoms are noted.

The pathophysiology of neurologic symptoms is most often attributed to cerebral edema, anoxia, hemorrhage, hyponatremia, hepatic or renal failure, microcapillary hemorrhage, and release of toxic substances. Yet there have been descriptions of clinical syndromes consistent with encephalitis, raising the possibility that some of the viruses are neurotropic, causing disease by direct infection and damage or bystander inflammatory injury. Due to the remoteness of sites of many of the epidemics, specimens would not have been available for confirmation by virus detection in CSF or brain (by culture, PCR, immunostaining, or electron microscopy ultrastructure). Where CNS data are available, they are presented with mechanisms of the viruses’ hemorrhagic effect. The nervous system complications of viral hemorrhagic fevers are summarized in Table 1.

Table 1. Viral Hemorrhagic Fevers and CNS Disease

Arenaviruses. Except for the congenital and organ-donor transmitted cases, the virus is transmitted from asymptomatic rodents. Virus enters the human host via mucosal surfaces, abraded skin, or parenteral or inhalation routes. Initial replication at the site of infection in nonreservoir hosts is followed by infection of macrophages. Infection spreads, involving additional cell types in liver, adrenal, spleen, lymphatic tissue, lung, and intestine. Widespread infection of submucosal macrophages causes secretion of cytokines, neuropeptides, and inflammatory mediators, which are thought to play an important role in evolution of shock syndrome and pulmonary edema (43). In the 2005 donor-transmitted cases of lymphocytic choriomeningitis virus, all organs came from a donor who had been exposed to a hamster infected with lymphocytic choriomeningitis virus (07).

Lassa virus was isolated from 3 out of 3 spinal fluids from patients with signs of meningitis (164) and a Lassa aseptic meningitis, diagnosed by CSF pleocytosis and positive serum PCR, was reported (278). In 1 patient with fever and a 30-minute generalized seizure followed by 2 days of disorientation and stupor, Lassa virus was cultured from CSF, not serum, and real-time PCR indicated higher virus load in CSF compared to serum. Lassa virus-specific IgM and IgG antibodies were present in CSF (134). Acute CNS syndromes (seizures, dystonia, tremor, ataxia, encephalitis/encephalopathy) with inflammatory CSF and convalescent syndromes (amnesia, psychosis, fatigue, depression, dementia) have been described (367), and close to one third of Lassa patients develop hearing loss during recovery or convalescence (83).

Nonhuman primates with Lassa also develop hearing loss as a convalescent phenomenon, and studies have linked deafness to a systemic immune-mediated vasculitis. The vasculitis is characterized by inflammatory vascular findings in tissues in the absence of circulating virus or evidence of viral antigen in tissues or vessel walls. The mechanism of hearing loss in surviving nonhuman primates is presumed inflammation and occlusion of anterior inferior cerebellar artery and downstream vessels resulting in cochlear hypoxia, as seen in polyarteritis nodosa in man. Perivascular inflammation around cochlear nerve branches had no viral staining in surviving nonhuman primate study subjects with BSER-demonstrated hearing loss, and serologic markers of autoimmune systemic vasculitis, such as elevated C-reactive protein, circulating immune complexes, elevated proinflammatory cytokines, and positive antineutrophil cytoplasmic antibodies, were present (58).

Thirty percent of Lassa patients cared for at Irrua Specialist Teaching Hospital, Nigeria from 2011 to 2015 were reported to have severe CNS symptoms: coma, seizures, irrational talk or behavior, altered sensorium, tremors, disorientation, and confusion, and they were diagnosed as encephalitis, meningitis, or encephalopathy. Surviving patients in this cohort, followed for 3 to 18 months, had no long-term neurologic sequelae. One patient who had Lassa virus nucleic acid in CSF during acute illness, followed for over 24 months, also had no sequelae. An additional 37 hospitalized Lassa patients were not included in the study; 5 (14%) had early sensorineural hearing loss, some of whom did not recover and had permanent hearing loss (279). Presumably, most or all patients were treated with intravenous ribavirin.

The authors note that the circulating virus in Nigeria is more diverse and genetically distinct than the virus elsewhere, and they suggest some of their clinical findings, such as CNS disorders or intrinsic kidney disease, may be a result. With up to 32% strain variation, it is comparable in variation to Crimean-Congo hemorrhagic fever (CCHF) (30% sequence variation among isolates), in contrast to Ebola (3% across all sequenced strains) and Rift Valley fever (5%). Lassa virus sequence data were not provided, which would help correlate Lassa virus variants with mortality or CNS disease.

A late neurologic syndrome of fever, headache, cerebellar ataxia, and cranial nerve palsies was reported after immune plasma treatment in about 10% of treated Argentine hemorrhagic fever patients (219). Presence of the Argentine hemorrhagic fever convalescent syndrome raises the possibility of prior invasion of CNS by virus and persistent infection. Whether this late neurologic syndrome is the result of CNS relapse with viral replication or immune reaction is not known from the 1979 report. One group has suggested neurologic diseases could be a function of pathogenic differences between Old and New World arenaviruses, with Old World arenaviruses such as Lassa causing disease through generalized immunosuppression and New World viruses such as Junin causing disease by cytokine storm (238). An effective T cell-mediated response is required for recovery from Lassa. Antibody response is ineffective in controlling Lassa infection but is important in Argentine hemorrhagic fever (238).

Studies are testing the hypothesis that Lassa fever virus is a driver of natural selection of genes associated with Lassa infectivity and immunity in West African populations where Lassa is endemic (10). Lassa fever virus uses aDG (alpha-dystroglycan—a cell surface molecule for attachment to the extracellular matrix) as a host cellular receptor. Lassa fever virus exposure results in positive selection on genes affecting expression, modification, and proper function of aDG. The International HapMap project has identified positive selection of 2 genes: LARGE and dystrophin; in a Nigerian population such that allelic variants may alter ability of Lassa fever virus Gp to bind aDG. LARGE (cellular like-acetylglucosaminyltransferase) glycosylates aDG, enabling its interaction with extracellular matrix or Lassa fever virus GP and dystrophin, is a cytosolic adapter protein for aDG (238).

Filoviruses.

Ebola/Marburg viruses. Virus enters the human host via mucosal surfaces, abraded skin, or a parenteral route.

The endosomal protein Neumann-Pick C1 (NPC1) is an intracellular receptor. EBOV-glycoprotein interacts with NPC1 at the endosomal membrane, triggering release of viral material into the host cell (53). T cell immunoglobulin mucin domain-1 (TIM-1) is a phosphatidylserine (PS) receptor mediating filovirus entry into cells through interactions with phosphatidylserine on virions. TIM-1 is widely expressed in epithelial cells of mucosal epithelial surfaces (42).

Macrophages and monocytes are early targets of infection and replicate virus. With dissemination via lymphatic and vascular systems, endothelial cells are infected next. Possibly, virus enters the CNS through infected monocytes, and movement across the blood-brain barrier is aided by barrier breakdown due to endothelial infection. Activation of endothelial and mononuclear phagocytic cells is triggered by direct viral infection and by binding of soluble viral and host cellular factors. Endothelial injury and dysfunction are induced by the cytopathic replication that follows direct endothelial cell infection and by mediator-induced inflammatory responses. Infection of endothelial cells, together with cytokine production, sustains the vascular injury and increases permeability, leading to hemorrhage and circulatory compromise. Liver function is eroded as hepatocytes, endothelial cells, monocytes, and Kupffer cells are overwhelmed by the replicating virus. Virus infection of kidneys, adrenals, intestines, skin, and the reproductive system also occurs. Efficient viral replication over a wide host-cell range, cytotoxic viral and host products, and filovirus immunosuppression through interference with innate immune responses culminate in vascular dysregulation, fluid-distribution problems, coagulopathy, and bleeding (109). Fatal outcome is characterized by impaired humoral responses, failure to generate specific IgG, and intravascular apoptosis of peripheral blood leukocytes (18; 431; 343). Highly elevated d-Dimer levels early in disease (0 to 2 days after onset), which signify disseminated intravascular coagulation, are also associated with fatal outcomes (337).

In cases from the 1967 Marburg outbreak, CSF contained between 0 and 24 white blood count and protein up to 68 mg/dL (376). Virus has been detected in CSF in Ebola patients (341; 149), potentially having gained access to the CNS via infected monocytes or macrophages. Immunostained glial nodules in the brain of Ebola-infected nonhuman primates (198) raise the possibility of CNS cells, perhaps microglia, as virus reservoirs. Marburg has been cultured from the anterior chamber of the eye in a recovered patient (120), and Ebola has been cultured from the aqueous humor of a convalescent individual with anterior uveitis 10 weeks after the onset of Ebola virus (397). Patterns of lesions of retinal ganglion cell nerve fibers seen in Sierra Leone in Ebola survivors raise the possibility of EBOV’s capacity for transneuronal spread (374).

Acute or convalescent neurologic syndrome signs and symptoms have been noted in Marburg patients (226; 366) and postmortem exams from patients surviving several days with Marburg show widely distributed microglial nodules, diffuse glial cell proliferation, perivascular inflammation, and scattered petechial hemorrhages (26; 158).

Bunyaviruses.

Hemorrhagic fever with renal syndrome. Hantaviruses are transmitted from the infected rodent reservoir to humans by inhalation of infectious aerosols produced from rodent excreta. The detection of hantavirus in mites raises the possibility of a role for arthropods in transmission as well (438). Viral RNA is present in blood at early stages of disease. Hantaviruses infect endothelial cells without cytopathic effect and interact with beta integrins present on platelets (272). The more pathogenic hantaviruses may utilize beta-3 cell surface integrins, whereas less pathogenic hantaviruses use beta-1 integrins for cell entry (119). Pathogenic effects could relate to integrin modulation of calcium signaling, but early events in infection are incompletely understood. Because hantavirus-specific antibodies and T cells are detected at onset of symptoms, immune changes, rather than virus-induced cell death, may drive early disease. Later, virus antigen is pervasive, present in endothelial cell layers throughout the body. Antigen predominates in kidney endothelial cells in postmortem cases of hemorrhagic fever with renal syndrome and lung endothelial cells in cases of hantavirus pulmonary syndrome.

Hantavirus (Asian/Korean/Seoul) hemorrhagic fever with renal syndrome complications include encephalopathy, characterized as depressed mental status and abnormal behavior, along with choreiform movements, rigidity, or finger tremors, CSF pleocytosis, radiologic features of reversible corpus callosum, splenial lesion or posterior reversible encephalopathy syndrome, pituitary dysfunction, and Guillain-Barre syndrome (183; 366; 17; 103; 162). MRI findings in hemorrhagic fever with renal syndrome-related encephalopathy patients associate with stage of disease. MRI changes of PRES (posterior reversible encephalopathy syndrome) are found in the oliguric phase, and splenial lesion pattern in the febrile phase (189). Postmortem findings include brain intraparenchymal hemorrhage, petechial hemorrhage, subdural hematoma pituitary necrosis, or cerebral edema (212; 306). Dobrava (HFRS) has caused focal seizures and hemiparesis in a Slovenian patient (64).

Severity of hemorrhagic fever with renal syndrome (HFRS) has been associated with human platelet alloantigen (HPA) alleles. Platelet and platelet-associated proteins are highly polymorphic, and up to 30% of natural variation in platelet reactivity is estimated related to genetic inheritance. There are 5 common HPA gene variants, with differential susceptibility to cardiovascular disease and stroke found among individuals with different alleles. One of these polymorphisms, the HPA-3b allele, is observed in patients with more severe HFRS (210).

Nephropathia epidemica of Puumala virus, generally the mildest form of hemorrhagic fever with renal syndrome, has encephalitis, seizures, Guillain Barré syndrome, cerebral hemorrhage, hypophyseal hemorrhage and panhypopituitarism, or MRI-demonstrated white matter lesions as rare complications. As in severe hemorrhagic fever with renal syndrome cases, a mechanism of pituitary injury can be hemorrhage or hypovolemic shock. The hypophyseal gland is vulnerable because of its anatomical location and organization of blood supply. The anterior lobe of the pituitary receives 10% to 20% of its blood supply from superior and inferior hypophyseal arteries, with the remaining 80% to 90% provided by hypophyseal venous portal circulation. Both ischemic and hemorrhagic damage of the pituitary have been found in radiologic images (29). Alternatively, accumulation of antigen in critical target tissues could yield inflammatory responses that enhance lesions. In 1 case, Puumala viral antigen was found in neuroendocrine stromal and vascular endothelial cells (144).

Severity of Puumala illness has been associated with HLA haplotype. Severe disease is seen in B8 DRB1*0301 patients and mild disease in HLA B27 allele patients (262), the same HLA associations found in rapid and slow progressors, respectively, in HIV patients (262).

The wide spectrum and variable intensity of Puumala disease has led to more genetic investigations. A new study finds that a genetic defect in innate antiviral immunity, analogous to the toll-like receptor 3 (TLR3) mutations seen in HSV-1 encephalitis vulnerability, severe influenza pneumonia, and VZV ophthalmicus may be related to PUUV hantavirus encephalitis risk. TLR3 recognizes dsRNA and activates antiviral immune responses by producing inflammatory cytokines and type I interferons. Exome sequencing of PUUV encephalitis patients identifies a TLR3p.L742F variant that showed reduced biological activity in functional studies of cellular immune response, potentially a factor in PUUV encephalitis predisposition (296).

Hantavirus pulmonary syndrome has been associated with single cases of ADEM and transverse myelitis (152). However, both patients received extracorporeal membrane oxygenation therapy, which carries a risk of cerebrovascular injury and complicates the interpretation of these cases. North American hantavirus pulmonary syndrome triggered a painful peripheral neuropathy in 1 patient (09).

Andes viruses, which are agents of HPS circulating in Argentina, Bolivia, Chile, and Uruguay, are now also implicated in encephalitic syndromes. In 2010, an Argentine patient with a febrile syndrome, respiratory distress, and shock developed, on day 13 of illness, an encephalitic syndrome characterized by headache, confusion, excitation, generalized seizure, and bilateral frontal and parieto-occipital T1 and T2 signal abnormalities on MRI, with greater cortical than underlying white matter involvement. The patient received ICU supportive care and empiric antibiotics. On follow-up 8 months later, the patient was well and the MRI was normal (383).

Rift Valley fever. Human transmission is mainly by mosquito bite but can occur after contact with infected animals or infected aerosols. After inoculation of virus by Aedes mosquito bite, virus is transported by lymphatic drainage to regional lymph nodes where local replication occurs. Virus spills over into the circulation causing primary viremia and spread of virus to major organs. Reduction of antithrombin function of endothelial cells initiates intravascular coagulation and necrosis of hepatocytes and other infected cells. Release of procoagulants into the circulation, liver damage that impairs synthesis of coagulation factors, and removal of circulating activated coagulation factors contribute to disseminated intravascular coagulation. Biomarkers of overall prognosis include cytokine profiles, soluble adhesion molecules, and coagulopathy. Primary encephalitis and retinitis have been described (216; 333; 182; 06), with 1 patient who died of meningoencephalitis having brain pathology showing perivascular cuffing and round-cell infiltration (236). Patients with encephalitis or the CNS complications of systemic disease can have altered mental status, seizures, amnesia, abnormal movements, vertigo, ataxia, paresis, meningeal signs, visual loss (218), and MRI changes of cortical and thalamic areas of diffusion restriction (06). Encephalitic complications may be as high as 8% of infections, and retinitis in 10% in recent epidemics (269). Lower percentages, less than 0.5% of human Rift Valley fever cases, develop encephalitis 1 to 4 weeks after recovery from the acute illness (272), but sequelae after delayed encephalitis are more frequent. CSF has normal protein, glucose, and lymphocytic pleocytosis. Both direct viral and immune-mediated cell injury are implicated in this complication (303). The potential for an autoimmune pathogenesis for retinitis is supported by the report of antibodies to retinal tissue in Rift Valley fever virus-exposed individuals with retinitis (269). Active ocular lesions resolve in 10 to 12 weeks, but resulting macular and paramacular scarring, vascular occlusions, and postinfectious optic atrophy cause visual loss (160). Underlying mechanisms leading to different diseases and different outcomes are incompletely understood. Viremic load correlates with severe disease, so that significant exposures with higher inoculation rate probably link to severe disease. Encephalitis is a complication in immunosuppressed patients, and single nucleotide polymorphisms of the host (TLR3, TLR7, TLR8, MyD88, TRIF, MAVS, and RIG-I) also associate with severe symptomatology (160). Confection with HIV or malaria has been associated with severe disease that can include encephalitis. Schistosomal hepatic disease and bacterial or fungal co-infection have been found in fatal cases (160). No serum biomarkers of CNS disease in humans have been found (234).

Crimean-Congo hemorrhagic fever. Crimean-Congo hemorrhagic fever (CCHFV) is spread principally by ixodid ticks of genus Hyalomma. Crimean-Congo hemorrhagic fever is maintained in vertical and horizontal transmission cycles in iodide ticks and a variety of wild and domestic vertebrates (ie, hedgehogs, hares, ground-feeding birds, sheep cattle, and other large mammals). Man is the only species to show clinical disease. On introduction of virus by tick bite, there is local replication followed by blood and lymph-borne spread of virus to endothelium and major organs, including liver. Congestion, edema, focal hemorrhage, and necrosis of infected tissue follow (379). Disseminated intravascular coagulation then plays a role in disease progression and encephalopathy (272). Experiments with Crimean-Congo hemorrhagic fever in humanized mice have shown viral antigen in astroglia, microglia, and meninges, with virus entry to the CNS in turn facilitated by activation of astrocytes and impairment of astrocyte contribution to blood-brain barrier function (371). Cerebral edema with herniation or intracerebral, subdural, and subarachnoid hemorrhages have been reported (04; 79; 185).

One study from Turkey reported reduced susceptibility to Crimean-Congo hemorrhagic fever disease in individuals with the CCR5Δ32 heterozygous genotype, such that protection would link to CCR5 receptor down regulation (339; 104).

Ilesha virus. Ilesha virus, first isolated from a patient in Western Nigeria in 1957 (280), and isolated from Anopheles gambiae mosquitoes in the Central African Republic (93), has been associated with fatal meningoencephalitis in the Central African Republic (74) and hemorrhagic fever in Madagascar (254; 288).

Severe fever with thrombocytopenia syndrome (SFTS). Severe fever with thrombocytopenia syndrome, recognized in central and northeastern China in 2009, is caused by a novel phlebovirus isolated from febrile patients (434). The tick Haemaphysalis longicornis is the vector of severe fever with thrombocytopenia syndrome, and person-to-person spread through direct contact with contaminated blood and other body fluids has also been reported. Infection ranges from asymptomatic or self-limited to life-threatening. Encephalitis complications were reported in 19% of patients (82). Severe fever with thrombocytopenia syndrome cases have been found in China, Japan, and South Korea. Severe fever with thrombocytopenia syndrome’s closest relative is Bhanja virus, a tick borne human pathogenic phlebovirus that causes fever and meningitis. Two cases of febrile illness caused by another closely related tickborne phlebovirus (Heartland virus) were reported in the United States (241). The discovery of the emergence of this virus in China (home to one fourth of the world's population and numerous wild and domestic animals living close to humans) reiterates that country's significant potential for emergence or reemergence of infectious diseases.

Flaviviruses.

Yellow fever. Yellow fever virus is the prototype of the Flavivirus genus, transmitted by Aedes aegypti mosquitoes. After inoculation, the virus replicates in regional lymph nodes then spreads to liver, spleen, bone marrow, and cardiac and skeletal muscle. Bleeding is promoted by decreased synthesis of vitamin K-dependent coagulation factors by injured liver, disseminated intravascular coagulation, and altered platelet function (45). Neurotropic yellow fever infection of mice has been used as a model system for studies of flavivirus encephalitis.

Dengue hemorrhagic fever. Dengue is transmitted predominantly by A aegypti and occasionally by A albopictus mosquitoes. All 4 dengue virus serotypes cause dengue hemorrhagic fever. Dengue follows 3 clinical stages: acute febrile phase, critical phase, and recovery/convalescent phase. The immune response promotes dengue fever clearance and disease resolution, but also has a role in pathogenesis.

One pathogenesis hypothesis is that severe disease has an immunopathologic basis and antibody-dependent enhancement, and it occurs in individuals previously sensitized by infection with a heterologous serotype. The first infection provides long-term or life-long immunity to the infecting serotype but only poor short-term immunity to the other 3 (heterotypic) serotypes. Later, with a next infection with a different serotype, antibodies from the first infection bind but do not inactivate a heterotypic virus.

Capillary permeability shock syndrome is a multistep process attributed in part to antibody dependent enhancement (cross-reactive antibodies from the first DENV infection bind but fail to neutralize the second DENV serotype). Instead, they form virus-antibody complexes that in turn facilitate the infection of myeloid cells expressing Fc-gamma receptors (309; 110). But this observation has been difficult to fully scientifically confirm in patients because the majority of populations in dengue-endemic regions are serologically positive for all 4 dengue serotypes before school ages. In addition, the frequent recording of severe primary infections in travelers to endemic regions suggests additional, unidentified factors may account for the severity of re-infection with dengue heterologous serotype (244). For instance, no difference in capillary leak is detected using abdominal sonography between primary dengue (in 32% of cases) and secondary dengue (in 40% of cases) (244). Following up on earlier literature showing that 50% of sera from DHF/DSS patients have detectable endotoxin (394), current investigators suggest that the levels of lipopolysaccharide in plasma of dengue patients have a significant role in disease severity (395). Shock seen in dengue patients may partly be due to the leakage of gastric substances, likely endotoxin or commensal bacteria, into the systematic circulation. Nevertheless, whether the general concept on antibody-dependent enhancement of viral replication may worsen the outcome or not remains to be further investigated. Under the current dogma, the high level of virus replication seen during both secondary infection with a heterotypic virus and during primary DENV infection in late infancy is a direct consequence of antibody-dependent enhancement of replication. Translated to clinical care, this observation means that vaccines will need to provide long-term protection against each of the 4 DENV serotypes (260), and vaccine strategies for Zika virus would minimize the generation of deleterious cross-reactive antibodies to avoid ZIKV vaccines sensitizing to severe DENV infections (110).

Studies with specimens collected during visible dengue hemorrhagic fever link the hemorrhagic disturbance to microvascular injury, liver injury, platelet dysfunction, thrombocytopenia, neutropenia, and disseminated intravascular coagulation. Hemostatic defects, therefore, are the major finding in dengue hemorrhagic fever. Thrombocytopenia may be due to direct attachment of the dengue virus to platelets, infection of platelets, antiplatelet antibody-mediated platelet lysis, reduced production of platelets from megakaryocytes, and direct infection of megakaryocytes by dengue virus during the acute phase of infection. Procoagulants released from affected platelets, monocytes, or endothelial cells activated by inflammatory cytokines promote disseminated intravascular coagulation, and cross-reactive antibodies have been found between dengue E peptide and plasminogen (45). Increased levels of soluble thrombomodulin in blood (markers of vascular endothelial cell injuries and dysfunction) and levels of cytokines predict dengue hemorrhagic fever and dengue shock syndrome (46). The sources of these measurable parameters and their relation to vascular leakage are incompletely understood. However, a proposal, based on in vitro investigations, is that dengue virus-infected dendritic cells trigger vascular leakage through metalloproteinase production (214). Other work supports platelet dysfunction due to direct infection by dengue virus (276; 274) or increased nitric oxide production (69).

In addition to immune response, viral and host factors have a role in of severe dengue. The NS (non-structural protein)1 protein can interact with multiple host proteins, protects virus from neutralization, and has toxic properties that disrupt the endothelial glycocalyx. In man, genetic polymorphisms in the activating Fc gamma receptor FcyRIIA, cytokines, HLA, MHC class I chain related protein B, phospholipase C epsilon I, and genes in lipid and steroid metabolism pathways, are associated with more severe disease. Polymorphisms in genes that encode proteins linking lipid metabolism pathways with immune response, such as oxysterol binding protein-like 10 and RXRA (retinoid x receptor alpha), may underlie reduced susceptibility of people of African descent to severe dengue (414).

Dengue virus is responsible for a wide spectrum of disease: fever, arthralgia, myositis, rash, neurologic and psychiatric syndromes, hemorrhage, and shock. Neurologic involvement as encephalitis, transverse myelitis, or a lower motor neuron syndrome can occur without hemorrhagic disease, and dengue hemorrhagic fever with encephalopathic symptoms is described (174; 229; 396). Historically, dengue virus was not thought of as a neurotropic pathogen. The incidence of CNS involvement in dengue virus infection was low, ranging from 0.88% to 5.4% in past studies; however, the fatality rate is high (192).

The newest WHO guidelines for dengue diagnosis include neurologic manifestations in the clinical case classification for severe dengue (422). Wide ranges of neurologic features in clinical manifestation under the term dengue have been noted in 0.5% to 21% of patients admitted to the hospital (56). Furthermore, with the new WHO guidelines, encephalitis-like illness, ranging from 4% to 47% of admissions in dengue endemic regions, has been registered (56).

The evolution of CNS disease is thought to involve direct crossing of the blood-brain barrier by dengue virus infected cells or the virus infected cells riding with immune cells into the CNS. Dengue-infected mice showed a virus-induced, cytokine-mediated breakdown of the blood-brain barrier, providing access to the brain, perhaps by infiltration of virus-containing cells such as macrophages (68) or platelets (274). Dengue IgM antibodies or dengue RNA detected by RT-PCR are found in CSF of dengue patients with or without hemorrhagic disease, and dengue viral antigens can be demonstrated by immunohistochemistry in CNS in postmortem cases (71; 213; 246; 369; 48; 432; 249; 263; 381; 220; 193). Some patients can have virus in the CSF (as dengue viral RNA) and not in serum (98). Currently, neurologic complications in dengue-associated illness are categorized as dengue encephalopathy, encephalitis, neuromuscular complications, immune-mediated syndromes, and neuroophthalmic involvement (56). The main features of dengue maculopathy are: intraretinal hemorrhage, venular sheathing, vascular leakage, yellow subretinal dots, retinal pigment epithelium mottling, macular edema, foveolitis, disc hyperemia, and disc edema. Additional eye complications are exudative retinal detachment and optic neuropathy (329).

Of all the flaviviruses, Zika virus ZIKV is most closely related to the 4 serotypes of DENV and shares 54% to 59% amino acid identity of the viral E protein, a major immunodominant target for antibody responses (110). DENV-1 through -4 have 60% to 70% genetic homology (139). The similarity presents problems for diagnosis and vaccination, and has implications for disease pathogenesis due to antibody cross-reactivity.

There is experimental evidence that cross-reactivity has the potential to be either immune-protective or be associated with an aggravated form of the disease. Infection by DENV1 can confer transient protection to DENV2 in humans, and in vitro and animal studies suggest the possibility of protection against ZIKV by cross-priming by DENV1 (334). However, most of the Guillain-Barre syndrome cases associated with Zika infection in 1 region of Colombia had serologic evidence of prior DENV infection by IgM-capture and/or IgG capture ELISA for DENV (295), and all Guillain-Barre syndrome patients in a second area of Colombia had both ZIKV and DENV IgG antibodies (08). These results are consistent with ZIKV infection being a secondary enhancing flavivirus infection, and support the hypothesis that prior or coinfection of dengue with other agents may promote emergence of complicated cases, especially neurologic cases.

Given that the 2015 ZIKV outbreak in the Americas and the Caribbean occurred in dengue-endemic areas, the biological significance of cross-reactivity on protection, virulence, and immunopathology of DENV and ZIKV infections is critical, but there are unanswered questions. Would cross-reactive antibodies produced during prior DENV exposure worsen the severity of Zika through mechanisms like antibody-dependent enhancement, the mechanism that increases the severity of dengue after a second DENV infection? Would this process have a role in Zika congenital syndromes?

In Brazil, preexisting high antibody titers to dengue virus generally were associated with reduced risk of ZIKV infection and symptoms. However, there was 1 exception. The presence of DENV NS1-reactive molecules of IgG3 subclass, antibodies with a short half-life and indicating recent DENV infection, was inversely correlated with the probability of Zika infection. Therefore, very recent exposure to DENV transiently increased susceptibility to ZIKV infection. Antibodies work together to protect against infection most of the time but sometimes worsen disease (336; 411).

Kyasanur Forest disease/Omsk hemorrhagic fever. Kyasanur Forest disease and Omsk hemorrhagic fever are other flaviviruses associated with hemorrhagic fever and neurologic syndromes. They belong to tick-borne encephalitis antigenic complex. Kyasanur Forest disease virus is endemic in several areas of Karnatka and Kerala states of India. Infection is by ixodid tick bite. The virus has been isolated from 16 species of ticks, with Haemaphysalis spingera the major vector. The virus produces systemic illness with fever, headache, myalgia, hypotension, gastrointestinal symptoms, and hemorrhages followed by an asymptomatic period of several days, then fever and signs of meningoencephalitis. In most patients, the illness is biphasic, with the second wave of symptoms at the beginning of the third week. Omsk hemorrhagic fever virus, which occurs in the western Siberian regions of Omsk, Novosibirsk, Kurgan, and Tyume, causes similar systemic and febrile illness, except for frequent sequelae of hearing loss and neuropsychiatric complaints (45). Human infections occur by direct contact with urine, feces, or blood of infected rodents: most commonly muskrat (Ondatra zibethica), water vole (Arvicola terrestris), and narrow-skulled voles (Microtus gregalis). Rodents are the primary host for Omsk hemorrhagic fever, which acquire the virus from the bite of an infected tick. Tick vectors include Dermacentor reticulatus, Dermacentor marginatus, and Ixodes persulcatus.

Alkhurma virus, a subtype of Kyasanur Forest disease, was isolated in 1994 from a patient in Saudi Arabia initially thought to have dengue hemorrhagic fever (435). Hemorrhagic and encephalitic syndromes associated with up to 25% mortality developed in sheep handlers, butchers, and meat and raw camel’s milk consumers. Infection is caused by tick bite, direct contact with infected blood, or by consumption of raw milk (66).

Tick-borne encephalitis. The tick-borne encephalitis complex consists of 17 antigenically related viruses, mainly represented by Russian spring-summer encephalitis virus (the Far Eastern variant of tick-borne encephalitis) or the milder Central European encephalitis virus. They are usually not on the viral hemorrhagic fever list, but sporadic reports of Far Eastern tick-borne encephalitis with encephalitic and hemorrhagic manifestations began appearing in the early 1990s (01). Far Eastern tick-borne encephalitis then “became,” or could be considered hemorrhagic, after further reports of tick-borne encephalitis with hemorrhagic syndrome in the Novosibirsk region Russia in 1999. The genome of hemorrhagic variants differed from all known tick-borne encephalitis and Omsk hemorrhagic fever strains. The hemorrhagic Far Eastern tick-borne encephalitis strain carried mutations in envelope protein E (a protein involved in receptor binding, virus entry, and virulence) and belonged to the Far Eastern genomic subtype. The mutations in protein E were not previously described for tick-borne encephalitis but found among other flaviviruses (yellow fever, dengue type 1,2,3, and 4, Kyasanur, and others) (387), suggesting the representation of certain mutations known previously only in other hemorrhagic flaviviruses predisposes to hemorrhagic phenotype. Ixodes persulcatus and Ixodes ricinus ticks are responsible for transmission in Russia and Europe, respectively. Other tick species, of genera Dermacentor and Haemaphysalis, have also been implicated in transmission in areas without Ixodes species ticks (45).

Togaviruses. Chikungunya, family Togaviridae, genus Alphavirus, is another arbovirus with infrequent hemorrhagic complications on the ICD-9 list of hemorrhagic arboviruses. It is endemic in India, Southeast Asia, the Philippines, and sub-Saharan Africa, where the usual presentation is fever and arthritis. In Africa, the virus is maintained in cycles similar to that of yellow fever virus, between Aedes africanus, A furcifer and wild primates, or Aedes aegypti and humans. Infrequently, a thrombocytopenic hemorrhagic version with shock occurs (129; 300). The extension and explosion of Chikungunya virus cases in Europe (Italy), which started with 2 cases in Ravenna Province, Italy, in July 2007 (12), is likely the result of the virus extending its host range from A aegypti to A albopictus (Asian tiger) mosquitoes due to a mutation in the viral envelope glycoprotein (E1). The mutation occurred when the virus was in circulation in India and the Indian Ocean nations during the preceding 2 years, as shown by studies of the outbreak in La Reunion, an island nation in the Indian Ocean, which involved one third of that island’s population. The (A226V) mutation, not present in the original 2005 to 2006 La Reunion epidemic viral isolates, increased infectivity for A albopictus mosquitoes and increased transmission efficiency to vertebrate species (393). By the close of the epidemic in La Reunion, the mutation was detected in more than 90% of viral isolates (345). Unfortunately, A albopictus is a species widely distributed in Europe and North America. The Chikungunya epidemic in the Caribbean that spilled over into the U.S. in returning travelers achieved endemic transmission through mosquitoes in Florida in 2014.

Epidemiology

Arenaviruses. Arenaviruses cause chronic asymptomatic infections of reservoir rodent hosts. The epidemiology of arenavirus diseases depends on the patterns of infection in the reservoir hosts and on factors that bring man in contact with rodents. Lassa virus is enzootic in Mastomys natalensis, peridomestic rats found throughout West Africa. Persistently infected rats excrete virus in urine and feces. Transmission of infection from rodent to man, an accidental host, occurs via contamination of superficial wounds by rodent urine or excreta, consumption of food contaminated by rats, or inhalation of aerosols or fomites from infected rodents. For Lassa fever, human disease occurs maximally during the dry season (winter months) in West Africa through contact with rats, genus Mastomys, and mice, Hylomycus pamfi, principally through agricultural, lumbering, or diamond mining activities or by contamination of food stores. There are an estimated 100,000 to 300,000 Lassa fever cases in West Africa each year, and there are approximately 5000 deaths. Junin virus infection increases in the fall in the Argentine pampas from harvest activities that bring man in contact with the small field rodent, Calomys musculinus. Machupo virus is carried by the field rodent C callosus. Disease activity is in the Beni province of Bolivia. Guanarito is carried by another field rodent, Zygodontomys brevicauda, in Venezuela. Venezuelan hemorrhagic fever was recognized in 1989 following forestry activity and clearing of land for small farms (43). To date, the International Committee for Taxonomy of Viruses recognizes that the family Arenaviridae contains the genus Arenavirus, which includes 24 viral species (67):

There are, in addition, about 10 arenaviruses that have been discovered for which taxonomic status is pending. Of these, Merino Walk virus, which was isolated from the rodent Myotomys unisulcatus in South Africa (290), Luna virus, associated with Mastomys natalensis in Zambia (157), and Morogoro with Mastomys natalensis in Tanzania (133) have not yet been associated with disease in man. The rodent host for Lujo virus in southern Africa is not known. Ocozocoautla de espinosa virus in the Tacaribe serocomplex, associated with P mexicanus deer mice, is thought to have been the cause of a hemorrhagic fever epidemic in Chiapas, Mexico, in 1967. Presently, it is the cause of illness clinically identical to dengue hemorrhagic fever and other severe febrile illnesses endemic in Chiapas state (47).

Although the arenavirus family has been thought to infect only mammals, 2 complete viruses and partial sequence from a related third virus have been isolated from cases of snake inclusion body disease, named for the large eosinophilic inclusions found in cytoplasm of cells, including brain, of infected animals. It is a fatal disease of captive snakes, characterized by behavioral abnormalities (abnormal body posture and movement), wasting, and secondary infections. The viruses share a typical arenavirus-like genome organization but have filovirus-like GP2 domains (373). These viruses show the arenavirus family and its hosts to be much broader than previously recognized. Identification of these highly divergent arenaviruses can be used for development of diagnostics, antivirals, multivalent vaccines, or vaccines based on development of immunity against conserved regions (34).

A conventional vaccine, a live attenuated strain against Argentine hemorrhagic fever, has been the most successful arenavirus vaccine. It is an attenuated vaccine strain Candid#1 (Cd1), derived from the pathogenic XJ strain of JUNV by serial passaging in guinea pigs, mouse brain, and cultured fetal rhesus monkey lung cells, and has an estimated efficacy of 98% (100).

Patients with arenaviral hemorrhagic fevers are thought to pose relatively limited risk of infection in the early stages of disease. As illness progresses, with more extensive infection of target organs and increasing blood virus titers, chances for dissemination increase. Person-to-person spread is uncommon except with Lassa fever virus, although nosocomial outbreaks and infection of multiple contacts, including sexual contacts, have occurred for each of the viruses. For Lassa fever, person-to-person transmission in hospitals and the community occurs by contact of mucous membranes with infected blood or body fluids. The tendency of disease to spread among contacts of acute cases suggests an aerosol mode of transmission as well. After recovery, a patient may continue to excrete virus in urine or semen for weeks. Lassa is the most frequently exported viral hemorrhagic fever. Its incubation period is 7 to 18 days (43).

Filoviruses. Filovirus disease was first recognized in Marburg in 1967 in individuals that had handled monkey blood, tissues, or cell cultures from the African green (vervet) monkeys (Cercopithecus aethiops) that came from Uganda. The first recorded human outbreaks of Ebola were in Democratic Republic of Congo and Sudan in 1976.

Despite multiple internationally-recognized outbreaks, filoviruses remain the only causative agents of viral hemorrhagic fever for which a natural maintenance cycle has not been definitely identified. Bats were suspected because of the association of index cases with caves (Marburg, Kenya 1980), a bat-infested gold mine (Marburg, Democratic Republic of Congo 1999), and a bat-infested factory (Ebola, Sudan 1976). Evidence of asymptomatic infection by Ebola virus was found in 3 species of fruit bat in Africa (206), and insectivorous bats are thought to have been the source for the West African Ebola epidemic (224). Duikers and great apes may be intermediate or amplifying hosts. Wild and domestic pigs may be a hidden source for Ebola-Zaire because virus transmission from pigs to cynomolgus macaques without direct contact has been observed (408).

The geographic range of known Ebola human cases has included most of the African rain forest. Prior to the 2013 to 2016 West Africa epidemic, with 28,616 cases and 11,310 deaths recorded, there had been 2317 human clinical infections and 1671 confirmed deaths caused by Ebola-Zaire and Ebola-Sudan documented since filoviruses were first identified more than 40 years ago. One half had occurred in the 8 years prior to 2013. A virulent Ebola species, Bundibugyo ebolavirus, caused the hemorrhagic fever outbreak in western Uganda in late 2007 (390). There are 5 species of Ebolavirus: Zaire Ebolavirus (ZEBOV), Sudan Ebolavirus, Tai Forest Ebolavirus, Bundibugyo ebolavirus, and Reston ebolavirus. Twice, Marburg virus has caused large outbreaks in Africa, including once in Angola, a country where filoviral hemorrhagic fever had never before been seen (39).

Ebola was newsworthy in January 2009 with the report of Ebola-Reston in an important livestock species, pigs, in the Philippines and detection of antibodies to Ebola-Reston in 1 healthy pig farmer (184). The presence of the virus on pig farms increases the odds of human exposure and infection.

Serologic and virus surveys of bats in Bangladesh completed in 2013 showed a fruit bat species (Rousettus leschenaultii) with a wide geographic range, from China to India, to have antibodies to Ebola Zaire and Reston viruses. The report establishes these bats as a potential reservoir for Ebola or Ebola-like viruses, and extends the range of filoviruses to mainland Asia (283).

In man, spread of filovirus infections is by close contact with another case through body fluid contact or direct contact with infected animals. Transmission by fomites is possible. Aerosol transmission, shown for nonhuman primate infections, may apply for humans. The incubation period is 7 to 9 days. Sexual transmission has been reported for Marburg and Ebola viruses; precautions should be taken during convalescence months (343).

It has been assumed that all known filovirus outbreaks in man have been the result of independent zoonotic transmission events, from reservoir bat species or an intermediate or amplifying host such as duikers and great apes. Now, however, 2 outbreaks, Guinea 2021 and DRC North Kivu Province 2021, are the result of transmission from humans infected in previous epidemics (178; 425).

West Africa’s first major Ebola outbreak began in December 2013. What was first thought to be a self-limited outbreak in a forested area of Guinea spread rapidly. By March 2014, the largest Ebola outbreak in history was erupting across Guinea, Sierra Leone, and Liberia. The severe Ebola epidemic of West Africa is thought to come from a single zoonotic transmission event to a 2-year-old boy in Meliandou, Guinea, followed by human-to-human transmission. The index case was likely infected when playing in a hollow tree housing a colony of insectivorous free-tailed bats (Mops condylurus). The report expands the range of possible Ebola virus sources (eg, fruit bats and susceptible hosts such as nonhuman primates and duikers) (282) to include insectivorous bats (224).

Ebola virus from Guinea forms a separate clade in relation to the known Ebola virus strains from the Democratic Republic of Congo and Gabon (19) and constitutes a different outbreak from the 69 cases that occurred in the Democratic Republic of Congo in July to Oct 2014 (285).

As of early 2016, the total numbers of cases from the West Africa epidemic were 28,000, and more than 11,300 deaths. Besides Guinea, Liberia, and Sierra Leone, affected countries were Nigeria 20 cases, Senegal 1, Spain 1, United States 4, Mali 8, UK 1. Prior to the 2013 to 2015 Ebola epidemic in West Africa, only 7 outbreaks were associated with more than 100 reported cases (285).

The usual Ebola incubation period is 3 to 8 days but can be as long as 3 weeks. Each new confirmed case pushed the end of the outbreak another 6 weeks into the future, twice the incubation period. The last new cases were found in Guinea and Liberia in March 2016. WHO declared the end of virus transmission in Guinea and Liberia in June 2016. West Africa's epidemic has shown more needs to be done to prioritize research in future outbreaks.

In May 2017, an Ebola outbreak was declared in the Democratic Republic of Congo in the northeast Bas-Uele province, bordering the Central African Republic and was over by July 2017. In May 2018, an Ebola outbreak, also in Democratic Republic of Congo, started in the northwest, Bikoro, Equateur Province and was over by July 2018.

For the first time, 2 epidemiologically and genetically distinct Ebola outbreaks emerged within weeks of one another. The Congo’s tenth Ebola epidemic since the disease was discovered in 1976 was declared on August 1, 2018 in the troubled north east of the Democratic Republic of Congo, where the affected provinces (Kivu Nord and Ituri) had never had the disease before. Despite deployment of interventions greater than any previous efforts, including vaccination of more than 90,000 people, things have not gone well. The Democratic Republic of Congo is the country with the world’s longest-running humanitarian crisis. The outbreak is occurring in areas of continued conflict among multiple armed groups at the time of troubled elections. People perceived the organized Ebola response as politicized and had little trust or willingness to cooperate in local activities. With dozens of armed civilian or rebel groups active in the northeast, security was a great concern, and health workers and facilities were repeatedly attacked (169).

As of July 2019, there were 2522 cases and 1698 deaths in the Democratic Republic of Congo (318). Against the background lack of community engagement and demilitarization, Ebola spread to the border city of Goma, a city of nearly 2 million. The high-risk for regional spread from this Rwanda-border city and detection of cases in Uganda prompted the World Health Organization to declare the northeast Democratic Republic of Congo the second largest of Ebola epidemics, a “public health emergency of international concern” on July 17, 2019. The declaration brought more international coordination, technical capacity, and funding. All neighboring countries, including Uganda, Rwanda, Burundi, and South Sudan, prepared for potential outbreaks. All together, 297,699 people were vaccinated.

With a total of 3458 Ebola cases and 2266 deaths, the 2017 to 2020 Democratic Republic of Congo epidemic in the northeast was the second largest Ebola outbreak in history. On Feb 17, 2020, once there had been no new cases of Ebola in the Democratic Republic of Congo, the outbreak would be declared over after waiting 2 full incubation periods (42 days) after the last person tested negative a second time (313). The country had 54 days without a new case and was ready to declare the official end April 13, 2020. However, starting on April 10 there had been 4 new cases, resetting the counter on this viral hemorrhagic fever epidemic (314). This was the 10th Ebola outbreak in the Democratic Republic of Congo and claimed the lives of 2287.

An 11th DRC outbreak in Equateur Province took 55 lives. The 12th outbreak was declared when new cases of Ebola resurfaced in Northeast Democratic Republic of Congo on February 7, 2021 (319). The 13th outbreak began in October 2021 in North Kivu Province, the same area in which the 2018 outbreak was declared over. The first case in the second North Kivu outbreak represented a new flare-up of the 2018 to 2020 outbreak, due to persistence of virus in the community (425). The Democratic Republic of Congo’s 14th outbreak, and third in Equateur province since 2018, began in April 2022. The index case was a student vaccinated in 2020, and genetic analysis was consistent with this being a new spillover from an animal reservoir (320).

A novel ebolavirus species discovery was reported in the July 27, 2018 ProMed post (316). The new virus, called Bombali virus, was found in insectivorous bats in Sierra Leone. It is not the virus that caused the 2013 to 2016 West African epidemic, which belongs to the species Ebolavirus Zaire. Whether Bombali virus has infected anyone or is pathogenic for man is not known.

Bunyaviruses. Hantaviruses are temperate-zone viruses transmitted to people mainly through inhalation of aerosolized rodent excreta or saliva. The epidemiology of Hantavirus diseases depends on the patterns of infection in the reservoir hosts, factors that bring man in contact with rodents, and population density. Hemorrhagic fever with renal syndrome is a group of clinically similar rodent-borne infections: Hantaan, Dobrava, Seoul, Saaremaa, and Puumala; characterized by the clinical triad of acute renal failure, fever, and hemorrhage (29).

Hantaan virus is found in parts of eastern China, Korea, and Far Eastern Russia and is present in Apodemus agrarius mantchuricus, a mouse common in agriculture fields. Hemorrhagic fever with renal syndrome cases are more common in fall, reflecting abundance of infected mice in fall, contact with mice during crop harvests, and movements of rodents into houses with winter. Seoul virus is associated with Rattus norvegicus and R rattus and is worldwide in distribution. Dobrava virus is found in Apodemus flavicollis and A agrarius in the Balkans, and the Far East. Puumala virus is associated with the bank vole, Clethrionomys glareolus, in Scandinavia. The incubation period is 7 to 21 days (272). Studies in Hungary show that Puumala, Dobrava, and Saaremaa, all pathogenic hantaviruses, co-circulate in the same geographic area and can be maintained side-by-side in different rodent species (305). Historically, hantaviruses were considered to be associated with specific rodent reservoir hosts due to a long coevolutionary history. However, the discovery of new hantaviruses in nonrodent mammalian hosts, including shrews, moles, and bats (409; 135), now challenges the idea of specific viruses associated with very specific rodent species (332). The addition of viruses of the hantavirus genus to the long list of bat-borne viruses, further supports bats as primordial reservoirs for the majority of viruses pathogenic for man. People infected with the North American strain of hantavirus pulmonary syndrome are not contagious to others. However, some outbreaks in South America have shown evidence of person to person transmission.

Rift Valley fever. The geographic distribution of Rift Valley fever is Africa and the Arabian Peninsula, where there is vertical infection of virus in floodwater by Aedes mosquitoes. In epidemics, there is horizontal transmission by many different mosquito species between domestic animals. Viremic livestock are amplifying hosts. Man acquires infection by mosquito bite, exposure to aerosols or fomites from infected cattle or sheep, or consumption of raw milk or meat. The incubation period is 2 to 6 days. Hemorrhagic fever cases in humans are usually seen 1 to 2 weeks after the appearance of abortions and disease in livestock. Laboratory infections occur because Rift Valley fever is highly infectious by aerosol (272). Rift Valley fever has been isolated or has been shown to infect mosquitoes of mainly Aedes and Culex, but also Anopheles, Coquillitidea, Eretmapoites, Mansonia, and Ochlerotatus genera, as well as sand flies. Over 40 species of mosquitoes are competent vectors; therefore, mosquitoes in North America and Europe could be competent vectors (240).

Crimean-Congo hemorrhagic fever. Crimean-Congo hemorrhagic fever occurs in roughly 30 countries and has the most extensive geographic range among the tick-borne viruses that affect human health. The overall global distribution of Crimean-Congo hemorrhagic fever: Africa, Middle East, the Balkans, southern former Soviet Union, and western China, corresponds most closely with the distribution of Hyalomma ticks, although members of the genera Dermacentor and Rhipicephalus may also become infected, pass the virus through the various stages of the life cycle, from larva to nymph to adult, and transmit. Human infections are acquired by bite from infected ticks, contact with ticks during their removal, or contact with blood or tissues of infected livestock (sheep, goats, cattle, or ostriches). The incubation period length depends on mode of acquisition of virus. Following tick bite infection, the incubation period is 1 to 3 days, with a maximum of 9 days. There have been several nosocomial outbreaks (272).

Crimean-Congo hemorrhagic fever is more prevalent in summer. The dates in the Islamic Calendar for Eid-Al-Adha are drifting 10 days earlier each year. Starting in 2015, and for the next 10 to 15 years, the festival will be celebrated in summer months. The summer season potentially increases risk of more human cases due to increased movements of animals, careless practices of animal slaughter, and inadequate knowledge of the disease (310).

Erve virus. Erve virus, a European Nairovirus distantly related to other Nairoviruses such as Crimean-Congo hemorrhagic fever virus, has been associated with severe “thunderclap” headaches without evidence of subarachnoid hemorrhage (392) and has been suspected in intracerebral hemorrhage in other cases. Named for the Erve Valley in northwest France, Erve virus is found in France, Germany, the Netherlands, and Czech Republic. Rodents, insectivores, wild boars, red deer, sheep, and seabirds are seropositive, and the mode of transmission to humans, whether ticks, mosquitoes, or sandflies, is unknown (392; 94).

Severe fever with thrombocytopenia syndrome virus. Severe fever with thrombocytopenia syndrome virus is a novel phlebovirus found in China in 2009, and then subsequently in Japan, South Korea, Taiwan, and Vietnam; it causes illness with symptoms of fever, thrombocytopenia, leukopenia, GI, muscle, neurologic symptoms, encephalitis, encephalopathy, coagulopathy, and hemophagocytosis. The virus is found in farm animals and domesticated deer and is spread by long-horned tick Haemaphysalis longicornis. There have been several nosocomial outbreaks. The indigenous Asian tick vector of severe fever with thrombocytopenia syndrome (bunya) virus was found in the United States in 2017 (25).

Flaviviruses.

Dengue. Dengue virus infections are 1 of the most common mosquito-borne viral diseases of humans worldwide. Initially, dengue infections were primarily recorded when they occurred as epidemics in tropical and subtropical countries. But over time, increasing globalization, rapid human movement, and unplanned urbanization coupled by the increase in the geographic area that Aedes species mosquito vectors Aedes aegypti and Aedes albopictus inhabit have supported the spread of dengue virus infection to nearly every corner of the world. By WHO estimates published in 1997, 4 out of every 10 people in the world are at risk for dengue virus infection (420). Undoubtedly, that number is larger now. Approximately 100 million people contract dengue fever annually, and about 200,000 to 500,000 contract dengue hemorrhagic fever. The mortality rate is about 5%, predominantly in children under 15 years of age (138; 276). Today, dengue is the leading cause of fever in returning travelers, having surpassed malaria for visitors to Southeast Asia. One in 6 tourists returning from the tropics is estimated to be infected with dengue, and a dengue fever outbreak in the Portuguese archipelago of Madeira spread cases to several other European countries in late 2012.

Since 2001, autochthonous dengue fever outbreaks have occurred in the continental United States and Hawaii: Hawaii (2001, 2005); Brownsville, Texas (2005); South Texas (2013); South Florida (Key West 2009 to 2011, Martin County 2013, Florida Keys 2020) (Centers for Disease Control and Prevention 2020). Twenty-six dengue cases were acquired in Key West, Florida in 2009 and 63 cases in 2010. Transmission was confirmed by detection of DENV-1 in local mosquitoes and Aedes aegypti mosquito pools (127; 03; 328). According to the statistics for the year 2012, there had been 3937 locally acquired dengue fever cases in the United States (4 in Florida and 3933 in Puerto Rico). Finding DENV-4 circulation in Aedes aegypti mosquitoes collected in Manatee County, Florida in 2016 and 2017 was significant for showing not only prolonged maintenance within a local mosquito population in the continental United States, but also that continued vector infection was occurring in the absence of a human index case in the region (38). Silent circulation is attributed to low but persistent vertical transmission in mosquito populations (78).

Blood donors with asymptomatic DENV infections are a potential source of virus spread by transfusion (Thomashek and Margolis 2011; 92).

A surprising outbreak of dengue in Tokyo in 2014 raised concerns for a potential outbreak during the 2020 summer Olympics and Paralympics in Tokyo, and led to development of a preparedness plan for dengue detection. Key features were the national infectious disease control system to detect dengue, training or certification program for physicians on tropical disease management, strengthening multilanguage communications methods, consideration for a tropical disease training program for accommodation staff, along with a contingency plan for infectious disease-suspected travelers (430).

Yellow fever. This is a zoonotic disease maintained by mosquito-monkey-mosquito transmission with occasional human infection when unvaccinated humans enter the forest. Alternatively, in urban epidemics, there is interhuman transmission by the domestic mosquito, A aegypti. Yellow fever occurs throughout much of tropical South America and sub-Saharan Africa. There are about 200,000 cases per year (45).

Kyasanur Forest disease. First described in 1957 in the state of Mysore, India, (renamed Karnataka in 1973), Kyasanur Forest disease is now also found in China (243; 406). The virus cycles between ixodid ticks and wild vertebrates, mainly rodents and insectivores. Monkey deaths may signal increased virus activity. Most human infections occur from tick bites (45).

Omsk hemorrhagic fever. This hemorrhagic fever occurs in western Siberia. It’s poorly understood cycle involves ticks, water voles, muskrats, and possibly water-borne transmission. Muskrats are epizootic hosts and human infections occur by direct contact with urine, feces, or blood (45).

Alkhurma virus. Alkhurma virus, endemic in the Arabian Peninsula, may have arisen from the introduction of Kyasanur Forest disease virus from India to Saudi Arabia (96). Sheep, goats, and camels are epizootic hosts. Like other tick-borne flaviviruses, it also infects humans via direct contact with infected blood or by consumption of raw milk. Alkhurma hemorrhagic fever virus (AHFV) infections in man, including syndromes of rhabdomyolysis and muscle weakness in European patients returning from Egypt, confirms the presence of AHFV in Egypt and supports the position that the current endemic area is extending in Africa via the animal trade (54; 331).

Chikungunya virus (Alphavirus, family Togaviridae). Chikungunya fever was first recognized in East African epidemics in 1952 and 1953. Since 2006, the disease has shown explosive emergence in nations of the Indian Ocean area, which then spread to India, China, and Europe.

Since 2012, Chikungunya virus also has been spreading to Pacific Island nations. Epidemics are sustained by human-mosquito-human transmission, with mosquitoes of Aedes species. In 2014, there were more than 600,000 cases in the Caribbean and 1471 cases in the United States. Not all were related to travel, signifying that mosquito transmission is taking place in Florida, where, like dengue, Chikungunya is likely to become endemic (404). Chikungunya virus’s outbreak history has illustrated long-distance movement of this virus by viremic individuals, with introductions of virus into areas where vector mosquitoes are abundant enough to allow continued transmission.

Aedes aegypti and Aedes albopictus are the main vectors of CHIK, Zika, and dengue.

Prevention

The best prevention is to minimize exposure to the virus. Prevention of rodent-borne arenaviruses and hantaviruses consists of preventing contact with rodents and their excreta. Effective measures include rodent proofing of homes, correct storage of food, disinfection, and removal of trapped rodents and rodent droppings. For filoviruses, contact with suspect species of fruit and insectivorous bats or with sick or dead monkeys should be avoided. Treatment of clothing with pyrethroid preparations that repel or kill ticks and tucking in pant legs are recommended for those likely to come in contact with Crimean-Congo hemorrhagic fever-infected ticks, including individuals with occupational exposure to livestock. Immunization of livestock is the most effective way to prevent Rift Valley fever epizootics and human cases. Yellow fever is prevented by yellow fever 17D, a live viral vaccine that confers immunity within 10 days. Areas containing the domestic form of A aegypti, at risk for introduction and urbanization of yellow fever, are water-treated and sprayed. Prevention of dengue epidemics depends on reduction or eradication of A aegypti by breeding site elimination, use of larvicides, and spraying of insecticides. However, resistance to widely used insecticides (pyrethroids, organophosphates, organochlorines) are developing in mosquito populations. When insecticide-treated bed nets are less effective, application of antiviral drugs to bed nets to inhibit viral infection of the vector may help (99). The next-generation tools to vector-control of mosquito-borne viral diseases include gene-drive-based mosquito population control and release of mosquitoes infected by the symbiotic bacterium Wolbachia to make mosquito populations refractory to viral infection (52).

Dengue vaccines or antiviral drugs are under development. Presently there are 6 vaccines for dengue at various clinical trial and approval stages (359). The farthest along and now licensed is Sanofi’s ChimeriVax, a tetravalent vaccine composed of 4 recombinant, live attenuated viruses (yellow fever 17D/DEN chimeric viruses), which completed phase III trials in highly endemic Latin American (402) and Asian countries (51), including roughly 30,000 children aged 2 to 16 years, and the vaccine has been licensed in several countries for those aged 9 years and above. Specific issues with the Sanofi vaccine were serotype-dependent efficacy in early clinical trials (145; 340), but the vaccine had higher efficacy in individuals with prior dengue infections and higher efficacy with increased age (416). Sanofi Pasteur’s licensing request was for CYD-TDV (Dengvaxia) in the 9 years and older age group (416). WHO originally recommended that countries consider vaccination against dengue with CYT-TDV in geographic locations where prior infection with any serotype was greater than 70% in the target age group, and it was not recommended where seroprevalence was less than 50% (156). CYD-TDV was licensed first in Mexico for individuals 9 to 45 years of age living in endemic areas to be given as a 3-dose series 6 months apart. Given the benefit of CYD-TDV to dengue-primed individuals, ie, vaccination against all 4 serotypes, now Dengvaxia is licensed in 20 dengue-endemic countries of Asia, Latin America, and Australia in persons aged 9 to 45 years.

Long-term safety follow-up has shown that the vaccine performs differently in seropositive versus seronegative individuals, with lower efficacy in seronegative individuals and increased risk to this group of hospitalization and severe dengue starting 30 months after the first vaccine dose. Dengvaxia presumably mimics an initial natural exposure in dengue-naive recipients, such that subsequent natural infection could cause severe disease by antibody-dependent enhancement. Therefore, SAGE (Strategic Advisory Group of Experts on Immunization) for WHO has stated a preference for “pre-vaccination screening strategy,” resulting in only dengue-seropositive persons being vaccinated. Currently, consideration of this strategy would be at the country level, taking into account sensitivity and specificity of available tests, local dengue epidemiology, and cost (424).

The development of dengue vaccines was delayed by the possible or theoretical risk of vaccine-related adverse events, such as immune enhancement of infection and the requirement to induce long-term protective immunity against all 4 dengue serotypes simultaneously. The overall efficacy of the tetravalent dengue vaccine in clinical trial did not quite live up to expectation (340), suggesting that alternative approaches toward dengue vaccine development are needed.

The Sanofi Pasteur (CYD-TDV) (Denvaxia) vaccine has not presented a straightforward solution and failed to meet the needs of a dengue-naive population. Estimations of vaccine efficacy varied by previous seropositivity to dengue, infecting serotype, clinical severity, and age. There was higher efficacy in trial participants aged 9 to 16 years than in those aged 2 to 8 years. A 3-dose schedule is required, but good neutralizing antibody concentrations alone may not be protective. Therefore, the results of the performance of second-generation dengue vaccines, based on live attenuated vaccines with nonstructural proteins of the dengue virus backbone partially present, or DNA, subunit, virus-like particles, and viral vector technologies, are eagerly anticipated (415).

Vaccine candidates farthest along in development are also live attenuated vaccines, differing in type of backbone and extent of chimerization from CYD-TDV. TAK-003 is Takeda’s live attenuated chimeric tetravalent dengue vaccine. Designed to induce both humoral and cellular immunity, it differs from CYD-TDV in being based entirely on DENV genome, using an attenuated DENV-2 strain plus 3 chimeric viruses (containing premembrane and E genes of DENV-1,3, and 4) cloned into an attenuated DENV-2 backbone, rather than yellow fever. The DENV-2 backbone may generate T-cell mediated responses to dengue infection, in addition to humoral responses to the premembrane and DENV proteins. However, problems remain. NS1 proteins from different serotypes differ in their amino acid sequences. Serotype immune dominance of DENV-2 might hinder the immune response to other serotypes, as shown by DENV-4 serodominance in the Sanofi-Pasteur trials (413). TV003/TV005 (developed at NIH NIAID) has 3 full length DENV 1,3, and 4 attenuated by deletions in the 3’ untranslated region, and DENV-2/DENV-4 chimera containing DENV 2 PM and E coding regions. TV005, a formulation of the same components as TV003 but having a higher dose of DENV-2/DENV-4 chimera, generated a balanced, robust response to all 4 serotypes. TV003/TV005 has been licensed to Instituto Butantan (Brazil), Indian and Vietnamese groups, for ongoing efficacy evaluation (172; 415).

Vaccines eliciting immune response have been proven to be one of the most successful medical interventions against viral infections. However, as shown above, dengue has presented difficulties. The morphology of the dengue virus obtained in tissue culture significantly differs from that of in vivo virus. Serum or plasma from patients during secondary infection show high levels of antibody-dependent enhancement (ADE), but no neutralizing activity, with culture-adapted virus strains; however, serum or plasma from the same patient have high levels of neutralization activity but are without ADE with autologous virus directly from patient plasma. Thus, antibody-dependent enhancement effect is with virus derived from culture-adapted virus strains (65). In addition, neutralizing antibody titers obtained with culture-adapted virus do not correlate with the protective effects in vivo (361).

This evidence clearly demonstrates the uniqueness of viral morphology in vivo. As such, considering the multiple serotypes of dengue virus, 1 of the strategies to develop vaccines that can minimize the potential adverse effects is to have an antibody with broad-spectrum neutralizing capacity. However, understanding the molecular bases of immune recognition is 1 of the big challenges in vaccine development. The observation that whole protein antigens are not necessarily essential for inducing immunity has led to the emergence of a new branch of vaccine design termed structural vaccinology. One of the goals of structural vaccinology is to enable the design and engineering of improved antigens, suggesting that elucidation of the three-dimensional (3D) structure of antigens and antigen-antibody complexes is central to structural vaccinology. If the 3D structure of the antigen alone is available, it is possible to use its atomic coordinates for the prediction of immunogenic regions, which can then be used for generating optimizing antigens in the form of synthetic peptide epitopes. As a result, resolving the 3D viral morphology of in vivo dengue viral morphology is urgently needed in order to obtain a viral epitope that can have properties responsible for the ability of antigens to elicit functional or protective antibodies, which can have broad neutralizing ability and would be an ideal dengue vaccine candidate. Whether other dengue vaccine candidates in the pipeline (inactivated, live attenuated, chimeric recombinant, subunit DNA) will have higher efficacy than CYD-TDV remains to be seen. Presently, CYD-TDV is the only dengue vaccine with WHO approval available internationally. Its moderate efficacy supports the strategy of integrating vaccination with improved vector control, which would have added value in helping contain other Aedes-transmitted diseases: CHIK, Zika, yellow fever (416).

In summary, Dengvaxia has been endorsed by the European Medicine Agency and the US Food and Drug Administration. Because of excessive risk of severe dengue in seronegative vaccinees, use is restricted to seropositive individuals. The FDA has approved use in people 9 to 16 years of age who have laboratory-confirmed previous dengue infection and live in endemic areas; youth are at greatest risk for severe symptoms from a second infection of another serotype. Endemic areas include United State territories of American Samoa, Guam, Puerto Rico, and United States Virgin Islands. Dengvaxia is not an ideal travel vaccine due to the absence of a validated rapid diagnostic test to screen dengue serostatus and the 3-dose regimen required for protection (414). Correlates of risk or protective immunity, or neutralizing antibodies or cell-mediated immune responses, have not been established, but the Dengvaxia trials offer possibilities for retrospective analysis to determine correlates of protection.

For Ebola, rVSV-ZEBOV vaccine has been licensed and used in emergency situations. Ad26.ZEBOV, MVA-BN-Filo, first available under investigational protocols, has EU marketing approval. Both are discussed in the Management section.

To prevent nosocomial outbreaks, patients should be nursed in a hospital with barrier precautions where sterile instruments and equipment for injections are provided.

For high-risk exposure to an agent sensitive to ribavirin, ribavirin has been recommended prophylactically or as expectant treatment at the first sign of fever. Close personal contacts should be monitored for fever for a period of 3 weeks.

South American arenaviruses, in contrast to Lassa fever virus, produce high-titer neutralizing antibodies and passive serum therapy has been effective. Candid #1, a live attenuated vaccine against Junin virus licensed in Argentina for adults greater than 15 years of age, also protects nonhuman primates against Machupo virus (Bolivian hemorrhagic fever) (179).

Several different inactivated virus vaccines for Hantaan and Seoul viruses have been used in Asia. A Crimean-Congo hemorrhagic fever formalin-inactivated vaccine has been developed in Eastern Europe. Formalin-inactivated Rift Valley fever vaccines have been used for immunization of laboratory and field workers at risk for exposure. The formalin inactivated vaccine, the Salk Institute-Government Services Division (TSI-GSD) 200, developed by the US Army, has been used to vaccinate lab workers through its Special Immunization Program (240). Currently, there is no commercially available FDA-approved RVFV human vaccine. The formalin-inactivated (TSI-GSD) 200 vaccine, live attenuated vaccines MP-12, rMP-12 variants, DDvax, RVFV-4s, and adenovirus vector ChAdOx1-GnGc are under investigation (155). Environmental safety profiles, such as the relative safety of vaccine candidates in the context of mosquito transmissibility, are considered for new vaccines (49). For now, preventing RVFV infection of livestock by vaccination is a key to breaking the links to human epidemics. No specific Omsk hemorrhagic fever vaccine has been made, but tick-borne encephalitis vaccines can provide cross-protective immunity and have been used in high-risk population groups. Knowledge of the geographic range of the diseases, identification of high-risk groups, and proof that candidate vaccines are protective in animal models will all be important should large-scale immunization programs be developed in the future. Integrated approaches to the prevention of zoonotic diseases with products developed for use in animals and humans will also be important in the future.

The multiagency Coalition for Epidemic Preparedness Innovations (CEPI) reviewed the vaccine-development response to the 2014 Ebola epidemic in West Africa and produced a list of agents to be made priorities for their funding of vaccine development. The list of diseases that should be urgently addressed under the WHO Research and Development blueprint included several VHFs: filoviruses (Ebola and Marburg), CCHF virus, Lassa fever virus, and Rift Valley fever virus. The category of serious diseases necessitating further action as soon as possible included severe fever with thrombocytopenia syndrome, Chikungunya, and congenital abnormalities and other neurologic conditions associated with Zika virus (338). This action raised hopes that more vaccines for more agents might be expedited and become available soon.

For Lassa fever, there are currently no licensed vaccines. Candidates most likely to progress to clinical trials and licensure are MOPV/LASV (Mopeia V/Lassa V) reassortment clone 29 and 5 vaccines endorsed by the Coalition for Epidemic Preparedness Innovations (CEPI) for accelerated development. The portfolio contains:

Concern for transfusion transmission of flaviviruses and alphaviruses, particularly Chikungunya virus, has been raised because of high population incidence of infection during outbreaks and high-titer viremia lasting about 6 days. Estimated transfusion risks may be as high as 150 per 10,000 donations during outbreaks, leading to recommendations to defer symptomatic carriers, discontinue donations in impacted areas, and screen blood for Chikungunya virus (304).

Inactivated vaccines are generally thought safe for multiple sclerosis patients, but live attenuated vaccines such as YF17D were reported to provoke multiple sclerosis relapses (107; 211).

Differential diagnosis

Associated or underlying disorders

Early disease. Early signs of disease, such as high fever, prostration, flushing, conjunctival injection, postural hypotension, myalgias, abdominal discomfort, and axillary petechiae, are shared by a wide spectrum of infectious, inflammatory, or autoimmune diseases. At this juncture, major treatable diseases such as malaria, rickettsial diseases, leptospirosis, shigellosis, typhoid, and N meningitides meningitis or meningococcemia should be excluded by specific serologic, microbiology, and CSF studies, and diseases with significant public health consequences, such as reemergent measles, considered. Lab chemistries and hematologic studies would not specify the diagnosis. Rickettsial diseases cause thrombocytopenia and leukopenia. Leptospirosis causes meningitis, myalgia, hepatic or renal disease, thrombocytopenia, and leukopenia.

Next, the differential diagnosis is expanded to include diseases prevalent in the region, such as measles, hepatitis, systemic lupus erythematosus, hemolytic-uremic syndrome, or trypanosomiasis. Both African and American trypanosomiasis are associated with thrombocytopenia and mechanisms of disease may be immune-mediated or associated with splenic sequestration (85; 222). The presentation of encephalitis in Asia can be dengue fever, dengue hemorrhagic fever, Japanese encephalitis, or Nipah encephalitis. A positive tourniquet test has been 1 of several clinical parameters considered by the World Health Organization to be important in the diagnosis of dengue hemorrhagic fever, but is now less frequently used. It may be useful in diagnosing dengue infection in busy rural health stations in dengue-endemic areas. A positive test should prompt close observation or early hospital referral (50). Encephalitic agents of the California serogroup (Orthobunyavirus, Bunyaviridae) are in the differential diagnosis of early febrile illness in regions of overlap with viral hemorrhagic fevers. For example, Inkoo and Chatanga viruses and mosquito-borne California serogroup viruses are in the differential diagnosis of Puumala virus infections in Finland (326). Acute febrile illness and encephalitis may be Zika, Chikungunya, or dengue in central and South America, with low platelets a feature of dengue.

Advanced disease. The development of a severe multisystem syndrome in which the overall vascular system is damaged, autoregulatory mechanisms are damaged, and there is bleeding signifies advanced disease. Signs and symptoms of advanced viral hemorrhagic fevers may be respiratory (chest pain, shortness of breath), vascular (hypotension, edema), gastrointestinal (vomiting, diarrhea), or neurologic (headache, confusion, coma), and there is frank bleeding (rash, ecchymoses, uncontrolled oozing from venipuncture sites, mucosal hemorrhages). Hematuria can signify malaria (as blackwater fever), yellow fever, Ebola, CCHF, Rift Valley fever virus, leptospirosis, or septicemia with disseminated intravascular coagulation.

Late disease is multiorgan failure and coagulopathy, indicated by shock, convulsions, severe metabolic disturbances, renal failure, and coagulation dysfunction. There is generally laboratory evidence of abnormal platelet function and there may be abnormal liver function tests.

The differential diagnosis of advanced disease includes hematologic disorders associated with fever, infectious, and parainfectious syndromes. Thrombotic thrombocytopenic purpura is precipitated by endothelial injury and is characterized by hemolytic anemia with fragmentation of erythrocytes and signs of intravascular hemolysis, thrombocytopenia, diffuse and nonfocal neurologic findings, decreased renal function, and fever. In thrombotic thrombocytopenic purpura there are hyaline thrombi in arterioles, capillaries, and venules without inflammatory changes in the vessel wall and minimal activation of the coagulation system. Pregnancy, certain drugs, and HIV infection are causes of thrombotic thrombocytopenic purpura.

Immunologic thrombocytopenia occurs when platelets coated with antibody, immune complexes, or complements are rapidly cleared from the circulation. The most common causes are viral or bacterial infections, drugs, or an autoimmune disorder (idiopathic thrombocytopenia purpura). Transient immunologic thrombocytopenia complicates some cases of infectious mononucleosis, acute toxoplasmosis, cytomegalovirus, viral hepatitis, viral exanthems, and initial infection with HIV.

Acute febrile illness with thrombocytopenia and leukopenia has been reported in tick-borne encephalitis, human granulocytic anaplasmosis, rickettsiosis, leptospirosis, and primary HHV-6 infection in infants. In HHV-6, thrombocytopenia is the result of bone marrow suppression rather than immune-mediated peripheral consumption (16).

Disseminated intravascular coagulation occurs in critically ill patients when blood starts to coagulate throughout the body, depleting platelets and coagulation factors. Laboratory markers for disseminated intravascular coagulation are low-platelet counts, prolonged prothrombin time, decreased fibrinogen level, and increased fibrin markers (D-dimers: cross-linked fibrin degradation product and fibrin split products), with fibrin(ogen) derivatives generally correlating with degree of blood coagulation disturbance (60). Laboratory evidence of disseminated intravascular coagulation can accompany Korean, Rift Valley, and dengue hemorrhagic fevers, but apparently not Argentine hemorrhagic fever. Microscopic evidence of disseminated intravascular coagulation or vasculitis is absent from the arenaviruses. In Korean hemorrhagic fever, Rift Valley fever, and dengue hemorrhagic fever, release of tissue factor from cells in which viruses replicate and increased levels of TNF alpha have been suggested as mechanisms for initiation of the tissue factor pathway (348).

Common viral infections, such as influenza, varicella, rubella, and rubeola, have rarely been associated with disseminated intravascular coagulation. Purpura fulminans associated with disseminated intravascular coagulation has been reported in patients with infections and either hereditary thrombophilias or acquired antibodies to protein S. Other extreme examples of sepsis-related disseminated intravascular coagulation are streptococcus A toxic shock syndrome, meningococcemia, Staphylococcus aureus, and Streptococcus pneumoniae infection. Shock not correlated with significant adrenal hemorrhage or necrosis distinguishes the shock-like state of hemorrhagic fever with renal syndrome from the Waterhouse-Friderichsen syndrome (adrenal apoplexy and hemorrhage) (348). A hemorrhagic shock and encephalopathy syndrome has been associated with severe influenza infections in Japan (382) and Europe (125).

Also in the differential diagnosis of viral hemorrhagic fever is acute hemorrhagic leukoencephalitis, a rapidly evolving focal or multifocal cerebral disorder with fever and obtundation. Acute hemorrhagic leukoencephalitis is the more severe and destructive form of para- or postviral acute disseminated encephalomyelitis. The CNS white matter of acute hemorrhagic leukoencephalitis shows necrotizing vasculitis involving venules and capillaries with perivascular accumulations of polymorphonuclear cells and red blood cells. The perivascular demyelinating lesions frequently coalesce to form large lesions. MRI shows extensive white matter abnormalities and hemorrhage distinct from a thrombotic thrombocytopenic purpura case, in which there is symmetric deep gray involvement as may be seen in metabolic abnormalities.

Hemorrhagic forms of smallpox (purpura variolosa) and hemorrhagic chickenpox are also in the differential diagnosis. Finally, there is snake bite. Several species of snakes belonging to the Viperidae family produce venoms that have a wide range of pharmacologic activities affecting hemostasis.

Biphasic disease. Dengue hemorrhagic fever, Kyasanur Forest disease, and Omsk hemorrhagic fever are biphasic diseases with a febrile or hemorrhagic period followed by CNS symptoms. The biphasic course is similar to tick-borne encephalitis and other members of the tick-borne encephalitis antigenic complex, except systemic hemorrhagic manifestations are not characteristic of the first phase of tick-borne encephalitis. Determinants of the biphasic clinical disease are incompletely understood, but may associate with the cycles of virus proliferation or physiological attempts to achieve homeostasis.

Transplant recipients. Lymphocytic choriomeningitis virus is the most commonly recognized arenavirus among solid organ transplant recipients and should be considered in the transplant patient with fever, hepatitis, meningitis/encephalitis, and/or multiorgan failure (11). In the United States from 2002 to 2013, clusters of encephalitis from infectious agents in transplant patients included lymphocytic choriomeningitis virus, West Nile virus, rabies virus, and Balamuthia mandrillaris (362). All reported cases of lymphocytic choriomeningitis virus in solid organ transplant patients had been acquired from an infected donor, with 1 exception. The transplant patient was infected with lymphocytic choriomeningitis virus after cleaning a basement contaminated with mouse excreta and developed a meningitic syndrome and communicating hydrocephalus (385).

Dengue, if transmitted through solid organ transplantation, most commonly presents with hepatitis. DENV RNA can be detected in cadaver corneoscleral tissue but has not yet been linked to donor-derived infections (256).

Congenital disease. Congenital lymphocytic choriomeningitis virus is characterized by microcephaly, periventricular calcifications, and hydrocephalus, for which the differential diagnosis is TORCH pathogens and Zika virus (88).

Peripheral nervous system. Dengue, as well as other flaviviruses Zika and West Nile, is associated with Guillain-Barre type syndromes. Acute hypokalemic paralysis, similar to other acute flaccid paralysis syndromes, is a rare complication of dengue.

Diagnostic workup

The diagnosis of viral hemorrhagic fever should be suspected in any febrile patient returning from an endemic area, particularly if there had been travel to rural areas during the times of seasonal epidemics. There may be a history of exposure to the animal vector or exposure to ticks or mosquitoes. Evidence for systemic or disseminated infection would be found in abnormal liver function tests, complete blood count, or CSF. Because the combination of fever and hemorrhage can be caused by a number of human pathogens, among them hepatitis viruses, malaria, rickettsia, shigella, typhoid, and leptospiral bacteria, diagnostic workup includes testing to exclude treatable causes. In puzzling cases, cutaneous biopsy of a rash may support an alternative diagnosis and demonstrate pathogens such as rickettsiae.

Early diagnostic tools, including immunochromatographic strips and viral protein detection, are critically needed for rapid on-site testing in areas where diagnostic facilities are limited. For example, rapid diagnostic tests under development for point-of-care application for dengue include NS1-antigen-based diagnostics for acute cases combined with IgM antibody detection for later stages (33). A rapid immunoassay is available for the Lassa virus subtypes found in Sierra Leone, and a similar test to detect all strains is being tested in Nigeria (35).

For Ebola, figuring out who has disease, isolating them, and practicing barrier nursing techniques, contains outbreaks. In 2014 to 2016, rapid diagnostic testing using Trombley assay, Real-time TaqManTM PCR, Real Star Filovirius Screen RT-PCR (Altona Hamburg Germany), and other nucleic acid amplification tests were deployed through portable labs and to health centers in the affected West African countries (90; 353). Because current molecular lab diagnostic methods such as PCR require trained personnel and lab infrastructure, development of rapid, accurate, point of care testing continues.

Lateral flow assays, often immunoassays, are the technology behind low-cost, simple, rapid, and portable point-of-care detection devices. They can be used for detection of proteins, haptens, nucleic acids, and amplicons. Although the concept behind the latera flow assays is straightforward, the device has a complex architecture, with critical elements of the assay, the antibodies and membrane. Results are most often qualitative (on/off) or semiquantitative (186). The tests have not been well-suited for differentiating among multiple febrile diseases with similar symptoms, Ebola, Lassa and malaria, for example, and other technologies for multiplex assays are in development (347).

The majority of diagnostic kits today are not simple and fast enough to get rapid results. Therefore, blood and other bodily fluids should be considered infectious, and great care must be taken when handling any potentially infectious materials. Specimens should be processed with appropriate precautions in biosafety labs. Consultation and guidelines for submitting specimens are available through State Health Departments in the United States and the Centers For Disease Control and Prevention (404-639-1510). Instructions are available at the National Center for Infectious Diseases, Specimen Submission Information and Forms website.

At specialized labs, diagnosis is by seroconversion, antigen detection, or PCR of serum (258). Most of the viral hemorrhagic fevers can be diagnosed from blood by a combination of detecting antigen by ELISA and IgM ELISA. The combination of tests yields a diagnosis within 24 to 48 hours of presentation in most patients. One of the potential pitfalls in the rapid ELISA diagnostic kits is the effect of fever or elevated temperatures on the expression of antigens. All diagnostic ELISA kits are developed under laboratory conditions, in other words, at normal temperature. Thus, although the specificity and sensitivity for antigens prepared at normal temperature appear to be great, the specificity and sensitivity are frequently different from expected in field site investigations. The reason may be due to an alteration of antigen expression at febrile temperature. Thus, if a diagnostic ELISA kit could include a test with antigens prepared at a higher temperature, the accuracy of diagnosis of virus infection might increase.

For greater sensitivity in mild cases or early infection when the patient is viremic, RT-PCR may be used. RT-PCR tests complement immunodiagnostic assays and allow genotyping of the virus. Acute serum RT-PCR has been successfully applied for Arenaviridae, Bunyaviridae, Filoviridae, and Flaviviridae families. Both antigen detection and amplification of virus RNA can be done on materials that have been rendered noninfectious, utilizing gamma irradiation from a cobalt-60 source, for example. Lassa virus and the South American hemorrhagic fever viruses can be isolated from blood or serum during illness from onset up to 12 to 14 days of illness. Virus can be isolated from blood or serum of Ebola and Marburg patients. Virus isolation from acute clinical specimens from hantavirus infections are generally negative, which may be related to the presence of a strong immune response during early illness that includes serum neutralizing antibodies. On the other hand, virologic diagnosis for Rift Valley fever and Crimean-Congo hemorrhagic fever can be made due to high viremias during acute disease. Yellow fever virus is most easily isolated from serum during the first 4 days of illness, but it may be recovered from serum up to the 14th day and occasionally from liver tissue at death. Kyasanur Forest disease and Omsk hemorrhagic fever viruses can be isolated during the acute hemorrhagic phase.

For dengue fever, virus can be isolated from blood in the early febrile phase of illness. Originally, inoculation into Toxorhynchites species mosquitoes or larva was used for primary virus isolation and the hemagglutination-inhibition test to classify dengue infections. Secondary infections were characterized by the presence of hemagglutination-inhibition antibodies in acute sera and high titers in convalescent sera (45). Nowadays, PCR or nonstructural protein NS1 testing is used for diagnosis during the first week of illness. It takes 4 to 7 days for IgM antibody to be produced. Similarly, PCR and NS1 testing would detect early secondary infection, as would elevated IgG to IgM ratios.

When dengue is suspected, serologic tests are carefully interpreted in populations where dengue and other flaviviruses are endemic. The plaque reduction neutralization test (PRNT) is used when there is high likelihood of cross reactivity between viral antibodies. Also, serologic results are interpreted with regard to the individual patient; whether an adult with probable prior flavivirus infection, an immigrant from a nonendemic area, a tourist, or child (261).

Detection of dengue IgM antibodies or NS1 antigen in CSF, as individual tests, has variable sensitivity and high specificity. The combined use of 2 markers (NS1Ag and dengue-specific IgM) in CSF increases the sensitivity of dengue diagnosis to an estimated 92% (322) and NS1 antigen detection is not believed to cross-react with Zika (261). The case definition for dengue encephalitis (365) has been fever, acute signs of cerebral involvement, positive IgM dengue antibody or dengue PCR in serum and/or CSF, and exclusion of other causes of encephalitis/encephalopathy, a distinction increasingly difficult because of reports of positive serum IgM for dengue and Japanese encephalitis (357), and high levels of Zika and dengue virus antibody cross-reactivity (309). The plaque reduction neutralization test (PRNT) is used when there is high likelihood of cross reactivity between viral antibodies; it is more specific, measuring virus-specific neutralizing antibodies, and is considered the gold standard for DENV by the World Health Organization when performed by industry standard validated labs. Substitution of chimeric dengue for wild-type virus in diagnostic plaque reduction neutralization tests has the potential to reduce lab requirements from BSL3 to BSL2 conditions (163). Evidence from current publications has been insufficient to draw definite conclusions on time course or kinetics of dengue virus in CSF, viral load in patients with probable/confirmed dengue encephalitis, and appearance of dengue-specific antibodies in CSF (270).

Serologic tests for SARS-CoV-2 need to be evaluated with care when assessing coronavirus disease spread and immunity in the tropics. There have been several reports of serologic cross-reactivity between SARS-CoV-2 and dengue virus (319). Nonspecific or cross-reactive antibody reactions have led to false positive Covid serology among dengue patients and false positive dengue serology among Covid patients. For example, in one nonendemic dengue area, 22% of Covid infections could be falsely identified as dengue, with cross reactivity attributed to structural similarities between dengue envelope protein and SARS-CoV-2 epitopes in HR2-domain of the spike protein (215). If point of care rapid test kits have low specificity, advanced diagnostic assays and serological analyzers would be solutions (354).

For Chikungunya, diagnosis is by acute and convalescent sera testing, and CNS disease by detection of anti-Chikungunya virus IgM in CSF (335). RT-PCR in serum is positive during the viremic phase lasting for 1 week after onset of symptoms. Alphaviruses such as CHIK are not believed to have IgM ELISA antibody cross-reactive with flaviviruses.

Next-generation sequencing, available in specialized laboratories, is becoming important both for detection of pathogens when standard lab methods fail (as in a suspected hemorrhagic fever outbreak in Uganda) (242), and for pathogen discovery. Next-generation metagenomic approaches follow an unbiased approach to disease analysis. The method does not require primers to amplify specific sequences or probes. Instead, with deep sequencing machines, large numbers of reads with high coverage of genomes contained in the clinical sample are produced. From established DNA or RNA databases and application of computational biology methods, viral genomes are reconstructed (199; 342; 258).

CRISP sequence together with Cas nucleases are the basis of CRISPR/Cas technology used for genome editing. A newer application of the CRISPR-Cas system is virus detection, when viral disease is diagnosed by nucleic acid based methods. CRISPR-based diagnostic methods identify a certain target sequence associated with a disease, then cleave it to produce a readable signal (167). CRISPR diagnostic tests, which can be performed with simple reagents and paper-based lateral flow assays, are providing rapid point of care diagnostics and decreasing healthcare and lab worker exposure. For example, CRISPR-Cas13a diagnostic tools to detect Ebola and Lassa virus cases have been developed and tested (23).

It is important to keep in mind that nucleic acid tests may fail if the virus has already been cleared or if the virus is low abundance or not normally present in the fluid tested. However, tests detecting antiviral antibodies can identify ongoing or cleared infections. Analysis of antiviral antibodies to cover the entire human virome is now available (427)

VirScan is another specialized research tool, a programmable high throughput method to comprehensively analyze antiviral antibodies that covers the complete human virome. A phage-display assay for viral immune responses enables human virome-wide exploration at the epitope level of immune responses (427). In 1 case, chronic dengue virus panencephalitis was diagnosed in a patient with progressive dementia based on VirScan identification of elevated dengue antibodies in CSF compared to serum when no virus was detected in serum or CSF (165).

ICD codes for various viral hemorrhagic fevers can be found in Table 2.

Table 2. ICD Codes for Viral Hemorrhagic Fevers

Management

Management is symptomatic, along with empiric antiviral and/or antibiotic treatment until a diagnosis is made. In all viral hemorrhagic fevers, treatment is to preserve function of vital organs by providing supportive care for bleeding, fluid and electrolyte imbalance, shock, and hypoxia. Cardiotonic pressor drugs are used for shock, along with cautious fluid administration because of the tendency to precipitate pulmonary edema. Estimates of central volume status and tracking of fluid resuscitation status is especially important in children with dengue shock syndrome and dengue hemorrhagic fever (255). Fluid management in dengue (154; 384) and use of a hydration tent during epidemics when there is a shortage of hospital beds are reviewed (225). Future studies in filoviruses may test pediatric ICU protocols in primates. Activated protein C, although U.S. FDA-approved for treatment of septic shock (27), has not been systematically tested for efficacy in viral hemorrhagic fevers. Usual precautions for patients with bleeding diatheses, such as avoiding intramuscular injections and aspirin, are practiced and hepatotoxic drugs are avoided. Strict isolation and barrier nursing procedures are followed. Disseminated intravascular coagulation is managed with platelet transfusions and fresh frozen plasma. Platelets are replaced if counts fall below 20,000/µL. Hepatic-induced coagulopathies are treated with vitamin K. Medication selection may be influenced by noting drug-induced immune thrombocytopenia that can be caused by quinine, sulfonamides, penicillins, vancomycin, cephalosporins, or thiazide diuretics. In hemorrhagic fever with renal syndrome, diuresis can be renal or central due to pituitary dysfunction. The endocrine status of every patient who survives hemorrhagic fever with renal syndrome should be checked.

Ribavirin is a broad spectrum antiviral that has been used in the treatment of hemorrhagic fevers for more than 30 years. Ribavirin is the therapeutic agent of choice in Lassa fever. It is efficacious when administered to patients within the first 7 days of illness but is contraindicated in pregnant women. It has been used principally in Lassa fever patients with a poor prognosis who have serum aspartate transaminase values above 150. The intravenous dose is a 2 g loading dose, followed by 1 g every 6 hours for 4 days, then 0.5g every 8 hours for another 6 days (233). Its penetration into the CNS is limited and its main side effects are anemia and serum amylase elevations. Results with intravenous ribavirin have also been positive in hemorrhagic fever with renal syndrome (151), Crimean-Congo hemorrhagic fever (61) and Argentine hemorrhagic fever (106). Convalescent serum (usually 2 to 3 units with high neutralizing titer) within the first 8 days of illness, was the original treatment of choice for Argentine hemorrhagic fever (105). When therapy of Junin virus infections with ribavirin was recognized effective for the visceral phase of the infection, ribavirin was then added to hyperimmune globulin (121). If time to diagnosis is over 8 days, considered too long to administer immune plasma, ribavirin plus oral favipiravir (2000 mg loading dose followed by 1200 mg twice a day, to a maximum of 1800 mg twice a day for 14 days) is used (398). Ribavirin, used in China for severe fever with thrombocytopenia syndrome, is reported effective for early-stage disease (113). Ribavirin is not effective in hanta pulmonary syndrome.

Monoclonal antibody therapy, based on anti-JUNV GP neutralizing mouse-human chimeric mAbs, has shown prevention of CNS infection and superiority to all other evaluated drugs in a guinea pig model, and therefore may be an alternative to immune plasma (437). Ways of increasing blood-brain barrier permeability to mAbs and other protein therapeutics are important issues for these classes of compounds. Currently, ribavirin is approved for use in treating viral hemorrhagic fevers caused by arenaviruses and bunya viruses but not filoviruses under the compassionate use provisions for investigational new drugs (121). Originally treated with intravenous ribavirin, Crimean-Congo hemorrhagic fever has been found to respond well to oral ribavirin. The oral dosage recommended by the World Health Organization for Crimean-Congo hemorrhagic fever is 2000 mg as an initial loading dose, then 1000 mg every 6 hours for 4 days and then 500 mg every 6 hours for 6 days (or 30 mg/kg as an initial loading dose, then 15 mg/kg every 6 hours for 4 days, and then 7.5 mg/kg every 8 hours for 6 days) (source 200 mg tablets made in the United Kingdom for Durbin PLC). Patients with nausea and vomiting receive the drug via nasogastric tube and/or with antiemetic drugs (421; 223; 287). Survival is improved if treatment begins within 4 days of onset of illness (386). Safety and pharmacokinetic data on intravenous ribavirin in a pediatric population, including CSF levels, are provided in a study on La Crosse encephalitis in children (237).

Opinions vary for the use of oral ribavirin for prophylaxis for primary contacts in the event of exposure to 1 of the viral hemorrhagic fevers susceptible to ribavirin (81). Some experts recommend immediate prophylaxis whereas others suggest observing contacts and treating if an individual becomes febrile (184). Intravenous ribavirin, used during the Rift Valley fever virus outbreak in Saudi Arabia in 2000, was quickly stopped when suspected to increase the chance of neurologic disease (142). Newer broad-spectrum antivirals such as favipiravir have shown promise in rodent models (142), including hamsters that have shown the same augmentation of neurologic disease with ribavirin (344).

Favipiravir (T-705), an investigational antiviral drug that inhibits the RNA-dependent RNA polymerase of influenza virus, also blocks the replication of many other RNA viruses. It has broad-spectrum antiviral activity against arenaviruses (Junin, Machupo, Pichinde), phleboviruses (Rift Valley fever, sandfly virus), hantaviruses (Dobrava), and flaviviruses (yellow fever) in vitro and in rodent models, making favipiravir a promising candidate drug for RNA viral diseases with no approved therapies (113). Successful treatment of advanced Ebola virus infection in a small animal model with favipiravir has also been reported. Initiation of T-705 treatment at day 6 postinfection induced rapid virus clearance, reduced biochemical parameters of disease severity, and prevented a lethal outcome in 100% of mice (lacking the type 1 interferon receptor) (277).

Investigational treatments of Ebola virus disease have included the use of monoclonal antibodies, plasma transfusions from convalescent patients, nucleoside and nucleotide analogues, nucleic acid-based drugs, small molecule antiviral agents, and vaccines.

West Africa, United States, Europe 2014-2017. Monoclonal antibody treatment combined with adenovirus-vectored interferon-alpha has rescued Ebola-infected experimental nonhuman primates after detection of viremia and symptoms (327). Before 2014, monoclonal antibody-based treatments were identified by several BSL4 labs as the preferred clinical option in response to accidental Ebola virus exposure (327). ZMapp is a cocktail of 3 humanized monoclonal antibodies manufactured in tobacco plants, which targets the Ebola virus glycoprotein. It was used to treat 7 patients, with 5 surviving (232), but available supplies were soon consumed. ZMapp targets only 1 strain of Ebola, the Zaire strain. A new set of broadly neutralizing monoclonal antibodies has been found in the blood of Ebola survivors, which proved effective against disease caused by several strains, Zaire, Bundibugyo, and Sudan Ebola viruses (124).

Randomized controlled trials of new drugs in people with Ebola continued until the end of the West Africa epidemic. Use of passive immune therapy (blood or plasma transfusions from convalescent patients to treat Ebola-infected patients) had World Health Organization approval in 2014 (132). Transfusion therapy has been successfully used on at least 3 patients in the United States but could not be used for the fatal Ebola case in the United States because of blood type mismatch (32). Although screening donated blood for pathogens is an issue in resource-limited settings, chemicals can be added and mixed with donated blood that, when exposed to UV light, irreversibly crosslink DNA and RNA of pathogens, thus preventing their replication.

Favipiravir (T-705), an oral nucleoside analogue, successfully treated 1 French nurse (32). Supplies are available because it had already advanced to late-phase clinical studies. It showed modest efficacy in a phase II trial that did not have a control group. Brincidofovir (CMX-001), an orally available lipid conjugate of cidofovir in clinical trials for double-stranded DNA viruses, was found to have activity against Ebola virus and has been used as a treatment on 5 patients infected in the U.S. (32). The clinical trial in Liberia begun in early 2015 was terminated because of the drop in numbers of new cases. TKM-Ebola is a small interfering RNA that affects 3 of Ebola's 7 proteins; it received FDA approval for emergency use during the West African epidemic.

Gilead’s GS-5734 (remdesivir), a prodrug nucleoside inhibitor with efficacy in infected primates, moved to phase I studies in healthy human volunteers. It was used to treat relapse in the Scottish nurse in late 2015, in whom it was reported to have eradicated virus from the CNS causing her meningoencephalitis. A survivor cohort study in Africa was planned to study eradicating virus from persistent sites by treatment for 1 or 2 weeks and checks of viral load in immune-privileged sites but timing is important, as the natural history of Ebola is a drop in viral RNA over time. Other investigational agents include: JK-05 (approved in China); AVI-7537 (a phosphorodiaminidate morpholino oligomer that blocks translation), inhibits VP24 protein and potentially improves host innate immunity; BCX-4430, an adenosine analogue; and the antiretroviral lamivudine (32; 181). Sequence-based candidate therapeutics may be strain-specific.

Other treatments have included fibrin-derived peptide for vascular leak syndrome and extracorporeal virus elimination by lectin affinity plasmapheresis; each used in Germany. A summary of treatments in West Africa and what patients have been given outside West Africa appeared in a newspaper article in late 2014 (197).

Doctors needing information on availability and use of experimental therapies should contact the U.S. Food and Drug Administration and drug manufacturers. On a case by case basis, emergency investigational new drug applications are awarded.

The West Africa outbreak ended as numerous Ebola medical countermeasures were being explored or developed. These studies were carried forward into the Democratic Republic of Congo outbreak.

Democratic Republic of Congo 2018-2020. A trial of 4 investigational therapies was conducted in the Democratic Republic of Congo outbreak, starting in 2018. Patients were randomly assigned in a 1:1:1:1 ratio to intravenous administration of the triple monoclonal antibody ZMapp (the control), the antiviral agent remdesivir (the nucleotide analogue RNA polymerase inhibitor GS-5734, a), the single monoclonal antibody MAb114 (a single human MAb derived from an Ebola survivor), or the triple monoclonal antibody REGN-EB3 (a coformulated mixture of 3 human IgG1 monoclonal antibodies). Both MAb114 and REGN-EB3 were superior to ZMapp in reducing Ebola virus mortality. Remdesivir had been dropped from the study after an interim analysis showed the superiority of MAb114 and REGN-EB3 to ZMapp and remdesivir. The superior agents had each reduced deaths to 33% to 35% compared to roughly 50% in the ZMapp group. Mortality in all groups highlights the need for further improvements in supportive care and a possible role for combination strategies that use agents with complementary mechanisms of action (257).

A compassionate use protocol, Monitored Emergency Use of Unregistered and Investigational Interventions, offered to all patients in the Democratic Republic of Congo with laboratory-confirmed Ebola virus infection covered these therapies and represented the first time several investigational treatments were available in the midst of an outbreak (169).

Relapses after MAbs appear to be rare but create questions as to whether passive immunotherapy could have a role in viral persistence and relapse (230). There have been 3 patients with documented cases of relapse: all had received antibody-based therapy as part of their initial infection treatment. A Scottish nurse treated with convalescent plasma and ZMapp relapsed 9 months later with meningoencephalitis (159). A man treated with mAb114 relapsed after 6 months with systemic disease (230).

A nonhuman primate Ebola study demonstrated similar recurrence of disease. Rhesus macaques with acute infection treated with monoclonal antibodies recovered, but went on to develop fatal CNS disease characterized by ventriculitis, choroid plexitis, and meningoencephalitis. At autopsy, infection and inflammation were limited to the brain: to the ventricular system and adjacent neuropil. Macrophages were the cellular reservoir of persistent infection, and EBOV persistence in brain ventricles originated from choroid plexus vasculature (209).

The natural and experimental cases of relapse after antibody treatment mean that antibody treatments may need to be monitored for evolving viral resistance, and future treatments may require a cocktail of mAbs targeting nonoverlapping epitopes of Ebola virus (231).

Vaccines. rVSV-ZEBOV vaccine has been licensed and Ad26.ZEBOV, MVA-BN-Filo is available under investigational protocols. As the only viral surface protein and mediator of virus attachment and cell entry, the Ebola glycoprotein had been of interest as a target for vaccines as well as antibody-based therapies noted above. rVSV-ZEBO (Newlink Genetics/Public Health Agency Canada, subsequently licensed to Merck) stimulates immune response to Ebola glycoprotein using recombinant vesicular stomatitis virus as a delivery vector. A phase III clinical trial in Guinea assessed its efficacy in preventing Ebola virus disease using a ring vaccination method inspired by the strategy used in smallpox eradication. Clusters of individuals at high risk of infection based on their connection to an index case were vaccinated. After confirmation of a new infection, clusters of contacts and contacts of contacts were assigned to immediate or delayed vaccination. Early results showed no cases of Ebola at 10 days after randomization in the immediate vaccination group, but 16 cases from 7 clusters in the delayed vaccination group (147). This trial discontinued randomization and continued with immediate vaccination of new clusters of patients. Data on the duration of humoral immunity induced by VSV-EBOV are not yet available. cAd3-EBO (GSK/USNIAID) stimulates antibody and T cell immune response to Ebola glycoprotein using chimpanzee adenovirus vectored ebolavirus vaccine encoding the glycoprotein from Zaire and Sudan species. Reactogenicity and immune response were dose-dependent (201). The vaccine is also under investigation as a primer in the 2 vaccine (prime-boost) study. Ebola vaccine trials are planned or underway in Liberia, Guinea, and Sierra Leone. Both vaccines appear safe in early reports of the phase II Liberian clinical trial. Johnson & Johnson (through its division Crucell NV, together with Bavarian Nordic and USNIAID) has also accelerated development of a vaccine program based on a prime-boost regimen with sequential use of 2 Ebola vaccines. The Ad26.ZEBOV uses an adenovirus vector to deliver genetic material from EBOV-Zaire (responsible for the West Africa outbreak) and MVA-BN-Filo uses a modified vaccinia virus Ankara vector (MVA) to deliver the Ebola genetic material, plus inserts for Sudan Ebolavirus, Tai Forest ebolavirus, and the related Marburg virus. All participants in the phase I trial who received the Ad26.ZEBOV vaccine first (the prime) followed by MVA-BN-Filo as a booster developed and maintained antibodies to Ebola 8 months after immunization and most of the group maintained vaccine-induced T cells (247). Vaccine updates and commentary can be found on Wellcome Trust-CIDRAP (Center for Infectious Disease Research and Policy) website (cidrap.umn.edu).

In 2015, Merck’s rVSV-ZEBOV vaccine (now Ervebo) stopped the spread of Ebola in Guinea (146). This vaccine was offered to international responders, local health workers, and contacts of people with Ebola in the Democratic Republic of Congo’s 2018 Ebola epidemic. Since the start of vaccination on August 8, 2018, 297,699 people (high risk contacts, contacts of contacts, and front-line providers) had been vaccinated (312). The main vaccination strategy was “ring vaccination”. “Targeted geographic vaccination” was also used and provided for “pop-up” vaccination sites in unsafe areas. The earliest studies have shown a 97.5% vaccine efficacy. Ebola patients in the Democratic Republic of Congo who had received rVSV-ZEBOV vaccination prior to infections had lower viral loads, better outcomes, and were less likely to have kidney injury (176). Vaccinating people already infected increased their chances of survival (423). The vaccine is estimated to have a 1-year efficacy. As of 2022, approximately 10% of patients admitted to Ebola Treatment Units in DRC were fully vaccinated. The vaccine is licensed by WHO in Democratic Republic of Congo, Burundi, Ghana, and Zambia (423), has European Commission marketing authorization, and is approved for medical use in Europe and by FDA in the United States.

The June 2019 revised World Health Organization vaccine strategy proposed the Johnson and Johnson vaccine (Ad26.ZEBOV/MVA-BN) to complement ring vaccination in the Democratic Republic of Congo. The vaccine is a 2-dose heterologous vaccination regimen (Zabdeno [Ad26.ZEBOV] and Mvabea [MVA-BN-Filo]), with a replication-deficient adenovirus type 26 vector-based vaccine expressing Zaire Ebola virus glycoprotein and a nonreplicating modified vaccinia Ankara (MVA) vector-based vaccine, encoding glycoproteins from Zaire Ebola virus, Sudan virus, and Marburg virus, and nucleoprotein from the Tai Forest Ebola virus. The vaccine has marketing authority by the European Medicines Agency.

Originally, the Johnson and Johnson vaccine, which requires 2 doses separated by 60 days, would be used outside of outbreak zones and would be a preventive vaccine in lower-risk not-yet-exposed groups. The Merck vaccine would continue as vaccination of high-risk individuals already exposed to the virus. The combined efforts would continue ring vaccination protocols around cases while increasing immunity in the general community with the Johnson and Johnson vaccine.

Merck’s rVSV-ZEBOV vaccine (now Ervebo) protects only against Zaire Ebolavirus species of Ebola virus and Janssen’s Ad26.ZEBOV/MVA-BN-Filo contains coding regions of several filoviruses. Both have regulatory approvals.

Altogether, at least 8 vaccines entered clinical trials between 2014 and 2016. However, the immunologic correlates of protection remain unknown. More research is needed. For all the candidate vaccines, long-term persistence of the vaccination response is unknown, along with whether an immune correlate of protection identified for 1 vaccine applies equally to immunity by another vaccine (364). More vaccine candidates also may be needed to provide protection against multiple Ebola species and to extend the length of protection.

There is no consensus for antiviral treatment in dengue. An uncontrolled trial of interferon was performed during an outbreak in Cuba in 1981 with some indication that fatalities were averted (45). General guidelines are in a World Health Organization publication (419). During a 2002 dengue hemorrhagic fever outbreak in Brazil, patients were treated for 5 days with 500 mg/kg per day intravenously administered IVIG (that had tested negative for any dengue antibody). Platelet counts recovered in 5 days, compared to 9 days in untreated patients in other series (286). The mechanism of IVIG effect was hypothesized to be limiting immune-mediated destruction of platelets and vasculitis caused by immune complexes. However, a report suggests that IVIG has no effect in hastening the recovery of platelet counts in patients with secondary dengue infections (95). The reason for the lack of IVIG efficacy is attributed to enhance platelets phagocytosis by macrophages through Fc gamma receptors (148). Desmopressin, a treatment for refractory bleeding in hemophilia and von Willebrand disease Type 1, was used in a dengue hemorrhagic fever/dengue shock syndrome case (298).

Post-dengue syndromes judged to be immune-mediated are treated with IVIG, plasma exchange, or steroids. In 1 study, hematocrit value, mean body temperature, and alanine aminotransferase (ALT or SGPT) levels during infection were predictors of immune-mediated syndromes (30).

Experimental therapies for critically ill severe fever with thrombocytopenia syndrome patients with encephalopathy have been designed with goals to mitigate early cytokine storm with steroid pulse therapy or plasma exchange and to decrease viral burden with convalescent plasma therapy or ribavirin (293; 292; 75; 266). Ribavirin is used as 30 mg/kg as an initial loading oral dose, then 15 mg/kg every 6 hours for 4 days, and then 7.5 mg/kg every 6 hours for 6 days. A comatose patient recovered after plasma exchange reduced IFN-a and IP-10 levels, and convalescent plasma therapy reduced viral load (75).

Reduction in immunosuppression and supportive care are fundamental management of transplant-associated lymphocytic choriomeningitis virus. One survivor had received ribavirin and reduction of immunosuppressive therapy.

Outcomes

Arenaviridae. Eight to 10% of Argentine hemorrhagic fever patients treated with Junin immune plasma developed nystagmus, ataxia, cerebellar tremor, and abnormal brainstem-evoked responses 2 to 3 weeks after discharge from the hospital (219; 80). Lassa fever is associated with a high incidence of hearing disability in West Africa. Thirty percent of those who survive Lassa fever develop sensorineural hearing loss during the recovery or convalescence phase. Hearing loss was an outcome unrelated to whether the patient had received antiviral treatment and was thought to be a consequence of the immune response that promotes clearance and disease resolution (83).

Filoviridae. Seven survivors of the Kikwit 1995 epidemic received hyperimmune serum (259). Transfusion therapy with hyperimmune serum was used successfully on at least 3 patients in the United States during the West African epidemic. ZMapp is a cocktail of 3 humanized monoclonal antibodies manufactured in tobacco plants, which targets the Ebola virus glycoprotein. It was used to treat 7 patients, with 5 surviving. Favipiravir (T-705), an oral nucleoside analogue, successfully treated 1 French nurse (32). Brincidofovir (CMX-001), an orally available lipid conjugate of cidofovir in clinical trials for double-stranded DNA viruses, was used as a treatment on 5 patients infected in the United States (32). Gilead’s GS-5734 was used to treat relapse in the Scottish nurse in late 2015, in whom it was reported to have eradicated virus from the CNS that caused her meningoencephalitis. Post-Ebola (neurologic, musculoskeletal or ocular) symptoms and syndromes were present 48 months after recovery in a portion of survivors (91).

Flaviviridae. One patient, 60 days after dengue encephalitis diagnosed by inflammatory CSF and positive serology, developed a behavior disorder treated with risperidone (20).

Togaviridae. Rarely, Chikungunya virus may affect the CNS, with severe encephalitis in individuals with advanced age and underlying diseases (59).

Special considerations

Pregnancy

Generally, viral hemorrhagic fevers cause more severe disease with higher mortality in pregnant than nonpregnant women due to pregnancy-induced immunotolerance/immunosuppression and complications of severe hemorrhage, vertical transmission, and intrauterine demise. For example, mortality among pregnant women with Ebola hemorrhagic fever (95.5%) was higher than overall mortality (77%) observed during the Ebola epidemic in Kikwit in 1995 (195).

Lassa fever carries a high risk to the fetus throughout pregnancy and to the mother, especially in the third trimester, with 30% or greater fatalities. Evacuation of the uterus significantly improved the mother’s chance of survival and patients seem to improve quickly after delivery or abortion (308). Active obstetric management with induction of labor, caesarian section, and evacuation of retained products of conception has been encouraged, and ribavirin is recommended after delivery. Lassa virus is present in breast milk of infected mothers.

Vertical transmission in humans is reported for Rift Valley fever (15); dengue, presenting as newborn or perinatal encephalitis (360; 168); hemorrhagic fever with renal syndrome (204; 370); and Lassa (251). Early-life Lassa infection causes “swollen baby syndrome,” with generalized edema, abdominal distension, and bleeding (251).

Infants born to dengue-immune mothers and first exposed to dengue when transplacentally-acquired maternal dengue antibodies are waning and risk symptomatic or severe disease (414). Viral hemorrhagic fevers are considered in the differential diagnosis of HELLP (hemolysis, elevated liver enzymes, low platelets) syndrome in gravid patients in endemic areas. General anesthesia may be a safer option than subarachnoid block in patients with bleeding diathesis (73).

Anesthesia

Upper airway edema may be severe enough to result in difficult intubation, in which case preparation of equipment for difficult airway management, such as endotracheal tubes of various small sizes and fiberoptic laryngoscopes, is required.