This article includes discussion of viral hemorrhagic fevers: neurologic complications and neurologic complications of viral hemorrhagic fevers. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
The viral hemorrhagic fevers are febrile illnesses accompanied by abnormal vascular regulation and vascular damage caused by members of the Arenaviridae, Filoviridae, Bunyaviridae, and Flaviviridae families of viruses. Found in diverse areas of the world, hemorrhagic fever viruses are potentially present in anyone who steps off a plane from an endemic area. Although the high mortality and exceptional epidemics of viral hemorrhagic fevers have been well publicized, neurologic aspects of these diseases are less familiar. Neurologic complications occur in many of the viral hemorrhagic fevers, and the CNS diseases are not simple reflections of what has started in the periphery. Some viral hemorrhagic fevers present as pure neurologic illness.
In this article, the dark history, role in human experimentation, global ecology and epidemiology, clinical and neurologic aspects, and treatment and prevention of viral hemorrhagic fevers are presented. The West African Ebola epidemic, declared in December 2013, is updated with attention to the reports of encephalitis cases, clinical research agenda, drug and vaccine pipeline, and descriptions of Ebola and post-Ebola syndromes with neurologic manifestations. Travel-acquired viral hemorrhagic fever cases and the widening geographic home range of important human and veterinary viruses are updated.
• Most of the viral hemorrhagic fever agents cause severe, life-threatening disease, and most of these viruses are handled in Biosafety Level 4 (BSL4) containment facilities.
• Excepting agents requiring mosquito intermediates, all other agents have a degree of aerosol infectivity. Body fluids should be considered infectious.
• There is no cure or established drug treatment for many of the viral hemorrhagic fevers. The exceptions are hyperimmune globulin for Argentinian hemorrhagic fever and ribavirin for Lassa fever, Crimean-Congo hemorrhagic fever, and Argentinian hemorrhagic fever. Several investigational drugs (ZMapp, Brincidofovir, TKM-Ebola, Favipiravir, AVI-7537, BCX4430, GS-5734) and blood transfusion or convalescent plasma from recovered patients have been used to treat Ebola patients during the current outbreak. Several Ebola vaccines are in clinical trials.
• The 2009 WHO case classification for severe dengue now includes CNS involvement, and dengue has been identified in 4% to 47% of hospitalized patients with encephalitis-like illness in endemic areas.
• The coming year brings a universal vaccine that includes all 4 serotypes of dengue, as Sanofi-Pasteur applies for licensure for CYD-TDV with an indication for use in individuals age 9 years and older, the (9-16 years) age group that would benefit most from the vaccine.
Historical note and terminology
Viral hemorrhagic fevers are febrile illnesses with abnormal vascular regulation and vascular damage. The combination of fever and hemorrhage can be caused by a number of human pathogens: viruses, rickettsiae, bacteria, protozoa, and fungi. However, the term hemorrhagic fever usually refers to a group of illnesses that are caused by 4 different families of viruses: Arenaviridae, Filoviridae, Bunyaviridae, and Flaviviridae. All viral hemorrhagic fever viruses are lipid-enveloped RNA viruses that persist in nature in an animal or insect host. Except for some viruses in the Flaviviridae family, humans are not normally the natural reservoirs but become infected after contact with infected vectors or natural hosts, usually arthropods or rodents. For some viruses, accidental infection of humans may be followed by human-to-human transmission. The viruses are geographically restricted to areas where the natural hosts live, usually rural areas but occasionally urban cities. Modern transportation can export viremic individuals, infectious host species, or vectors to any location.
Except for the dengue viruses, yellow fever virus, and Chikungunya (Togaviridae family, genus Alphavirus) that require a mosquito intermediate, all of the other agents have a degree of aerosol infectivity. Because of the manner of infection, high virulence, and associated high mortality, these viruses are handled in biocontainment level 4 laboratories and facilities.
Epidemic hemorrhagic fever, a possible manifestation of hantavirus infection, may have existed in China as early as 960 CE (Common Era) (Hart and Bennett 1994). In early China, there were also descriptions of a dengue-like disease called “water poison,” because of its linkage with water-associated flying insects and fever, rash, arthralgia, myalgia, and hemorrhage, in Chinese literature from the Chin (CE 265-420), Tang (CE 610), and Northern Sung (CE 992) dynasties (Gubler 1997). Other diseases known before modern understanding of pathogens and transmission were named after appearances of the afflicted or sites of epidemics. For example, “yellow fever” applied to jaundiced individuals. “Dengue” was a Spanish attempt at the Swahili phrase “ki denga pepo,” translated as “cramp-like seizure caused by an evil spirit” during a Caribbean outbreak in 1827 (Anonymous 2006). The naming of diseases after the location of first encounter yielded the geographic eponyms of Ebola (Ebola River, Zaire 1976), Marburg (Marburg, Germany 1967), Lassa fever (Lassa, Nigeria 1969), as well as Argentine, Bolivian, Rift Valley, Crimean-Congo, Kyasanur Forest, Omsk, Hantaan, Seoul, or Korean hemorrhagic fever.
Hantaviruses have an interesting and dark history. Records of outbreaks of fever, hemorrhage, and severe renal disease occurring in spring and early summer and again in fall starting in 1913 were retrieved from the archives of a hospital in Vladivostok (Casals et al 1970). The Russian and Japanese medical workers who encountered these sporadic epidemic hemorrhagic fever cases in the early 1900s went on to develop a practical working knowledge of the disease. Russian workers produced the Far Eastern disease in humans by parenteral injection of bacteria-filtered serum and urine from patients with natural disease. Experimental subjects were hospitalized, and psychiatric patients required pyrogenic therapy (Gajdusek 1962). A Japanese group working in Manchuria between 1938 and 1945 also isolated a filterable agent from field rodents and reproduced the disease. The sudden close of World War II interrupted the group's work and their transmissible agent was lost at the time of surrender (Hullinghorst and Steer 1953). In Scandinavia in the 1930s, an acute renal disease was seen, similar to hemorrhagic fever described in the Far East. Sixty Finnish and 1000 German frontline troops in Lapland had nephropathia epidemica (Puumala virus) in 1942 (Lahdevirta 1971). Over 3000 United Nations soldiers serving in Korea between 1950 and 1953 contracted Korean hemorrhagic fever (Lee and Dalrymple 1989). Today, hantaviruses, agents well adapted in nature to a number of common rodents, are becoming a worldwide problem. Actual human cases or seropositive rodents have been found on every continent except Antarctica, so that the potential for Hantaan human disease exists in any area of the world where rodent-human contact is common (LeDuc et al 1984; LeDuc et al 1986).
Viral hemorrhagic fevers can cause neurologic disease. Neurologic complications independent of (ie, not directly linked to) metabolic or hemorrhagic complications have been recognized and reported for members of every viral hemorrhagic fever family. Lassa, Argentine hemorrhagic fever, Marburg, hantavirus, Puumala virus, and dengue virus infections have neurologic manifestations or sequelae. Although the neurologic illness may be overshadowed by systemic or hemorrhagic illness, CNS disease is not a simple reflection of what has started in the periphery. Notably, South American hemorrhagic fever and Rift Valley fever may cause encephalitic disease without the hemorrhagic fever (Zaki and Peters 1997), and occasionally, dengue virus infection may have a similar presentation (Kumar et al 2008; Liou et al 2008b).
Importantly, the viral hemorrhagic fevers are prime examples of viral emergence in recent history. Work on viral hemorrhagic fevers represents much of the pioneering work identifying factors that shape viral emergence: factors such as human behavior, demographics, economic development, land use, technology and industry, international trade and travel, microbial adaptation and change, and global warming (Lederberg et al 1992; Morse 1994; Halstead 2008; Barclay 2008). Today, some of the most innovative work in structure-based antiviral drug design is in viral hemorrhagic fever laboratories. As with ribavirin, a promising agent against 1 or more viral hemorrhagic fevers may become a successful broad-spectrum antiviral of the future against multiple classes of viruses. However, a caveat is that fever is 1 of the critical clinical features in viral hemorrhagic fevers. Thus, antiviral drugs developed for treatment and based on the crystal structure of a protein prepared from normal body temperature may not fit perfectly in the target space of a protein at fever temperature. This may potentially induce a more virulent virus through viral gene mutation.
Work on the viral hemorrhagic fevers has also yielded critical recommendations for medical staff safety when implementing and maintaining viral containment measures (Centers for Disease Control and Prevention 1998). Looking back on the outbreaks, we find the effective use of nonpharmaceutical barrier nursing measures mitigated the impact of epidemics. Because epidemics may begin in resource-limited settings, lessons from the early Marburg, Ebola, and Lassa fever outbreaks can be as relevant today as in the 1960s and 1970s.
Currently, an estimated 49% of emerging viruses are characterized by encephalitis or serious neurologic clinical symptoms (Olival and Daszak 2005). The world is a small place, with constant reminders of the need for greater international monitoring of new viruses, increased investigating of virus ecology, and a broader understanding of basic viral and host factors. The viral hemorrhagic fevers outbreak experience helped crystallize these ideas and initiatives. When the viral hemorrhagic fevers emerged, they were brand new or new to man. What was learned from the viral hemorrhagic fevers can prepare us for future outbreaks, particularly management of the explosive outbreaks with high fatality rates, provided we pay attention.
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