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Infection of the central nervous system caused by free-living amoebae is rare but usually lethal. Granulomatous amebic encephalitis is a subacute infection caused by Balamuthia mandrillaris or Acanthamoeba species, with the latter occurring almost exclusively in immunocompromised patients. In contrast, primary amebic meningoencephalitis is a fulminant infection caused by Naegleria fowleri that most commonly affects healthy children and teenagers. Early diagnosis and prompt treatment with combination antimicrobials has proven lifesaving for what had previously been an almost uniformly fatal disease. In this article, the author reviews the clinical presentation and diagnostic evaluation for amebic meningoencephalitis, with an emphasis on new treatment options.
Primary amebic meningoencephalitis presents similarly to bacterial meningitis and is usually due to intranasal inoculation of water containing Naegleria.
Granulomatous amebic encephalitis is a subacute infection that mimics granulomatous infections, often causing CNS mass lesions.
Early treatment with combinations of antimicrobial regimens that include miltefosine may save lives and improve outcome.
In cases of suspected amebic meningoencephalitis, the U.S. Centers for Disease Control and Prevention should be contacted (770-488-7100) for diagnostic and clinical assistance.
Free-living amoeba were first recognized as causes of mammalian infection in 1958, when monkeys inoculated with Acanthamoeba culbertsoni developed fatal encephalitis (55), with the first human case of granulomatous amebic encephalitis recognized shortly thereafter. This was followed by the first report of a patient with primary amebic meningoencephalitis in Australia in 1965 (16). Balamuthia mandrillaris was first isolated in 1986 from brain tissue obtained from a baboon who succumbed to encephalitis. Balamuthia, named after the parasitologist Willam Balamuth, was initially grouped as a leptomyxid amebae, but in 1993 B mandrillaris was recognized as a distinct genus and species (56). The most recently identified free-living amoeba to cause human neurologic infection is Sappinia pedate, identified in 2001 in a previously healthy man with a temporal lobe mass lesion (18). However, there have been no subsequent reports of this as a human pathogen.
Estimation of the frequency of these infections is challenging given underdiagnosis and lack of an established surveillance system. A report identified 109 laboratory-confirmed cases of Balamuthia granulomatous amebic encephalitis in the United States between 1974 and 2016 (10). Primary amebic meningoencephalitis is a reportable disease in the United States, with 148 cases identified between 1962 and 2019. More information can be found on the CDC website:https://www.cdc.gov/parasites/naegleria/graphs.html.
CNS infection with free-living amebae can be broadly categorized under 2 clinical syndromes: primary amebic meningoencephalitis or granulomatous amebic encephalitis. These presentations differ with respect to causative agents, clinical presentation, and host risk factors. Primary amebic meningoencephalitis is caused by infection with Naegleria fowleri. This organism is abundant in fresh water and thrives in warm temperatures. Not surprisingly, most cases of primary amebic encephalitis occur in the summer months, with a history of swimming in fresh water within the week prior to the onset of symptoms. The clinical presentation of primary amebic meningoencephalitis mimics bacterial meningitis and is characterized by rapid progression, leading to the popular description of Naegleria as by a brain-eating parasite in the lay press. The disease course for primary amebic meningoencephalitis is usually fulminant with an abrupt onset involving headache, nausea, vomiting, and fever, with rapid evolution to coma. Death typically ensues within 5 days of symptom onset, usually due to overwhelming encephalitis and cerebral edema (06). The CSF shows an elevated opening pressure, a neutrophilic pleocytosis, an elevation in protein, and, not infrequently, hypoglycorrhachia.
In contrast, granulomatous amebic encephalitis caused by Acanthamoeba species and Balamuthia species presents as a subacute or chronic process, progressing over weeks to months. Some cases, however, may be more acute with coma and death within 7 to 9 days. Patients may manifest a prodromal flu-like illness with headache and fever. Subsequently, patients with granulomatous amebic encephalitis often develop headache, vomiting, altered mental status, and convulsions, suggesting the possibility of a mass lesion. The CSF usually shows a lymphocytic pleocytosis with a normal or near-normal protein and glucose concentration. Patients with immune compromise such as AIDS, severe malnutrition, and hematopoietic stem cell/organ transplants are more susceptible to granulomatous amebic encephalitis due to Acanthamoeba species (28). In addition, Acanthamoeba spp may also be carriers of endosymbiotic bacteria (Legionella and Legionella-like pathogens) and have been implicated in outbreaks of pneumonia in debilitated hosts (47). However, Acanthamoeba species are increasingly recognized as pathogens in immunocompetent patients as well (13; 02).
Granulomatous amebic encephalitis due to Balamuthia mandrillaris is an increasingly recognized infection occurring in both immunocompromised and immunocompetent hosts, particularly in children (57). A summary of patients reported to the CDC identified 109 laboratory-confirmed cases documented between 1974 and 2016, 99% of whom had encephalitis, with a disproportionately high number of cases among people of Hispanic ethnicity (10). The clinical picture for Balamuthia is similar to that seen with Acanthamoeba encephalitis, with progressive neurologic decline variably associated with focal signs (eg, hemiparesis), cranial nerve palsies, and a CSF mononuclear pleocytosis; the time from symptom onset to death averages 24 days (10). The focal signs may be a result of infarctions caused by a chronic necrotizing vasculitis with trophozoite and cyst forms of amoebae within the blood vessel wall (22).
Mycotic aneurysm associated with Balamuthia infection has also been reported (41). Concurrent cutaneous disease, identified in 6% of Balamuthia encephalitis cases, presents as an erythematous indurated plaque, often involving the face and may present a more easily accessible site for diagnostic biopsy (10). Of interest, dual concurrent infection with Balamuthia mandrillaris and Acanthamoeba amebic encephalitis was reported in a patient with advanced AIDS coinfected with cerebral toxoplasmosis (43).
The significance of Sappinia species in causing human disease is controversial. To date there has been a single patient identifieda 38-year-old immunocompetent man with visual disturbances, headache, and a seizure. Brain images showed a solitary mass in the posterior left temporal lobe. He underwent a surgical resection, and the mass was composed of necrotizing hemorrhagic inflammation that contained free-living amoebae. This was ultimately identified as Sappinia pedatadiploidea, an organism usually found in soil and tree bark (44). This patient was treated successfully with a sequential regimen of antiamebic drugs (azithromycin, pentamidine, itraconazole, and flucytosine) after the surgical excision (19; 18).
Primary amebic meningoencephalitis and granulomatous amebic encephalitis are extremely lethal diseases, and often the diagnosis is only recognized postmortem. Early recognition and initiation of combination therapy, including miltefosine, has led to a slight improvement in outcomes. Of 148 cases of reported primary amebic meningoencephalitis the United States, 144 cases have been fatal. There have been 5 patients in North America (1 in Mexico, 4 in U.S.) who have survived. Three of these cases occurred since 2013, all of whom were 16 years or less in age with favorable neurologic recovery (30). In a case series of Balamuthia granulomatous amebic encephalitis, mortality was 90% (10).
A 13-year-old boy was admitted with a 2- to 3-day history of headache, lethargy, and vomiting. His parents recalled a trip to a lake 1 week prior to presentation, where he engaged in water sports and went cliff diving. On physical examination he was febrile to 39.5ºC with meningeal signs, increased reflexes, and lethargy. Brain CT and MRI scan were normal. The CSF showed increased protein of 300 mg/dL, glucose of 45 mg/dL, and 160 white blood cells/mm3 (65% polymorphonuclear cells). Motile amoebae were observed in a wet mount of the CSF. Despite treatment, he died within 3 days of admission.
Free-living amoebae are single-cell organisms that are ubiquitous in the environment.
Naegleria are frequently present in fresh water, especially warm, stagnant water. Naegleria exist in 3 forms, which are all nucleated: trophozoite, flagellate, and cyst (33; 30). The trophozoite is a motile form that resembles a eukaryotic cell, and it is the stage that causes human infection. After placement into distilled water, trophozoites can transform into a distinctive flagellate form; the latter stage appears pear shaped, with 2 to 4 flagellae protruding from the anterior end, providing the capacity for rapid motility. Under conditions of stress, such as nutritional deprivation or desiccation, Naegleria form resistant cysts. Clinical specimens typically contain the trophozoite form.
Acanthamoeba are common soil inhabitants that only exist as trophozoites or double-walled cysts, with no flagellate form. The trophozoite form has fine pseudopodia at the leading edge as it moves; these thorn-like spikes give the genus its name, from the Greek word akantha for thorn. The cyst form of Acanthamoeba has a more wrinkled appearance than the cyst of Naegleria; this dormant phase can survive in the environment for decades.
Balamuthia mandrillaris is indistinguishable from Acanthamoeba in formalin-fixed tissue examined at the light microscope level; however, there are distinctive differences with respect to ultrastructure, physiology, and antigenicity (22).
Pathophysiology. A large number of free-living amoebae are of various virulence, perhaps related to their possession of particular pathogenic enzymes. Naegleria thrive in warm, stagnant water. Raised temperatures during the hot summer months, water in hot spring recreational facilities, or warm water from power plants facilitate the growth of N fowleri (53). Classically, primary amebic meningoencephalitis occurs through inoculation of infected water through the cribiform plate. The classic scenario is cliff diving, where water under pressure enters the nasal cavity. Infection through tap water exposure is also a risk factor, and cases have been reported after Neti Pot nasal irrigation, ritual nasal ablution, use of a Slip-n-Slide supplied with tap water, and rarely, ingestion of contaminated tap water (08). The agent then enters the meninges along the extensions of the olfactory nerve or blood vessels, spreading throughout the meninges and within the brain. Amoebae are known to be cytopathogenic for nerve cells in tissue culture, and presumably this property contributes to brain pathology (32; 34; 30). This means of spread was demonstrated to occur in experimentally infected mice (36). The presence of severe amebic involvement of the olfactory regions and frontal regions of the cerebral hemispheres further supports the postulated route of spread. Preclinical evidence demonstrated that N fowleri strongly adhered to host extracellular matrix (26) and disrupted epithelium by changes in tight junction proteins such as claudin-1 (51). The microscopic pathology is characterized by intense polymorphonuclear (neutrophil and eosinophil) infiltration resulting in necrotizing encephalitis.
The pathogenesis of Acanthamoeba encephalitis is less well understood. Acanthamoeba colonizes the nares in up to 24% of cases (55), suggesting intranasal spread through inhalation of airborne dust as a route. Cutaneous inoculation leading to hematogenous spread is another postulated mechanism of infection. An experimental model demonstrated that Acanthamoeba disrupt the blood-brain barrier by secretion of proteases to invade the brain (15). Organisms then multiply intracranially, with resulting edema, hemorrhage, and necrosis; perivascular cuffing by neutrophils, trophozoites, and cysts is often present (55). Granulomatous inflammation is typically present, although may be atypical or absent in the profoundly immunocompromised.
Balamuthia mandrillaris and Acanthamoeba species have been isolated from soil, dust, and water. The pathogenesis of CNS disease caused by this organism is unclear; however, hematogenous spread is suggested based on clustering of amebae around blood vessels. Preclinical evidence demonstrated interaction of Balamuthia and human brain microvascular endothelial cells via a galactose-binding protein (38). Brain infarction may occur secondary to a necrotizing vasculitis or mycotic aneurysm formation with amoeba trophozoite and cyst forms in the blood vessel wall (22; 25). Pathology of brains from patients with Balamuthia infection showed a spectrum of inflammation that ranged from neutrophilic infiltrates to necrotizing granulomas. Diffuse meningitis with massive coagulative necrosis and perivascular infiltration by lymphocytes and histiocytes was reported in an autopsy case (23; 03). Trophozoites and cysts can be found in subarachnoid and Virchow-Robin spaces.
Naegleria have been found in thermal discharges of power plants, heated swimming pools, hot springs, hydrotherapy pools, sewage, and even tap water (35; 50). In sporadic cases of primary amebic meningoencephalitis, patients usually have a history of swimming in fresh water within the week before the onset of the illness. Two fatal cases in Louisiana were reported in association with the use of contaminated tap water for nasal irrigation (59). Naegleria fowleri has been isolated from a treated public drinking water system in the U.S. (11). Several cases were reported in Pakistan following ritual nasal ablution in Muslim communities (50). Occasional epidemic outbreaks have occurred, usually because of swimming in an inadequately chlorinated swimming pool (27; 35; 42). As of 2019, a total of 145 cases of primary amebic meningoencephalitis have been reported in the United States, with most cases occurring in Southern states during summer months. More information can be found on the following site:CDC.gov. However, a few cases have occurred in Northern states such as Minnesota and Indiana (06). The median age of the patients was 12 years. Most cases (76%) were male. More information about gender can be accessed at the following site:CDC.gov.
Granulomatous amebic encephalitis caused by Acanthamoeba is reported primarily in immunocompromised patients (37; 14; 04; 01; 45; 17; 12). As this is not a reportable disease, scant data exist regarding the epidemiology of Acanthamoeba granulomatous amebic encephalitis, with most information coming from case reports or small case series. There is no clear geographic localization for this infection.
A registry of Balamuthia granulomatous amebic encephalitis in the United States reported 109 infections between 1974 and 2016 (10). Several important features in this report include male predominance (68%), overrepresentation of Hispanics (55%), and documented recreational or occupational soil exposure in 85% of cases where information was available. Although previously believed to be a disease of immunocompetent hosts, this study revealed 39% of cases were immunocompromised. Transmission through organ transplant has also been described (31).
Environmental measures may decrease the risk of infection. Adequate chlorination of swimming pools and water supplies prevents growth of Naegleria. Because the location and concentration of Naegleria in the water can vary over time, environmental sampling, testing, and posting of warning signs are unlikely to be effective in preventing infections (07). Although supporting data are absent, risk for infection might be reduced by measures that minimize water entering the nose when using warm freshwater lakes or rivers in southern-tier states. It is possible that wearing nose clips or avoiding jumping into water will reduce the likelihood of the agents' entry into the nasal cavities (37; 20). Currently, no vaccines are available for prevention of these infections.
The clinical syndrome of primary amebic meningoencephalitis resembles that of acute bacterial meningitis. Patients usually have a history of swimming in pools or lakes within 7 days of the onset of symptoms. Diagnostic confusion between these 2 entities is compounded because the CSF formula of primary amebic meningoencephalitis is similar to that seen with bacterial meningitis. Failure to identify a bacterial pathogen on culture or molecular studies might suggest the diagnosis of primary amebic meningoencephalitis. The identification of amebic trophozoites in fresh, warm CSF specimens is diagnostic (54).
In infections secondary to Acanthamoeba or Balamuthia mandrillaris, focal CNS signs and imaging is often suggestive of other causes of mass lesions, such as bacterial brain abscess, tuberculosis, fungal infection, toxoplasmosis, or tumor. Cases caused by B mandrillaris can resemble progressive or atypical strokes. The absence of amoebae in the CSF in cases of granulomatous amebic encephalitis makes the diagnosis even more elusive. Cerebral biopsy (or autopsy) with an astute neuropathologic evaluation is necessary for the proper definitive diagnosis. A diagnosis of granulomatous amebic encephalitis should be considered in patients with subacute onset of unexplained encephalitis, particularly if there are focal mass lesions on imaging (07).
Patients with primary amebic meningoencephalitis caused by Naegleria have a clinical syndrome resembling that of acute bacterial meningitis. The CSF shows features characteristic of acute bacterial meningitis with a neutrophilic pleocytosis, elevated protein, and decreased glucose levels. Visualization of motile trophozoites in fresh and warm CSF specimens is considered diagnostic. Giemsa-Wright stain or modified trichrome stain of CSF may also demonstrate Naegleria trophozoites. Culture of the organism and detection by polymerase chain reaction are possible but these tests are not uniformly available outside of research settings. Identification of Naegleria in tissue by immunohistochemistry or immunofluorescence can be performed by the Centers for Disease Control and Prevention, with diagnostic assistance available (CDC.gov).
In patients with granulomatous amebic encephalitis, neuroimaging may suggest the diagnosis, although findings are often nonspecific and include single and multifocal parenchymal lesions, ring-enhancing lesions, hydrocephalus, nonspecific white matter lesions, meningeal exudates, hemorrhagic infarcts, or necrosis (29; 24; 52; 48). In contrast to primary amebic meningoencephalitis, organisms are rarely seen in the CSF. Serologic testing is of limited value given the ubiquity of these organisms in nature (30). Diagnostic testing is limited through commercial laboratories, and when granulomatous amebic encephalitis is suspected the CDC free-living and intestinal ameba laboratory can provide assistance through telediagnosis or testing of tissues (CDC.gov). PCR testing of formalin-fixed tissue may be useful for detecting free-living amebae in brain tissue (40). Metagenomic deep sequencing may play a role in diagnosis, with a report of Balamuthia mandrillaris detected in the CSF of a patient with an eye infection complicated by meningoencephalitis (58).
Early recognition and prompt aggressive management of primary amebic meningoencephalitis and granulomatous amebic encephalitis is critical given the poor prognosis of these infections. There have been no clinical trials to guide optimal treatment, and most recommendations are based on anecdotal reports of success in survivors.
Until 2013, mortality for primary amebic meningoencephalitis approached 100% (49; 05; 21). Since this time, there have been 4 reported survivors, all of whom were treated with a regimen that included miltefosine. This medication was previously only available through the CDC but is now available commercially, with consignment programs allowing on-site stocking of medication (www.impavido.com). Studies show variable penetration of this medication into the cerebrospinal fluid (39). Other agents active against Naegleria include amphotericin B, rifampin, fluconazole, and azithromycin (09). The CDC recommends use of both intravenous and intrathecal amphotericin B deoxycholate for this infection, which is significantly more active than liposomal preparations (30). Dexamethasone is often indicated for treatment of cerebral edema. Specific regimens and dosing should be determined in consultation with experts at the CDC (770-488-7100).
Most cases of granulomatous amebic encephalitis are fatal, with combination therapy offering the best chance of survival for CNS infections with Acanthamoeba and Balamuthia. The use of drug combinations addresses resistance patterns that may exist or develop during treatment, ensuring that at least 1 of the drugs may be effective against the amoebae (47; 46). Because of the complexity of treatment, consultation with an expert at the CDC is strongly recommended (770-488-7100). Based on reports of survivorship, the recommended regimen for treatment includes pentamidine IV, sulfadiazine, flucytosine, fluconazole, azithromycin, and miltefosine (30). The specific regimen and dosing recommendations are available online (CDC.gov). Miltefosine can be obtained commercially (www.impavido.com), although is not FDA-approved for this indication.
All contributors' financial relationships have been reviewed and mitigated to ensure that this and every other article is free from commercial bias.
Karen C Bloch MD MPH
Dr. Bloch of Vanderbilt University Medical Center has no relevant financial relationships to disclose.See Profile
Christina M Marra MD
Dr. Marra of the University of Washington School of Medicine has no relevant financial relationships to disclose.See Profile
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