This article includes discussion of amebic meningoencephalitis, amebic encephalitis, granulomatous amebic encephalitis (GAE), and primary amebic meningoencephalitis. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
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 Acanthamoeba species or Balamuthia mandrillaris, with the former occurring almost exclusively immunocompromised patients. In contrast, primary amebic meningoencephalitis is a fulminant infection caused by Naegleria fowleri that most commonly effects 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 update on new treatment options.
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• Primary amebic meningoencephalitis presents similarly to bacterial meningitis and is usually due to intranasal inoculation of water containing Naegleria.
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• Granulomatous amebic encephalitis is a subacute infection that mimics granulomatous infections, often causing CNS mass lesions.
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• Early treatment with combinations of antimicrobial regimens that include including miltefosine may save lives and improve outcome.
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• 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.
Historical note and terminology
Free-living amoeba were first recognized as causes of mammalian infection in 1958, when monkeys inoculated with Acanthamoeba culbertsoni developed fatal encephalitis (48), 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 (13). Balamuthia mandrillaris was first isolated in 1986 from brain tissue obtained from a baboon who succumbed to encephalitis. Balamuthia was initially grouped as a leptomyxid amebae, but in 1993 B mandrillaris was recognized as a distinct genus and species (49). 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 (15).
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 (09). Primary amebic meningoencephalitis is a reportable disease in the United States, with 145 cases identified between 1962 and 2016 (CDC.gov).