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  • Updated 07.01.2023
  • Released 01.12.1994
  • Expires For CME 07.01.2026

Amebic meningoencephalitis



Infection of the central nervous system caused by free-living amebae is rare but usually lethal. There are two distinct presentations of encephalitis due to these organisms. Primary amebic meningoencephalitis is a fulminant infection caused by Naegleria fowleri that most commonly affects healthy children and teenagers. In contrast, granulomatous amebic encephalitis is a subacute infection caused by Balamuthia mandrillaris or Acanthamoeba species, with the latter occurring almost exclusively in immunocompromised patients. Early diagnosis and prompt treatment with combination antimicrobials has proven lifesaving for what had previously been almost uniformly fatal infections. In this article, the author reviews the evaluation and management of amebic meningoencephalitis, with an emphasis on new diagnostic modalities and treatment options.

Key points

• Granulomatous amebic encephalitis is a subacute infection that mimics granulomatous infections, often causing CNS mass lesions.

• Primary amebic meningoencephalitis presents similarly to bacterial meningitis and is usually due to intranasal inoculation of water containing Naegleria.

• 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.

Historical note and terminology

Free-living amebae were first recognized as causes of mammalian infection in 1958, when monkeys inoculated with Acanthamoeba culbertsoni developed fatal encephalitis (39), with the first human case of granulomatous amebic encephalitis recognized shortly thereafter. 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 recognized as a distinct genus and species in 1993 (40). The most recently identified free-living ameba to cause human neurologic infection is Sappinia pedate, identified in 2001 in a previously healthy man with a temporal lobe mass lesion (16). However, there have been no subsequent reports of this organism as a human pathogen.

Estimation of the frequency of granulomatous amebic encephalitis is challenging given underdiagnosis and lack of an established surveillance system. There were 109 laboratory-confirmed cases of Balamuthia granulomatous amebic encephalitis in the United States between 1974 and 2016 (07). A case series based on reports to the CDC Free-Living Ameba database identified 122 patients with Acanthamoeba encephalitis in the U.S. between 1956 and 2020 (23). In contrast, primary amebic meningoencephalitis is a reportable disease in the United States, with 157 cases identified between 1962 and 2019. More information can be found on the CDC website:

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