Balamuthia Amebic Encephalitis — California, 1999–2007

Balamuthia mandrillaris is a free-living ameba that causes encephalitis in humans (both immunocompetent and immunocompromised), horses, dogs, sheep, and nonhuman primates. The ameba is present in soil and likely is transmitted by inhalation of airborne cysts or by direct contamination of a skin lesion. Approximately 150 cases of balamuthiasis have been reported worldwide since recognition of the disease in 1990. Balamuthiasis is difficult to diagnose because 1) the clinical symptoms mimic those of several other types of encephalitis, 2) few laboratories perform appropriate diagnostic testing, and 3) many physicians are unaware of the disease. The lack of recognition and subsequent delay in diagnosis might be a factor in its high mortality. Since 1998, the California Encephalitis Project (CEP) has been testing encephalitis cases for both common and uncommon agents known to cause encephalitis, including Balamuthia. This report describes the 10 balamuthiasis cases identified by CEP during 1999-2007. The preliminary diagnoses in these cases included neurotuberculosis, viral meningoencephalitis, neurocysticercosis, and acute disseminated encephalomyelitis. All but one patient died. These findings underscore the importance of increasing awareness among clinicians, epidemiologists, and public health officials for timely recognition and potential treatment of Balamuthia encephalitis.