From 1997 to 2007, the California Encephalitis Project identified ten cases of encephalitis that were caused by the free-living ameba Balamuthia mandrillaris, according to findings appearing in the Morbidity and Mortality Weekly Report for July 18.
As the name suggests, B. mandrillaris was initially recognized and isolated from the brain of a mandrill baboon that had died at the San Diego Wild Animal Park in 1989. Since the first human infections with the ameba that occurred in 1990, there have been about 150 cases worldwide. The ameba is found in the soil and thought to be spread via inhalation of airborne cysts or through direct skin contact.
Cases caused by the ameba are rare and carry a high mortality rate because the symptoms are nonspecific and so the diagnosis is usually delayed, C. Glaser, from the California Department of Public Health, and colleagues note in their report. Compounding the problem is the fact that many physicians are simply unaware of the diagnosis.
In the current series, the preliminary diagnoses included viral meningoencephalitis, neurotuberculosis, neurocysticercosis, and acute disseminated encephalitis. The disease proved fatal in all but one of the ten patients.
Of roughly 3000 encephalitis cases that were referred to the California Encephalitis Project for evaluation during the study period, 500 were selected for Balamuthia testing based on clinical symptoms, elevated cerebrospinal fluid levels of protein and leukocytes, abnormal neuroimaging, or occupational or recreational contact with soil.
In the ten cases identified, serologic testing made the presumptive diagnosis, which was then confirmed by other methods, such as indirect immunofluorescence of brain tissue.
Five of the patients had coexisting medical conditions that may have increased the risk of balamuthiasis, including diabetes, status post splenectomy, gout and heart disease, steroid-treated nephrotic syndrome, and possible lymphoma. Potentially relevant soil exposure was documented in five cases.
"The full spectrum of clinical disease (caused by the ameba) is unknown," the researchers conclude. "Balamuthiasis should be considered in patients with unexplained encephalitis, especially those with lymphocytic pleocytosis, elevated CSF protein (especially > 100 mg/dL), and focal lesions on neuroimaging."
Mor Mortal Wkly Rep CDC Surveill Summ 2008;57:768-771
