Infection due to environmental protozoan parasites. Pathogens include Naegleria fowleri, Acanthamoeba species, and Balamuthia mandrillaris. These organisms can invade the central nervous system (CNS) after humans are exposed by swimming in fresh water. CNS disease due to N. fowleri is severe and nearly always fatal.

Patients present with symptoms usually associated with acute bacterial meningitis including fever, headache, mental status changes, and seizures. Analysis of the cerebral spinal fluid (CSF) reveals a high white blood cell count, low glucose, and high protein (as is commonly seen in bacterial meningitis). If a CSF wet mount is performed, trophozoites can occasionally be identified. Infection with Acanthamoeba species or B. mandrillaris can present in a subacute manner with focal neurological deficits. Patients may also have changes in their mental status. These deficits are due to mass lesions in the CNS. Infections with these pathogens are also frequently fatal.

In addition to disease of the CNS, infection with Acanthamoeba species and B. mandrillaris has been reported to occasionally involve other body sites including the sinuses, lungs, and skin. Additionally, Acanthamoeba species can cause a keratitis. Optimal therapy for these severe infections has not been established.