Infection by Naegleria is acquired by swimming in freshwater ponds and lakes, in the southern United States and other subtropical and tropical regions. The amebas enter through the nasal mucosa and spread to the CNS via the olfactory nerves. The organisms initially cause an acute purulent meningitis but then rapidly spread beyond the leptomeninges to cerebral tissue, where they produce an acute hemorrhagic necrotizing encephalitis. The pathogenesis of meningoencephalitis caused by Acan thamoeba species is not well understood. In general, this form of amebic infection tends to be subacute or chronic in its onset and course.
Acanthamoeba meningitis occurs frequently in immunosuppressed or debilitated patients.
It is often associated with multiple hemorrhagic necrotic granulomatous abscesses of the brain, especially in midline and posterior fossa structures. Five or six days elapse between contact with amebas and onset of symptoms, which include bifrontal headache, high fever, and nausea and vomiting. Olfactory abnormalities and dysgeusia are frequent symptoms, and meningeal signs appear rapidly. Neurologic findings include bizarre behavior, diplopia, ataxia, seizures, and coma. If the illness is not treated or if treatment is unsuccessful, it progresses rapidly to death in three to 11 days.