Free-living amebas are protozoan environmental parasites, mainly in freshwater, lakes, and rivers, without needing a definitive host. In humans, there are four main genera of amebas that can cause disease, namely Naegleria (only Naegleria fowleri), Acanthamoeba (several species), Sappinia (only S. pedata), and Balamuthia (only Balamuthia mandrillaris). The amebas have a propensity to cause central nervous system infections, but Acanthamoeba is well-known to cause keratitis and disseminated infections as well.
These free-living amebas can cause two distinct clinical syndromes, primary amebic meningoencephalitis (PAM) and granulomatous amebic encephalitis (GAE). The initial symptoms of PAM are indistinguishable from bacterial meningitis, while the symptoms of GAE can mimic a brain abscess, encephalitis, or meningitis. These infections are almost uniformly fatal. The mortality rate is above 90%, despite antimicrobial therapy.
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