Causal Agents:
Naegleria fowleri and Acanthamoeba spp., commonly found in lakes, swimming pools, tap water, and heating and air conditioning units.  While only one species of Naegleria is known to infect humans, several species of Acanthamoeba are implicated, including A. culbertsoni, A. polyphaga, A. castellanii, A. astronyxis, A. hatchetti, and A. rhysodes.  An additional agent of human disease, Balamuthia mandrillaris, is a related leptomyxid ameba that is morphologically similar in light microscopy to Acanthamoeba.

Life Cycle:

Free-living amebae belonging to the genera Acanthamoeba, Balamuthia, and Naegleria are important causes of disease in humans and animals.  Naegleria fowleri produces an acute, and usually lethal, central nervous system (CNS) disease called primary amebic meingoencephalitis (PAM).  N. fowleri has three stages, cysts , trophozoites , and flagellated forms , in its life cycle.  The trophozoites replicate by promitosis (nuclear membrane remains intact) .  Naegleria fowleri is found in fresh water, soil, thermal discharges of power plants, heated swimming pools, hydrotherapy and medicinal pools, aquariums, and sewage.  Trophozoites can turn into temporary flagellated forms which usually revert back to the trophozoite stage.  Trophozoites infect humans or animals by entering the olfactory neuroepithelium and reaching the brain.  N. fowleri trophozoites are found in cerebrospinal fluid (CSF) and tissue, while flagellated forms are found in CSF.
Acanthamoeba spp. and Balamuthia mandrillaris are opportunistic free-living amebae capable of causing granulomatous amebic encephalitis (GAE) in individuals with compromised immune systems.  Acanthamoeba spp. have been found in soil; fresh, brackish, and sea water; sewage; swimming pools; contact lens equipment; medicinal pools; dental treatment units; dialysis machines; heating, ventilating, and air conditioning systems; mammalian cell cultures; vegetables; human nostrils and throats; and human and animal brain, skin, and lung tissues.  B. mandrillaris however, has not been isolated from the environment but has been isolated from autopsy specimens of infected humans and animals.  Unlike N. fowleri, Acanthamoeba, and Balamuthia have only two stages, cysts and trophozoites , in their life cycle.  No flagellated stage exists as part of the life cycle.  The trophozoites replicate by mitosis (nuclear membrane does not remain intact) .  The trophozoites are the infective forms and are believed to gain entry into the body through the lower respiratory tract, ulcerated or broken skin and invade the central nervous system by hematogenous dissemination .  Acanthamoeba spp. and Balamuthia mandrillaris cysts and trophozoites are found in tissue.

Geographic Distribution:
While infrequent, infections appear to occur worldwide.