Naegleria infection epidemiology and demographics
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Epidemiology and Demographics
Naegleria is a free-living ameba commonly found in the environment in water and soil. Only one species of Naegleria has been found to infect humans, Naegleria fowleri.
Naegleria fowleri is found worldwide. Most commonly, the ameba is found in:
- Warm bodies of fresh water, such as lakes, rivers
- Geothermal water such as hot springs
- Warm water discharge from industrial plants
- Minimally chlorinated swimming pools
- Soil
Although Naegleria is commonly found in the environment, infection occurs rarely. Only 23 infections were documented in the U.S. between 1995 and 2004.
When is Naegleria most common?
Infection with Naegleria is very rare. However, when it does occur, infection is most common during the dry, summer months, when the air temperature is hot, the water is warm, and water levels are low. The number of infections increase during years characterized by heat waves.
Etiologic agent:
Naegleria fowleri and Acanthamoeba spp., are commonly found in lakes, swimming pools, tap water, and heating and air conditioning units. While only one species of Naegleria, N. fowleri, is known to infect humans, several species of Acanthamoeba, including A. culbertsoni, A. polyphaga, A. castellanii, A. astronyxis, A. hatchetti, A. rhysodes, A. divionensis, A. lugdunensis, and A. lenticulata are implicated in human disease. An additional agent of human disease, Balamuthia mandrillaris, is a related free-living ameba that is morphologically similar to Acanthamoeba in tissue sections in light microscopy.
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 1, trophozoites 2, and flagellated forms 3, in its life cycle. The trophozoites replicate by promitosis (nuclear membrane remains intact) 4. 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 non-feeding flagellated forms which usually revert back to the trophozoite stage. Trophozoites infect humans or animals by entering the olfactory neuroepithelium 5 and reaching the brain. N. fowleri trophozoites are found in cerebrospinal fluid (CSF) and tissue, while flagellated forms are occasionally 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 has only recently been isolated from the environment and has also been isolated from autopsy specimens of infected humans and animals.
Unlike N. fowleri, Acanthamoeba and Balamuthia have only two stages, cysts 1 and trophozoites 2, 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) 3. 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 4. Acanthamoeba spp. can also cause severe keratitis in otherwise healthy individuals, particularly contact lens users 4. Acanthamoeba spp. and Balamuthia mandrillaris cysts and trophozoites are found in tissue.
Naegleria infection cannot be spread from person-to-person contact.