Naegleria Fowleri Life Cycle
During two of its three life stages, the aquatic amoeba Naegleria fowleri can parasitize humans, causing a fatal, meningitis-like inflammation. The amoeba enters the body through nasal membranes and along olfactory (smell) nerves to the brain. Symptoms appear 3 to 10 days later. Diagnosis is made at autopsy. Water testing requires weeks to culture the amoeba, but routine water tests suggest that Naegleria fowleri is common in warm rivers and lakes. For the years 1998 to 2007, 33 infections were diagnosed in the United States.
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Trophozoite
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The trophozoite life stage is a slug-shaped amoeba, actively mobile with wide, rounded pseudopods. N. fowleri feeds during this stage and will consume brain tissue in a human host. Trophozoites can survive in the cerebrospinal fluid (CSF), as well as the brain and olfactory nerves. Trophozoite contains vacuoles (fluid filled sacs) in the cell plasm and has all of the sub-cellular organelles typical of eukaryotes (cells with a nucleus). Electron micrographs reveal cup-like extensions of the cell surface used for feeding on bacteria. As the cell moves, the posterior end is sticky and may have trailing filaments.
Reproduction
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During the trophozoite stage, N. fowleri reproduces by asexual cell division. At all stages, this amoeba has a distinct cell nucleus, with a large dense nucleolus that contains genetic material. During nuclear division, the nuclear membrane remains intact while the nucleolus divides to form polar bodies for the daughter cells.
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Flagellate
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In fresh water between 80.6 to 98.6 degrees Fahrenheit (27 to 37 degrees Centigrade) the trophozoite can grow two flagella within 1 to 20 hours. Flagella are a central filament with a cytoplasm coating. Agitation of growth medium and age of cell also affect enflagellation. Once complete, the flagella persist for as many as 24 hours, enabling the cell to swim. Swimming allows a wider dispersal of the organism in fresh water or cerebrospinal fluid. The flagellate stage does not feed. It has a large "contractile vacuole" (squeezing sac) that can eliminate liquid from the cell. In the laboratory, flagella disappear when the amoeba is placed into distilled water.
Cyst
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The cyst (protected) stage of the N. fowleri life cycle is a time of reorganization. The shape of the organism becomes spherical and the cell wall thickens, but retains one or two minute pores. Inside the cell wall, the amoeba resembles a smaller version of the trophozoite stage, with a defined nucleus. Cysts form in water, soil and culture media, but not in human tissue. In dry conditions, cysts can survive for about 5 minutes, so that human infection from an airborne cyst is unlikely.
Exposure
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Naeglaria fowleri can survive in any warm fresh water that is not chlorinated. It is not found in ocean water. Of the recorded deaths in the United States, one Arizona child was exposed in a hot tub and another was exposed in a swimming pool filled from a residential deep well. Similar pool exposures occurred in Australia. A Georgia child was exposed while playing in a small warm stream. N. fowleri has been found at hot springs, in sewage sludge, HVAC condensing units and in an Illinois lake thermally polluted from cooling waters of a nuclear power plant.
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