|the stages flagellate, trophozoite and cyst (seen from upper left to lower left to right) of Naegleria fowleri|
Naegleria fowleri is a species of the genus Naegleria, belonging to the phylum Percolozoa. It is a free-living pathogenic protist that causes the disease naegleriasis also known as primary amoebic meningoencephalitis. With a length of 8 micrometers to 15 micrometers in size, depending on its life stage and environment. By comparison, a human hair is 40 to 50 micrometers wide.
This microorganism is typically found in bodies of warm freshwater ponds, lakes, rivers, and hot springs. It is also found in the soil, near warm-water discharges of industrial plants, and in poorly or unchlorinated swimming pools, in an amoeboid or temporary flagellate stage. There is no indication of N. fowleri living in salt water.
Naegleria fowleri is a thermophilic, free-living ameba. N. fowleri is found in warm freshwater ponds, lakes and rivers, and in the very warm water of hot springs. There is an increase in numbers as water temperature rises. It was first discovered in 1965, and first identified in Australia. N. fowleri occurs in three forms – as a cyst, a trophozoite (ameboid), and a biflagellate. It does not form a cyst in human tissue. Only the amoeboid trophozoite stage exists in human tissue. The flagellate form can exist in the cerebrospinal fluid (CSF).
The cyst form is spherical and about 7-15 µm in diameter. They are smooth, have a single-layered wall with a single nucleus. Cysts are environmentally resistant in order to increase the chances of survival until better environmental conditions occur. Trophozoites encyst due to unfavorable conditions. Factors that induce cyst formation include a lack of food, overcrowding, desiccation, accumulation of waste products, and cold temperatures. When conditions improve, the ameba can escape through the pore, or ostiole, seen in the middle of the cyst. N. fowleri has been found to encyst at temperatures below 10 °C (50 °F).
The trophozoite is the feeding, dividing, and infective stage for humans. The trophozoite attaches to olfactory epithelium, where it follows olfactory cell axon through cribriform plate to brain. This reproductive stage of the protozoan organism, which transforms near 25 °C (77 °F) and grows fastest around 42 °C (106.7 °F), proliferates by binary fission. The trophozoites are characterized by a nucleus and a surrounding halo. They travel by pseudopodia, temporary round processes which fill with granular cytoplasm. The pseudopodia form at different points along the cell, thus allowing the trophozoite to change directions. In their free-living state, trophozoites feed on bacteria. In tissues, they phagocytize red blood cells and white blood cells and destroy tissue.
The flagellate is pear-shaped and has two flagellum. This stage can be inhaled into nasal cavity during swimming or diving. This biflagellate form occurs when trophozoites are exposed to a change in ionic concentration, such as placement in distilled water. (The flagellate form does not exist in human tissue, but can exist in the cerebrospinal fluid). Once inside nasal cavity, flagellated form transforms into a trophozoite. The transformation of trophozoite to flagellate occurs within a few hours.
N. fowleri can cause a lethal infection of the brain called naegleriasis (also known as primary amoebic meningoencephalitis (PAM), amebic encephalitis, or Naegleria infection). Infections can occur when water containing N. fowleri is breathed in through the nose, where it then invades through nasal tissue and olfactory nerve tissue to enter the brain through the cribriform plate.  N. fowleri normally eat bacteria but when the amoeba get into humans, it uses the brain as a food source. It takes 2-15 days for symptoms to appear after amoebas enter the nose. The initial symptoms may include headache, fever, nausea, or vomiting. Later symptoms can include stiff neck, confusion, lack of attention, loss of balance, seizures, and hallucinations. Once the trophozoites ingest brain tissue and symptoms begin to occur, death will come within 1-2 weeks. There is no effective treatment. A person infected with N. fowleri cannot spread the infection to another person.
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"Life Cycle and Morphology". http://web.stanford.edu/group/parasites/ParaSites2004/N.%20Fowleri/life%20cycle%20&%20morphology.htm