Pathogen & Environment
Naegleria fowleri is a heat-loving (thermophilic), free-living ameba (single-celled microbe), commonly found around the world in warm fresh water (like lakes, rivers, and hot springs) and soil 1, 2. Naegleria fowleri is the only species of Naegleria known to infect people. Most of the time, Naegleria fowleri lives in freshwater habitats by feeding on bacteria. However, in rare instances, the ameba can infect humans by entering the nose during water-related activities. Once in the nose, the ameba travels to the brain and causes a severe brain infection called primary meningoencephalitis (PAM), which is usually fatal 1, 2, 3.
The first PAM infections were reported in 1965 in Australia. The ameba identified caused a fatal infection in 1961 and turned out to be a new species that has since been named Naegleria fowleri after one of the original authors of the report, M. Fowler 4. The first infections in the U.S., which occurred in 1962 in Florida 5, were reported soon after. Subsequent investigations in Virginia using archived autopsy tissue samples identified PAM infections that had occurred in Virginia as early as 1937 6.
The Pathogen and Life Cycle
Naegleria fowleri has 3 stages in its life cycle: , ameboid trophozoites , flagellates, and cysts. The only infective stage of the ameba is the ameboid trophozoite. Trophozoites are 10-35 µm long with a granular appearance and a single nucleus. The trophozoites replicate by binary division during which the nuclear membrane remains intact (a process called promitosis) . Trophozoites infect humans or animals by penetrating the nasal tissue and migrating to the brain via the olfactory nerves causing primary amebic meningoencephalitis (PAM).
Trophozoites can turn into a temporary, non-feeding, flagellated stage (10-16 µm in length) when stimulated by adverse environmental changes such as a reduced food source. They revert back to the trophozoite stage when favorable conditions return 7. Naegleria fowleri trophozoites are found in cerebrospinal fluid (CSF) and tissue, while flagellated forms are occasionally found in CSF. Cysts are not seen in brain tissue. If the environment is not conducive to continued feeding and growth (like cold temperatures, food becomes scarce) the ameba or flagellate will form a cyst. The cyst form is spherical and about 7-15 µm in diameter. It has a smooth, single-layered wall with a single nucleus. Cysts are environmentally resistant in order to increase the chances of survival until better environmental conditions occur 8.
Naegleria fowleri is normally found in the natural environment and is well adapted to surviving in various habitats, particularly warm-water environments. Although the trophozoite stage is relatively sensitive to environmental changes, the cysts are more environmentally hardy. There are no means yet known that would control natural Naegleria fowleri levels in lakes and rivers.
Drying: Drying appears to make trophozoites nonviable instantaneously and cysts nonviable in <5 min 9.
Temperature: Naegleria fowleri is a heat-loving (thermophilic) ameba able to grow and survive at higher temperatures, such as those found in hot springs and in the human body, even under fever temperatures. Naegleria fowleri grows best at higher temperatures up to 115°F (46°C) 10. Although the amebae may not be able to grow well, Naegleria fowleri can still survive at higher temperatures for short periods of time. The trophozoites and cysts can survive from minutes to hours at 122-149°F (50-65°C) with the cysts being more resistant at these temperatures 9, 11. Although trophozoites are killed rapidly by refrigeration, cysts can survive for weeks to months at cold temperatures above freezing, although they appear to be sensitive to freezing 9, 11. As a result, colder temperatures are likely to cause Naegleria fowleri to encyst in lake and river sediment where the cyst offers more protection from freezing water temperatures.
Disinfection: Naegleria fowleri trophozoites and the more resistant cysts are sensitive to disinfectants like chlorine 9, 11-15 and monochloramine 15, 16 which are used for disinfection of drinking water and swimming pools if adequate levels are maintained and monitored. The chlorine sensitivity is moderate and in the same range as the cysts from Giardia intestinalis, another waterborne pathogen 17, 18. Specifically, chlorine at a concentration of 1 ppm (1 mg/L) added to 77°F (25°C) clear water at a pH of 7.5 will reduce the number of viable Naegleria fowleri trophozoites by 99.99% (4 logs) in 12 minutes 14. The more resistant Naegleria fowleri cysts are reduced by 99.99% (4 logs) in 53 minutes under the same conditions 14.
In general, CDC does not recommend testing untreated rivers and lakes for Naegleria fowleri because the ameba is naturally occurring and there is no established relationship between detection or concentration of Naegleria fowleri and risk of infection. Environmental testing may be warranted for investigations in which Naegleria fowleri detection may be useful for establishing geographical distribution in new environments 19, survival in disinfected water bodies, or in household water systems 20.
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