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Primary amebic meningoencephalitis (PAM) is a very rare form of parasitic meningitis that causes a fatal brain infection. The parasite enters the body through the nose and is caused by the microscopic ameba (a single-celled living organism) Naegleria fowleri.
Naegleria fowleri infects people by entering the body through the nose. This typically occurs when people go swimming or diving in warm freshwater places, like lakes and rivers. The Naegleria fowleri ameba travels up the nose to the brain where it destroys the brain tissue.
You cannot be infected with Naegleria fowleri by drinking contaminated water. In very rare instances, Naegleria infections may also occur when contaminated water from other sources (such as inadequately chlorinated swimming pool water or heated tap water <47°C) enters the nose, for example when people submerge their heads or cleanse during religious practices (1), and, possibly, when people irrigate their sinuses (nose).
Naegleria fowleri is found around the world. In the United States, the majority of infections have been caused by Naegleria fowleri from warm freshwater located in southern-tier states (2). The ameba can be found in:
Factors that can increase your risk of viral meningitis include:
- Bodies of warm freshwater, such as lakes and rivers
- Geothermal (naturally hot) water, such as hot springs
- Warm water discharge from industrial plants
- Geothermal (naturally hot) drinking water sources
- Swimming pools that are poorly maintained, minimally-chlorinated, and/or un-chlorinated
- Water heaters with temperatures less than 47°C (3, 4)
Naegleria fowleri is not found in salt water, like the ocean.
Naegleria fowleri infects people by entering the body through the nose. This typically occurs when people go swimming or diving in warm freshwater places, like lakes and rivers. In very rare instances, Naegleria has been identified in water from other sources (such as inadequately chlorinated swimming pool water or heated tap water <47°C).
The Naegleria fowleri ameba causes primary amebic meningoencephalitis (PAM) when it travels up the nose to the brain and destroys the brain tissue. Naegleria fowleri infections are very rare. In the 10 years from 2001 to 2010, 32 infections were reported in the U.S.
PAM cannot be spread from one person to another.
Signs & Symptoms
Naegleria fowleri causes the disease primary amebic meningoencephalitis (PAM), a brain infection that leads to the destruction of brain tissue. In its early stages, symptoms of PAM may be similar to symptoms of bacterial meningitis.
Initial symptoms of PAM start 1 to 7 days after infection. The initial symptoms include headache, fever, nausea, vomiting, and stiff neck. Later symptoms include confusion, lack of attention to people and surroundings, loss of balance, seizures, and hallucinations. After the start of symptoms, the disease progresses rapidly and usually causes death within 1 to 12 days.
PAM is very rare. The early symptoms of PAM are more likely to be caused by other more common illnesses, such as bacterial or viral meningitis. People should seek medical care immediately whenever they develop a sudden onset of fever, headache, stiff neck, and vomiting, particularly if they have been in warm freshwater recently.
Several drugs are effective against Naegleria fowleri in the laboratory. However, their effectiveness is unclear since almost all infections have been fatal, even when people were treated.
Naegleria fowleri is found in many warm freshwater lakes and rivers in the United States, particularly in southern tier states. It is likely that a low risk of Naegleria fowleri infection will always exist with recreational use of warm freshwater lakes, rivers, and hot springs. The low number of infections makes it difficult to know why a few people have been infected compared to the millions of other people using the same or similar waters across the U.S. The only certain way to prevent a Naegleria fowleri infection is to refrain from water-related activities in or with warm, untreated, or poorly-treated water.
If you do plan to take part in water-related activities, some measures that might reduce risk include:
- Avoid water-related activities in warm freshwater during periods of high water temperature and low water levels.
- Hold the nose shut or use nose clips when taking part in water-related activities in bodies of warm freshwater.
- Avoid digging in, or stirring up, the sediment while taking part in water-related activities in shallow, warm freshwater areas.
If you are irrigating, flushing, or rinsing your sinuses (for example, by using a neti pot), use water that has been:
- previously boiled for 1 minute (at elevations above 6,500 feet, boil for 3 minutes) and left to cool;
- or filtered, using a filter with an absolute pore size of 1 micron or smaller.
Rinse the irrigation device after each use with water that has been distilled, sterilized, filtered, or previously boiled and leave the device open to air dry completely.
- Shakoor S, Beg MA, Mahmood SF, Bandea R, Sriram R, Noman F, et al. Primary amebic meningoencephalitis caused by Naegleria fowleri, Karachi, Pakistan. Emerg Infect Dis. 2011;17(2):258-61.
- Yoder JS, Eddy BA, Visvesvara GS, Capewell L, Beach MJ. The epidemiology of primary amoebic meningoencephalitis in the USA, 1962-2008. Epidemiol Infect. 2010;138(7):968-75.
- Griffin JL. Temperature tolerance of pathogenic and nonpathogenic free-living amoebas. Science. 1972;178(63):869-70.
- Stevens AR, Tyndall RL, Coutant CC, Willaert E. Isolation of the Etiological Agent of Primary Amoebic Meningoencephalitis from Artificially Heated Waters. Applied and Environmental Microbiology. 1977;34(6):701-705.
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