Primary amebic meningoencephalitis (PAM) is a rare brain infection that is usually fatal and caused by Naegleria fowleri. Naegleria fowleri is a free-living ameba* (a single-celled living organism that is too small to be seen without a microscope.) From 1962–2019, 148 U.S. infections have been reported to CDC with no more than 8 cases reported each year.
Naegleria fowleri lives in soil and warm freshwater around the world. It grows best at higher temperatures up to 115°F (46°C) and can survive for short periods at higher temperatures.1, 2, The ameba can be found in:
- Bodies of warm freshwater, such as lakes and rivers
- Geothermal (naturally hot) water, such as hot springs
- Warm water discharge from industrial plants
- Untreated geothermal (naturally hot) drinking water sources
- Swimming pools that are poorly maintained or minimally-chlorinated
- Water heaters
Naegleria fowleri does not live in salt water, like the ocean.
In the United States, most infections have been linked to swimming in southern states, like Florida and Texas.3 In very rare instances, Naegleria infections may also occur when contaminated water from other sources enters the nose. For example, when people submerge their heads or cleanse their noses during religious practices.4, 5 People have also become infected after irrigating their sinuses (nose) using contaminated tap water.6
Naegleria fowleri infects people by entering the body through the nose, usually while swimming. The ameba travels up the nose to the brain where it destroys the brain tissue.
People cannot get infected with Naegleria fowleri from drinking water contaminated with Naegleria. People also do not spread the ameba or PAM to others.
In its early stages, symptoms of PAM are similar to symptoms of bacterial meningitis.
Initial symptoms of PAM start 1 to 9 days after infection. The initial symptoms include:
- Stiff neck
Later symptoms include:
- Altered mental status (confusion)
- Lack of attention to people and surroundings
- Loss of balance
After the start of symptoms, the disease progresses rapidly and usually causes death within about 5 days.
The early symptoms of PAM are similar to other more common illnesses, such as bacterial or viral meningitis. PAM is difficult to diagnose because of the rarity of the infection and the non-specific early symptoms. Doctors diagnose PAM using specific laboratory tests that are only available in a few U.S. laboratories. People should seek medical care immediately if they suddenly develop a fever, headache, and stiff neck, and start vomiting. This is especially true 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. In the 10 years from 2009 to 2019, 34 infections were reported in the United States and there were 3 survivors.
Personal actions to reduce the risk of Naegleria fowleri infection should focus on limiting the amount of water going up the nose.
Please visit the following pages for information on lowering your risk of infection in specific situations:
- Stevens AR, Tyndall RL, Coutant CC, Willaert E. Isolation of the Etiological Agent of Primary Amoebic Meningoencephalitis from Artificially Heated Waters. Appl Environ Microbiol. 1977;34(6):701-705.
- Yoder JS, Eddy BA, Visvesvara GS, Capewell L, Beach MJ. The epidemiology of primary amoebic meningoencephalitis in the USA, 1962-2008Externalexternal icon. Epidemiol Infect. 2010;138:968-75.
- Griffin JL. Temperature tolerance of pathogenic and nonpathogenic free-living amoebas. Science. 1972;178(63):869-70.
- CDC. Primary amebic meningoencephalitis associated with ritual nasal rinsing — St. Thomas, U.S. Virgin Islands, 2012. MMWR Morb Mortal Wkly Rep. 2013;62(45):903.
- Shakoor S, Beg MA, Mahmood SF, Bandea R, Sriram R, Noman F, et al. Primary amebic meningoencephalitis caused by Naegleria fowleri, Karachi, Pakistan.Cdc-pdf[PDF – 4 pages]Externalpdf iconexternal icon Emerg Infect Dis. 2011:17;258-61.
- Yoder JS, Straif-Bourgeois S, Roy SL, Moore TA, Visvesvara GS, Ratard RC, Hill V, Wilson JD, Linscott AJ, Crager R, Kozak NA, Sriram R, Narayanan J, Mull B, Kahler AM, Schneeberger C, da Silva AJ, Beach MJ. Deaths from Naegleria fowleri associated with sinus irrigation with tap water: a review of the changing epidemiology of primary amebic meningoencephalitis.Externalexternal icon Clin Infect Dis. 2012;1-7.
*About the Term “Ameba”
In U.S. English, the single-celled living organism described here is an ameba. The word amoeba, with an “o”, is used as part of a scientific genus name (such as Amoeba or Acanthamoeba). In British English, both the generic organism term and genera names are spelled amoeba with an “o”.