Primary amebic meningoencephalitis (PAM) is a rare brain infection that is caused by Naegleria fowleri and is usually fatal. Naegleria fowleri is a free-living ameba* (a single-celled living organism that is too small to be seen without a microscope). From 1962 to 2021, 154 U.S. infections have been reported to CDC, with an average of 2–3 per year. There have only been four U.S. survivors.
Naegleria fowleri lives in warm fresh water and soil around the world, and infects people when the ameba enters the body through the nose. 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:
- Warm fresh water, such as lakes and rivers
- Geothermal (naturally hot) water, such as hot springs
- Warm water discharge from industrial or power plants
- Untreated geothermal (naturally hot) drinking water sources
- Swimming pools, splash pads, surf parks, or other recreational venues that are poorly maintained or minimally chlorinated
- Tap water
- 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 However, the geographical areas where Naegleria fowleri infections occur in the United States are changing. Evidence suggests that the range of Naegleria fowleri is expanding northward as the climate warms.
Although most Naegleria infections occur after people swim or submerge their heads underwater, PAM may also occur when people cleanse their noses during religious practices or irrigate their sinuses (sending water up the nose) using contaminated tap water.4,5,6
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 and causes swelling of the brain.
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.
Symptoms of PAM start 1 to 12 days after infection.
The initial symptoms include:
Later symptoms include:
- Stiff neck
- Altered mental status (confusion)
- Lack of attention to people and surroundings
After the start of symptoms, the disease progresses rapidly and usually causes death within about 5 days (range 1 to 18 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, stiff neck, or start vomiting, especially if they have been swimming 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. PAM is treated with a combination of drugs that often include amphotericin B, azithromycin, fluconazole, rifampin, miltefosine, and dexamethasone. These drugs are used because they are thought to have activity against Naegleria fowleri and have been used to treat patients who survived.
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, 1962-2008External. 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]External 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.External 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”.