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Primary Amebic Meningoencephalitis

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Disease Condition

Primary Amebic Meningoencephalitis


Primary Amebic Meningoencephalitis (PAM) is caused by an ameba called Naegleria fowleri.

For information on all types of amebic central nervous system infections including PAM, see


Amebic meningitis/encephalitis infections are not spread from person to person.

Naegleria fowleri infects people when water containing the ameba enters 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 infections may also occur when contaminated water from other sources (such as inadequately chlorinated swimming pool water or contaminated tap water) enters the nose, such as when people submerge their heads or cleanse their noses during religious practices, and when people irrigate their sinuses (nose) using contaminated tap water. The Naegleria fowleri ameba then travels up the nose to the brain where it destroys the brain tissue. Naegleria fowleri has not been shown to spread via water vapor or aerosol droplets (such as shower mist or vapor from a humidifier), and you cannot be infected with Naegleria fowleri by drinking contaminated water. 

While infections with Naegleria fowleri are rare, they occur mainly during the summer months of July, August, and September. Infections are more likely to occur in southern-tier states. Infections usually occur when it is hot for prolonged periods of time, which results in higher water temperatures and lower water levels.


In its early stages, symptoms of PAM may be similar to symptoms of bacterial meningitis. Initial symptoms of PAM start about 5 days (range 1 to 9 days) after infection. The initial symptoms may include headache, fever, nausea, or vomiting. Later symptoms can include stiff neck, 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 about 5 days (range: 1 to 18 days).

PAM disproportionately affects males and children. The reason for this distribution pattern is unclear but may reflect the types of water activities (such as diving or watersports) that might be more common among young boys. The extremely low occurrence of PAM makes epidemiologic study difficult.


Although infections are severe, the risk of Naegleria fowleri infection is very low. There have been 30 reported infections in the U.S. during the 10 years from 2000-2009, despite millions of recreational water exposures each year. By comparison, during the ten years from 1996 to 2005, there were over 36,000 drowning deaths in the U.S. 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 some people have been infected compared to the millions of other people using the same or similar waters across the U.S. The only way to prevent Naegleria fowleri infection is to refrain from water-related activities. If you do plan to take part in water-related activities, here are some measures that might reduce risk:

  • Avoid water-related activities in bodies of 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 such as lakes, rivers, or hot springs.
  • Avoid putting your head under the water in hot springs and other untreated thermal waters.
  • Avoid digging in or stirring up the sediment while taking part in water-related activities in shallow, warm, freshwater areas. 
  • Use only sterile, distilled, or lukewarm previously boiled water for nasal irrigation or sinus flushes (e.g., Neti Pot usage, ritual nasal ablution, etc.). 

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Recent Texas Trends

Other Amebic Meningitis and Encephalitis including Granulomatous Amebic Encephalitis (GAE):

From 1972-2016, 13 other amebic meningitis/encephalitis cases were reported to DSHS, eight caused by Balamuthia mandrillaris, three caused by Acanthamoeba species, one by a Leptomyxid species and one by a Sappinia species. Texas’ 1998 Sappinia pedata encephalitis case is the only known case of amebic encephalitis caused by Sappinia, worldwide. In 2012, Texas confirmed a case of Acanthamoeba healyi, a species not previously reported to cause human amebic encephalitis. 

Although GAE infections and other amebic meningitis and encephalitis cases have always been reportable to Texas health departments as exotic diseases, before 2012, PAM was the only amebic infection of the central nervous system that was listed on the Texas Notifiable Conditions list.

    Last updated October 18, 2017