Brain-Eating Amoeba FAQ

Rare, Fatal Amoeba Infection: Your Questions Answered

Medically Reviewed by Laura J. Martin, MD on August 18, 2011
From the WebMD Archives

Aug. 18, 2011 -- Brain-eating amoebas have killed three young Americans this summer.

What is this scary bug? How does it get to the brain? Where is it, and how can you avoid it? WebMD answers these and other questions.

What Is a Brain-Eating Amoeba?

Amoebas are single-celled organisms. The so-called brain-eating amoeba is a species discovered in 1965 and formally named Naegleria fowleri. Although first identified in Australia, this amoeba is believed to have evolved in the United States.

There are several species of naegleria, but only the N. fowleri species causes human disease.

Like other amoebas, naegleria reproduce by cell division. When conditions are less than optimal, amoebas become inactive cysts. When conditions are favorable, the cysts turn into trophozoites -- their feeding form. These trophozoites can also temporarily grow tails that allow them to swim. In this tailed form they cannot eat, so they soon revert to the trophozoite stage.

Where Are Brain-Eating Amoebas Found?

Naegleria love warm temperatures and are able to survive in water as hot as 113 degrees Fahrenheit.

These amoebas can be found in warm places around the globe. They are found in:

  • Warm lakes, ponds, and rock pits
  • Mud puddles
  • Warm, slow-flowing rivers, especially those with low water levels
  • Untreated swimming pools and spas
  • Untreated well water or untreated municipal water
  • Hot springs and other geothermal water sources
  • Thermally polluted water, such as runoff from power plants
  • Aquariums
  • Soil, including indoor dust

Naegleria can't live in salt water and cannot survive in properly treated swimming pools or in treated municipal water.

Most cases of N. fowleri disease occur in Southern or Southwestern states. Over half of all infections have been in Florida and Texas. However, a recent case in Minnesota suggests either that the amoebas are more common in Northern states than previously known, or that they are spreading into these states.

How Do Amoebas Get in the Brain?

The moniker "brain-eating amoeba" makes naegleria sound like tiny zombies wandering about looking for a way into your skull. But brains are accidental food for them, says Jonathan Yoder, MPH, who tracks the deadly amoeba for the CDC.

"It is normally eating bacteria in its natural environment, but for some reason it does use the brain as a food source when it gets into humans," Yoder tells WebMD.

If you were to drink a glass of water infested with naegleria, you would not get a brain infection. Infection occurs only after water (or perhaps dust) containing the amoeba gets into the nose.

This appears to happen most often when people are diving, water skiing, or performing water sports in which water is forced into the nose. However, infections have occurred in people who dunked their heads in hot springs or who used untreated tap water to cleanse their nostrils.

Studies suggest that N. fowleri amoebas are attracted to the chemicals that nerve cells use to communicate with one another. Once in the nose, the amoebas travel through the olfactory nerve into the frontal lobe of the brain.

How Frequently Do People Get Infected by Brain-Eating Amoebas?

Even though N. fowleri amoebas are relatively common, they only rarely cause brain disease. N. fowleri disease, known as primary amoebic meningoencephalitis or PAM, occurs from zero to eight times a year, almost always from July to September.

Worldwide, there have been some 400 reported cases. There have been 35 reported cases in the U.S. since 2001. Yoder and colleagues were able to identify 111 PAM reports in the U.S. from 1962 to 2008.

However, some cases may be unreported. A study in Virginia that looked at more than16,000 autopsy records from patients who died of meningitis found five previously unreported cases of PAM.

"I am sure we are missing some cases," Yoder says. "But these are pretty tragic infections, often involving children, so doctors and pathologists are motivated to find the cause."

Studies suggest that many people may have antibodies to N. fowleri, suggesting that they became infected with the amoeba but that their immune systems fought it off. It's not at all clear how often this happens.

"We have asked ourselves, 'Is this a rare infection that is always fatal, or a more common one that is only sometimes fatal?' We don't know the answer," Yoder says.

But in a 2009 study, Yoder and colleagues suggested that the common finding of antibodies to the amoeba in humans and the frequent finding of N. fowleri in U.S. waters indicate "that exposure to the amoeba is much more common than the incidence of PAM suggests."

How Long Do Brain-Eating Amoebas Take to Kill You?

It takes two to 15 days for symptoms to appear after N. fowleri amoebas enter the nose. Death usually occurs three to seven days after symptoms appear. The average time to death is 5.3 days from symptom onset.

Only one U.S. patient survived brain infection with these amoebas. This patient, a 9-year-old California girl, was successfully given anti-amoeba antibiotics and, after a month in the hospital, recovered completely. Worldwide, there have been seven reports of survival.

What Are the First Symptoms Someone Might Have?

Symptoms of PAM are not specific to this disease and resemble those of viral meningitis. Symptoms include headache, fever, stiff neck, loss of appetite, vomiting, altered mental state, seizures, and coma. There may also be hallucinations, drooping eyelids, blurred vision, and loss of the sense of taste.

How Do Amoebas Dissolve Brain Tissue?

One study suggests that N. fowleri amoeba produce two enzymes that dissolve protein.

Are Certain Groups Affected More Than Others?

Over 60% of U.S. cases are in children age 13 or younger. About 80% of cases are in males. It's not at all clear whether children or males are more susceptible to the amoeba, or whether young males are more likely to engage in activities that expose them to the amoeba.

How Can I Protect Myself Against Brain-Eating Amoebas?

It makes sense to avoid swimming underwater, diving, water skiing, and jumping in warm, still waters during the late summer. It also makes sense to wear a nose clip when swimming, boating, or playing in or on warm waters.

However, there's no scientific proof that these measures will prevent N. fowleri infection. Millions of people play in warm waters every summer without having their brains infested by amoebas.

It's a waste of time to put up signs warning that a body of water contains N. fowleri amoebas. There may be more or fewer amoebas depending on the time of year and other factors. More importantly, putting up such signs might imply that bodies of water without signs are safe -- and brain-eating amoebas are relatively common.

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Jonathan S. Yoder, MPH, epidemiologist, division of parasitic diseases, CDC, Atlanta.

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