Brain-Eating Amoeba

Medically Reviewed by Brunilda Nazario, MD on September 29, 2021
6 min read

Amoebas are single-celled organisms. The so-called brain-eating amoeba is a species discovered in 1965. Its formal name is Naegleria fowleri. It usually lurks in warm freshwater bodies or untreated, contaminated waters. When it finds its way inside the human body, it causes a rare, yet deadly infection and inflammation in the brain and eventually destroys the brain tissue by “eating” it. Doctors call this disease primary amebic meningoencephalitis (PAM). You can only get it when contaminated water with the brain-eating amoeba travels up your nose.

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

There are several species of Naegleria but only the fowleri species causes PAM. It was first identified in Australia, but this amoeba is believed to have evolved here in the U.S.

N. fowleri is microscopic: 8 micrometers to 15 micrometers in size, depending on its life stage and environment. By comparison, a hair is 40 to 50 micrometers wide.

Like other amoebas, Naegleria reproduces by cell division. When conditions aren't right, the amoebas become inactive cysts. When conditions are favorable, the cysts turn into trophozoites -- the feeding form of the amoeba.

Naegleria loves very warm water. It can survive in water as hot as 115 F.

These amoebas can be found in warm places around the globe. N. fowleri is 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
  • Splash pads for children
  • Water parks

Naegleria can't live in saltwater. It can't survive in properly treated swimming pools or in properly treated municipal water. You also can’t get infected by drinking contaminated water.

Most cases of N. fowleri disease occur in Southern or Southwestern states. Over half of all infections have been in Florida and Texas.

Symptoms of PAM are not specific to this disease. At first, PAM may seem like viral meningitis. Symptoms include:

There may also be hallucinations, drooping eyelid, blurred vision, and loss of the sense of taste.

It takes two to 15 days for symptoms to appear after N. fowleri amoebas enter the nose. Death usually occurs 3 to 7 days after symptoms appear. The average time to death is 5.3 days from symptom onset. Only a handful of patients worldwide have been reported to have survived an infection.

The term "brain-eating amoeba" makes the amoeba sound like a tiny zombie stalking your skull. But brains are accidental food for them.

According to the CDC, N. fowleri normally eats bacteria. But when the amoeba gets into humans, it uses the brain as a food source.

The nose is the pathway of the amoeba, so infection occurs most often from diving, water skiing, or performing water sports in which water is forced into the nose. But infections have occurred in people who dunked their heads in hot springs or who cleaned their nostrils with neti pots filled with untreated tap water.

A person infected with N. fowleri cannot spread the infection to another person.

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 (the nerve connected with sense of smell) into the frontal lobe of the brain.

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

It's considered a rare infection. But some cases may be unreported. A study in Virginia that looked at more than 16,000 autopsy records from patients who died of meningitis found five previously unreported cases of PAM.

Studies show that many people may have antibodies to N. fowleri. That suggests that they became infected with the amoeba but that their immune systems fought it off.

It's not at all clear whether N. fowleri is a rare infection that always causes PAM and is almost always fatal, or a more common infection that only sometimes causes PAM.

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

Brain-eating amoeba infections are difficult to diagnose. If your doctor suspects PAM, they will use specific lab tests that look for samples of the amoeba in your cerebrospinal fluid, biopsy, or tissue specimens. However, only few labs in the U.S. can run the specialized tests.

Because PAM is rare and difficult to diagnose and detect, 75% of the diagnoses are usually made after the disease causes death.

There is no rapid test for infection with brain-eating amoeba. But researchers are working to develop one. Until such tests come along, it can take weeks to identify the amoeba.

Yes. Experts can collect water samples from a lake or a pool, concentrate it and grow it in the lab to check for N. fowleri. The samples can then be tested for the amoeba using certain lab tests.

The right treatment isn't clear. A number of drugs kill N. fowleri amoebas in the test tube. But even when treated with these drugs, very few patients survive.

PAM caused by the brain-eating amoeba is considered a fatal infection. More than 97% of the people infected with N. fowleri die. In the U.S., between 1962 and 2019, only four out of 128 infected people survived.

One study suggests that N. fowleri amoebas produce two proteases -- enzymes that dissolve protein.


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.

To steer clear of the brain-eating amoeba, you should stay away from untreated splash pads at all times. You should also avoid certain water sports and activities in warm still waters during late summer, such as:

  • Swimming underwater
  • Diving and jumping in
  • Water skiing

It also makes sense to wear a nose clip when swimming, boating, or playing in or on warm waters. It's also a good idea to avoid stirring up mud while taking part in such activities.

And if you are cleansing your nostrils, be sure to fill your neti pot or squeeze bottle with distilled or sterile water -- not tap water. You can also use water that has been boiled for one minute (3 minutes at high elevations) and then cooled. And you can filter the water using filters with pores no larger than 1 micron (1 micrometer).