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By Daniel J. DeNoon

WebMD Health News

August 19, 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.

DANGER? Many organisms can thrive in warm fresh water, including the Naegleria fowleri.

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…




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