Image by D.T. John & T.B. Cole, Visuals Unlimited

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The feeding structures of the amoeba Naegleria fowleri have a face-like appearance.

Image by D.T. John & T.B. Cole, Visuals Unlimited

What We Do—and Don't—Know About Brain-Eating Amoebas

CDC scientist breaks down infection risk and story behind Arkansas girl's illness.

A 12-year-old Arkansas girl has been in a hospital for over a week after being infected with a typically fatal parasite that enters through the nose and consumes brain tissue.

A news release Friday from the Arkansas Department of Health says the source of the parasite is most likely a sandy-bottom lake at Willow Springs Water Park in Little Rock. A similar case was linked to the park in 2010.

This rare form of parasitic meningitis—primary amebic meningoencephalitis (PAM)—is caused by an amoeba called Naegleria fowleri. That microscopic amoeba—part of the class of life called protozoans—is a naturally occurring organism that normally feeds on bacteria and tends to live in the sedimentary layer of warm lakes and ponds.

To find out more about Naegleria fowleri, National Geographic got in touch with Jonathan Yoder, an epidemiologist at the Centers for Disease Control and Prevention who collects and analyzes data on the microscopic amoeba.

How does this amoeba called Naegleria fowleri infect a human?

Under certain conditions, Naegleria fowleri can develop flagella—threadlike structures that enable it to rapidly move around and look for more favorable conditions. When people swim in warm freshwater during the summer, water contaminated with the moving amoeba can be forced up the nose and into the brain.

This causes headache, stiff neck, and vomiting, which progresses to more serious symptoms. Between exposure and onset, infection generally results in a coma and death after around five days.

Where is it found?

We see it in warm freshwater or in places with minimal chlorination. It is not uncommon to detect the amoeba if you sample freshwater in warm weather states.

Can it live in swimming pools?

There have been no evident cases of contamination in the United States in well-maintained, properly treated swimming pools. Filtration and chlorination or other types of disinfectant should reduce or eliminate the risk.

But it does get a bit trickier—there was a case in Arizona about ten years ago where a kid swam in a pool filled with water from a geothermal hot water source before it was treated. Unfortunately, the kid became ill and died.

Are cases of infection becoming more common?

We don't have data that says infection from Naegleria fowleri is becoming more common. In the last few years there have been four to five cases per year.

What has changed recently is that cases have appeared in places we had never seen before—like Minnesota, Indiana, and Kansas. This is evidence that the amoeba is moving farther north. In the past it was always found in warmer weather states.

Why does the amoeba enter the nose of some people but not others?

That is a very good question we don't know the answer to. Millions of people swim in these bodies of water every year and don't become ill. So it is difficult for us to say why one person would become ill and other people who swam in the same place and did the same activities did not. It certainly can affect anyone.

What is the chance of survival?

Since 1962, there have been 128 cases of Naegleria fowleri [infection] and only one survivor, not including the current case. Back in 1978, a patient survived after being treated with antibiotics. The same regimen has been tried unsuccessfully on other patients.

How can people stay safe?

If people want to reduce their risk of becoming infected—even though this is a rare event— the thing to think about is holding their nose shut or wearing nose clips when swimming in warm, untreated freshwater. Keep your head above water in hot springs or other thermally heated bodies of water, and during activities where water is forced up the nose, like water sports and diving.

Another way to reduce the risk of infection is to avoid stirring up the sediment in lakes and ponds, where the amoeba may live.

This is a tragic event for someone who becomes infected, as well as their family. We feel it is important for us to be involved even though it does not affect lots of people each year.

This interview has been edited and condensed.

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