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5 things to know about the brain-eating amoeba that infected a swimmer in Iowa

Iowa officials closed the beach at Lake of Three Fires State Park in Taylor County last week, after an infection by the Naegleria fowleri amoeba was confirmed in a swimmer at the park.
Iowa Department of Natural Resources
Iowa officials closed the beach at Lake of Three Fires State Park in Taylor County last week, after an infection by the Naegleria fowleri amoeba was confirmed in a swimmer at the park.

Its name alone is terrifying. Add the fact that it kills most people it infects — and that while infections are rare, the parasite is fairly common — it's not surprising that a confirmed case of Naegleria fowleri infection in a swimmer in Iowa is drawing attention.

Iowa officials closed the beach at Lake of Three Fires State Park on Thursday after confirming that a person who swam there was infected with Naegleria fowleri, an amoeba that causes a disease called primary amebic meningoencephalitis (PAM). It's both extremely rare — and extremely deadly.

"The fatality rate is over 97%," the Centers for Disease Control and Prevention says of PAM infections. "Only four people out of 154 known infected individuals in the United States from 1962 to 2021 have survived."

Details about the Iowa case have not yet been released. The person was visiting from Missouri, which is just over the border from the park in Iowa's southwest.

Iowa's Department of Health and Human Services says it's working with the CDC to confirm whether Naegleria fowleri is present in the lake — a process that takes several days. The state agency is also in contact with the Missouri Department of Health, an Iowa representative told NPR.

"It's strongly believed by public health experts that the lake is a likely source," Missouri's health department said on Friday. But it added, "Additional public water sources in Missouri are being tested."

NPR contacted the Missouri Department of Health on Monday for an update on the patient's condition and other details, but had not received a response at the time of publication.

Here are five things to know about the microscopic brain-eating amoeba:

The amoeba doesn't mainly eat brains

Naegleria fowleri is commonly known as the "brain-eating amoeba" — and it does indeed destroy brain tissue. But the amoeba mainly eats bacteria, not brains, and those organisms are plentiful in the sediment of lakes and rivers. Infections in humans are devastating but rare.

"To get infected, the amoeba has to get to the ceiling of your nose – way, way up there," the late epidemiologist Raoult Ratard told NPR in 2013, when the amoeba was found for the first time in a city water supply in the U.S.

"At the top of the nose you have a little paper-thin plate made of bone with a bunch of holes, a little bit like a mosquito net," Ratard said. "The holes are for the olfactory nerve. So the amoeba is crawling up the nerve and gets into the brain."

Symptoms can start with a sharp headache

The first stage of symptoms can include a severe frontal headache, fever, nausea and vomiting, the CDC says. The disease can quickly progress to the second stage, which ranges from a stiff neck, seizures and mental issues to a coma.

The mechanism that causes people to die is actually swelling of the brain, as a result both of the amoeba's release of toxic molecules and the immune system's attempts to fight off the invader.

The disease moves quickly: the median time for death is just five days, the CDC says. And scientists are still trying to develop a rapid test that can detect Naegleria fowleri in water.

"PAM is difficult to detect because the disease progresses rapidly so that diagnosis is usually made after death," according to the agency.

Infection risk is 'very low,' the CDC says

"There have been 31 reported infections in the U.S. in the 10 years from 2012 to 2021, despite millions of recreational water exposures each year," the CDC said.

It compares that figure to the nearly 4,000 people who died from unintentional drowning deaths in each year of the same period, according to estimates.

You can't get infected with PAM from drinking contaminated water. And an infected person can't spread the disease to another person. But there have been unusual cases, including people who were infected after using contaminated tap water to rinse their nasal passages with a neti pot. And a case in Louisiana was traced to a Slip'n'Slide connected to a household faucet.

The brain-eating amoeba loves the heat

Naegleria fowleri becomes more prevalent as temperatures rise in freshwater lakes and rivers. Most U.S. cases have been found in Southern states, and in the height of summer.

"Infections usually occur when it is hot for prolonged periods of time, which results in higher water temperatures and lower water levels," according to the CDC.

In the area near Lake of Three Fires State Park, a weather station recorded high temperatures of around 95 degrees on two consecutive days over the July 4 holiday.

New treatments have saved lives in recent years

Most of the known cases in which people survived PAM in the U.S. have come in the past 10 years. They include a 12-year-old girl and a 16-year-old boy who, in separate cases, made full recoveries and were able to return to school.

Those success stories relied on fast diagnosis and aggressive treatment, including the use of therapeutic hypothermia — cooling the body below its normal temperature to ease swelling in the brain.

A range of antifungal and other drugs are used to fight the infection, including amphotericin B, miltefosine azithromycin, fluconazole, rifampin, and dexamethasone, according to the CDC.

Copyright 2022 NPR. To see more, visit https://www.npr.org.

Bill Chappell
Bill Chappell is a writer and editor on the News Desk in the heart of NPR's newsroom in Washington, D.C.