Brain-eating amoebas are rare. But hot weather increases the risk.
High temperatures send many of us straight to the nearest source of water to cool off, whether that’s a pool, beach, lake or backyard splash pad. But if you’ve heard reports about a “brain-eating” amoeba that lives in fresh water, you might be questioning how safe it really is to go for that dip.
The amoeba, Naegleria fowleri, is found around the world in soil and warm fresh water. It can also appear in tap water and less often in swimming pools that haven’t been properly chlorinated. The amoeba hasn’t been detected in salt water.
Naegleria fowleri is a free-living amoeba, which means it doesn’t need a host to survive. “There are lots of free-living amoeba in the environment,” said Dennis Kyle, director of the Center for Tropical & Emerging Global Diseases at the University of Georgia, “but Naegleria fowleri is one we have to worry about for causing disease in humans.”
Most of the time, humans coexist with Naegleria fowleri without issue - the amoeba is harmless if swallowed, Kyle said. But if it gets into a person’s nasal passages, the amoeba can become dangerous. From there, Naegleria fowleri can make its way to the frontal part of the brain, where a fast-acting infection called primary amebic meningoencephalitis, or PAM, develops.
Of the 164 reported cases in the United States over the past 60 years or so, only four people who developed PAM have made a full recovery. Once it enters the brain, Naegleria fowleri works quickly, consuming brain tissue before many doctors even think to test for the amoeba. “It’s a rapid disease that is often missed in the diagnosis,” Kyle said. A person might experience vague symptoms such as headache, fever and nausea, which escalate to brain swelling and coma - and death within a few days.
As scary as this is, it’s important to remember that Naegleria fowleri infections are very unusual. Fewer than 10 cases are reported in the U.S. each year, according to the Centers for Disease Control and Prevention. (As a comparison, accidental drownings are responsible for 400 times as many deaths annually, making them a far greater danger when it comes to water safety.)
But even though PAM is exceedingly rare, it’s still important to know how to lower your risk, especially if you swim in freshwater lakes or rivers. Here are strategies to help you stay safe.
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Try not to get fresh water in your nose.
Naegleria fowleri flourishes in warm freshwater lakes, rivers and hot springs. But the amoeba becomes harmful only if it manages to travel up the nasal passages. “The best protection is to avoid getting water up your nose,” said Eric Altneu, an infectious-diseases doctor at the Temple University Lewis Katz School of Medicine.
This means the safest strategy - aside from not swimming at all or sticking to salt water - is to “keep your head above water” while you swim, said James Morris, a professor in the Department of Genetics and Biochemistry and a founding member of the Eukaryotic Pathogens Innovation Center at Clemson University. Submerging your head while swimming or participating in aquatic sports that cause water to splash on your face (think wakeboarding, water skiing and whitewater rafting) increase your risk.
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Don’t jump in.
Not getting your face wet is best, though not always realistic. If you do go underwater during a freshwater swim, it’s safer not to jump or dive in, experts said. “That is going to force water up your nose, and it’s that route of entry that we should avoid,” said David Siderovski, a professor of pharmacology and neuroscience at the University of North Texas Health Science Center at Fort Worth. To be safe, the CDC recommends holding your nose or using a nose clip when jumping or diving into water that might be harboring Naegleria fowleri.
Jumping could also help explain why so many PAM infections have occurred in young boys: Some experts, including the CDC, have theorized that this demographic may be more likely to engage in higher-risk activities like jumping and diving when playing in fresh water.
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Avoid disturbing the soil.
We know Naegleria fowleri likes to live in freshwater silt and dirt. Disrupting this bottom sediment layer, such as by kicking or digging it up while you swim in shallow water, could allow the amoeba to circulate more freely. “You’re disturbing its resting place,” Siderovski explained. By leaving the sediment alone, “you can cut down on the amount literally in the water.”
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Keep in mind that this amoeba can live in other water sources, too.
Swimmers are most likely to be exposed to Naegleria fowleri in warm freshwater lakes, rivers and ponds. But the amoeba has also been found in swimming pools, canals, water parks, household tap water, sprinklers on a front lawn, even puddles.
Swimming pools that weren’t chlorinated properly once caused a greater number of Naegleria fowleri infections, but this is less of a risk today. A 2022 global review identified only one infection from a pool since 1987. Still, “really knowing where you’re swimming and making sure somebody is taking care of the chlorination or the disinfection of the water” is a good idea, Altneu noted.
Because Naegleria fowleri has also been found in tap water, kiddie pools, splash pads and Slip ’n Slides could accommodate the amoeba. Kyle said he’s been mindful of this when caring for his grandchildren: “We’d fill up their splash pad with fresh water, they would play in it, and I’d dump it out and not let it sit for three or four days,” he said, “because then the soil gets into it, and it has more chance for amoeba to grow.”
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Be extra aware when your risk is highest.
There are times when the risk is probably heightened. The majority of infections occur during summer or in places with a year-round tropical climate, the 2022 global review found.
In the U.S., most PAM cases have been reported in southern states. Climate change is likely to change this, said Siderovski. “We can no longer assume that Texas, California, Florida and Louisiana are the only four states we need to worry about.”
Unfortunately, there isn’t a specific formula experts can offer in terms of temperature or number of too-warm days before fresh water becomes less safe. This amoeba has been found on every continent except Antarctica, and research has suggested warming waters as a result of climate change means more humans will encounter Naegleria fowleri.
But in general, “the warmer the water is for a long period of time, the higher chance the amoeba are proliferating more rapidly,” Kyle said. Shallow water can also be risky, since it tends to be warmer and closer to that sediment layer where amoeba thrive.
“If I were going to a lake, I would always assume that the water has Naegleria in it,” Altneu said.
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Use neti pots safely.
Swimming in fresh water poses the highest risk for Naegleria fowleri. But a small number of cases have been reported after people used nasal irrigation devices such as neti pots or bulb syringes to rinse out their sinuses.
Nasal irrigation can be an effective way to get relief from symptoms such as sinus pain and congestion. But the water you use always needs to be distilled, sterile or boiled and cooled, since it’s possible to be exposed to Naegleria fowleri and other germs through unsterile water. Recently, a 71-year-old Texas woman died of a suspected PAM infection after using a nasal-irrigation device she’d filled with tap water from her RV.
Safe options include store-bought water that says “distilled” or “sterile” on the label, as well as water from your faucet that you’ve boiled for one minute (three minutes if you live in a high-elevation location, above 6,500 feet) and let cool. “You absolutely should not use tap water” that hasn’t been sterilized to rinse your nose, Morris said.
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The bottom line: Naegleria fowleri infections are thankfully rare. But they’re serious when they do occur, so taking simple precautions can help you and loved ones stay safe. And if you or your child develops a headache, fever, nausea or vomiting after swimming in warm fresh water or using a neti pot, seek medical care and tell your doctor about the potential exposure.
“It’s something that gets a lot of headlines because of the name, and I think awareness is a very important thing,” said Kyle, “but if, say, you get one drop of water up your nose from your shower, you’re very unlikely to be infected.”