“Brain-Eating Amoeba” Found in Yellowstone, Grand Teton, and Lake Mead

If you're planning a summer trip to some of America's most iconic national parks, there's something worth knowing before you jump in any hot springs.

A multi-year study conducted between 2016 and 2024 found Naegleria fowleri — the organism known as the brain-eating amoeba — in water samples from three of five popular western US recreation areas. The findings were published in the American Chemical Society's journal ES&T Water and involved researchers from the US Geological Survey, Montana State University, and the Commonwealth Scientific and Industrial Research Organization.

Out of 185 samples taken from 40 thermally impacted areas across five sites, 63 of them — about 34% — came back positive. No infections or deaths have been tied to any of these sites. But the presence is documented, and the locations are ones millions of people visit every year.

Where It Was Found

Yellowstone National Park, the country's first national park and third most visited as of 2025, had multiple positive sites. Naegleria fowleri was detected at Boiling River across multiple years between 2018 and 2023, at the Firehole Canyon Swimming Area in 2018 and 2019, along the Firehole River near Goose Lake in 2023, and at multiple hot springs at Lewis Lake in 2023.

Grand Teton National Park, the eighth most visited park in the country, had three hot spring areas test positive. Granite Hot Spring tested positive in 2019. Polecat Hot Springs and Huckleberry Hot Springs both tested positive consistently from 2019 through 2024.

Lake Mead National Recreation Area just outside Las Vegas was also a hotbed for activity. Between 2018 and 2019 four of its five sample sites tested positive — Blue Point, Boy Scout, Nevada, and Roger's springs.

Olympic National Park in Washington and Newberry National Volcanic Monument in Oregon both came back clean.

What This Thing Actually Does

Naegleria fowleri lives in warm freshwater — lakes, rivers, ponds, hot springs, and soil. It thrives when water temperatures are high, which is why geothermal areas like Yellowstone are particularly hospitable.

The amoeba causes a condition called primary amebic meningoencephalitis, or PAM. It gets into the body through the nose — not by drinking contaminated water, but by water entering the nasal passages while swimming, diving, or soaking. Once inside it travels up to the brain and destroys brain tissue.

The fatality rate is 98%. Of the 167 reported US cases between 1962 and 2024, only four people survived. Early symptoms include headache, fever, and nausea before progressing to confusion, seizures, and coma. Death typically occurs within seven to ten days of infection.

But Here's the Thing About Actual Risk

Before you cancel your Yellowstone trip, the actual risk of getting infected is extremely low — even in water where the amoeba is present.

The CDC reports fewer than 10 Americans die from PAM every year despite millions of people swimming and playing in warm freshwater lakes and hot springs across the country annually. Exposure to the amoeba doesn't automatically mean infection. The organism has to enter specifically through the nose and make it to the brain — casual water contact, accidental splashing, or even brief submersion doesn't guarantee that happens.

Drinking water containing Naegleria fowleri is not dangerous — stomach acid destroys it. The risk is specifically nasal water entry. People swim in warm lakes and hot springs containing this organism their entire lives without ever getting infected. The cases that do occur tend to involve prolonged submersion, diving, or activities that forcefully push water up into the nasal cavity.

The researchers and CDC aren't saying stay out of all warm water. They're saying be aware and take some simple precautions — especially in hot springs and geothermal areas where the amoeba is most likely to be present and water temperatures are highest.

What You Can Do

The simplest protection is keeping water out of your nose. That sounds obvious but it makes a real difference.

Hold your nose shut or use nose clips when submerging in warm freshwater — especially hot springs. Avoid putting your head underwater in warm shallow areas. Try not to stir up sediment at the bottom of lakes or rivers since the amoeba can live in the soil. Avoid warm freshwater activities when water temperatures are particularly high, since Naegleria fowleri thrives above 80 degrees Fahrenheit and grows more rapidly as temperatures climb.

Cold water is generally considered safe — the amoeba doesn't survive in it. Properly chlorinated pools are also safe since chlorine kills it.

The Bigger Picture

The reason researchers are paying closer attention now is that Naegleria fowleri has been steadily migrating northward since 1962 — a trend directly tied to warming water temperatures. It used to be primarily a concern in the southern US. Now it's showing up in Yellowstone, the Tetons, and the Pacific Northwest.

The researchers are pushing for better monitoring systems, clearer public awareness campaigns, and stronger risk management strategies at recreational water sites before the range continues to expand.

For now — enjoy the parks, be smart about hot springs, and keep water out of your nose.

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