A federal study has detected Naegleria fowleri, often called the “brain-eating amoeba,” in warm recreational waters at Yellowstone National Park, Grand Teton National Park, and Lake Mead National Recreation Area.
The amoeba was found in hot springs, thermal runoff, and other thermally influenced waters, not ordinary campground beaches. The study included water samples from five western National Park Service sites, but positive detections were reported at Yellowstone, Grand Teton, and Lake Mead.
Researchers from the U.S. Geological Survey and Montana State University analyzed 185 water samples from 40 thermally influenced recreational water sites between 2016 and 2024. The amoeba was detected in 34% of samples, according to the study.
Amoeba Found in Warm, Thermally Influenced Waters
The study focused on recreational waters affected by geothermal heat, including hot springs and places where warm thermal water mixes with rivers or other surface waters.
According to the reporting reviewed, positive detections were found in several named areas, including Polecat Hot Springs, Huckleberry Hot Springs, and Granite Hot Springs in the Grand Teton area. In Yellowstone, detections were reported in the Boiling River area, Lewis Lake springs on the southern shore, and Firehole River areas including Goose Lake and the Firehole Canyon Swimming Area. At Lake Mead, positive samples were reported at Boy Scout, Nevada, Blue Point, and Roger’s Hot Springs.
The study also included Olympic National Park and Newberry National Volcanic Monument, but the available reporting says no positive detections were found at those two sites.
Risk Is Low, But Infection Is Usually Fatal
Naegleria fowleri lives in warm freshwater and soil. Infection can happen when contaminated water enters the nose and travels to the brain. The amoeba does not infect people from swallowing contaminated water, according to the CDC.
The infection it causes is called primary amebic meningoencephalitis, or PAM. It is extremely rare, but usually fatal. The CDC says the U.S. recorded 167 reported PAM cases between 1962 and 2024. Only four people survived. Fewer than 10 cases are typically reported in the U.S. each year.
No human infections were reported from the tested park locations as part of this study.
What Campers and RVers Should Know
The findings do not mean visitors need to avoid Yellowstone, Grand Teton, or Lake Mead. They do show that warm freshwater and thermal areas deserve caution, especially in places where people soak, swim, dive, or splash.
The main risk comes from water going up the nose. Campers should be careful around warm freshwater, hot springs, and thermal runoff areas. Avoid jumping or diving into warm stagnant water, and do not intentionally submerge your head in hot springs or warm freshwater where the amoeba could be present.
Visitors should also follow park rules and posted closures. Some thermal areas are unsafe for reasons beyond amoebas, including extreme heat, unstable ground, and chemical hazards.
Warming Waters May Expand the Amoeba’s Range
Researchers noted that warming temperatures may allow Naegleria fowleri to appear farther north than it has historically been found. The study authors called for more monitoring, public awareness, and risk management at thermally influenced recreational waters.
For most visitors, the risk remains very low. Still, the study is a reminder that not all scenic hot springs, warm rivers, or shallow thermal waters are safe places to soak or swim.
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