A brain-eating amoeba known as Naegleria fowleri has turned up in roughly one-third of water samples taken from popular recreational sites at five national parks and monuments across the western United States this year, according to the U.S. Geological Survey and its partner agencies.
Scientists collected 185 samples from Yellowstone National Park, Grand Teton National Park, Olympic National Park, Newberry National Volcanic Monument and Lake Mead National Recreation Area. The microscopic organism was detected in about a third of them, with Lake Mead, Grand Teton and Yellowstone recording the highest contamination levels. Millions of people visited these parks last year, raising the stakes for public awareness.
What the discovery means for visitors
The amoeba – formally Naegleria fowleri – lives in fresh water such as lakes and rivers, and can also survive in poorly maintained pools and water systems, explained Dr. Lyssette Cardona, an infectious disease specialist with Cleveland Clinic Florida, in a statement last week. Infections typically occur when infected water enters the nose and travels up to the brain, triggering a condition called primary amebic meningoencephalitis, or PAM.
Experts stress that simple precautions can drastically reduce risk. Dr. Cardona advised avoiding submerging your head in hot springs and holding your nose shut before jumping into fresh water. Even if you are unsure about the water’s safety, she said, it is best to take protective measures.
The extreme fatality rate of PAM
Although PAM remains rare in the United States, its consequences are devastating. According to the U.S. Centers for Disease Control and Prevention, 97 percent of people who develop PAM die from it. Between 1962 and 2024, only 167 cases were reported – most of them involving young boys – and just four people survived. Most victims die within one to 18 days after symptoms appear, but the infection can lead to coma and death in as few as five days.
Symptoms include fever, headache, nausea, vomiting and confusion. As the disease progresses, patients may experience hallucinations, seizures, stiff neck, lack of attention to people and surroundings, and loss of balance. Dr. Cardona warned that early treatment is critical because the signs can vary widely and escalate rapidly as brain swelling increases.
Fewer than ten people contract PAM each year in the U.S., and exposures outside bodies of fresh water are rare. Importantly, the CDC notes that people cannot be infected by swallowing contaminated water, through contact with an infected person, or by breathing water vapor or aerosol droplets. Properly cleaned, maintained and disinfected swimming pools pose no risk.
Climate change is expanding the amoeba’s reach
Human-caused climate change is making conditions more favourable for Naegleria fowleri, experts say. Infections typically occur during warm summer months when water temperatures have been high for an extended period and water levels are low. A 2021 CDC study showed that the amoeba is spreading northwards from the U.S. South into the Midwest and as far as Minnesota.
“As air temperatures rise, water temperatures in lakes, ponds and other fresh water also rise. These conditions provide a more favorable environment for Naegleria fowleri to grow,” the CDC states. The finding at western national parks adds a new dimension to this geographic shift, underscoring that even iconic, cool-mountain destinations are not immune as the climate warms.