When it comes to potential dangers in the outdoors, most people think of getting lost, slipping and falling, or encountering wildlife such as bears and mountain lions. A new study, however, shows that there’s a significant danger we can’t see with the naked eye.
Researchers recently found high concentrations of a dangerous, brain-eating amoeba in several National Park Service (NPS) managed sites in the Western U.S. The findings indicate that this microscopic parasite could be moving farther north as temperatures warm with climate change.
Brain-Eating Amoeba in National Parks: The Study
The study was published in ACS ES&T Water, a scientific journal devoted to research on water quality, treatment, and protection. It focused on analyzing 40 bodies of water used for outdoor recreation across five NPS-managed sites in the West: Yellowstone National Park, Grand Teton National Park, Newberry National Volcanic Monument, Olympic National Park, and Lake Mead National Recreation Area. The study analyzed 185 water samples collected from 2016 to 2024.
Scientists were looking for the presence of Naegleria fowleri, a type of amoeba that is the main cause of primary amoebic meningoencephalitis (PAM). The condition has a fatality rate of 98%. Essentially, when a person goes swimming, the amoeba can enter the brain through the nose. It then causes brain swelling and the destruction of brain tissue. Death normally occurs within 1 to 7 days of infection.
Typically, this amoeba lives in soil and warm water in southern states. As the climate warms, however, signs show that it is spreading northward. It thrives in hot springs and water with temperatures ranging from 60 to 115 degrees Fahrenheit.
The Results
According to the study, N. fowleri was found in 63 of the 185 samples (or 34%).
In Yellowstone, it was present at Firehole River, Boiling River, and Lewis Lake hot springs. Several popular hot springs in Grand Teton National Park also showed elevated concentrations, including Polecat, Huckleberry, and Granite Hot Springs. Additionally, Lake Mead National Recreation Area tested positive in multiple locations, including Blue Point, Boy Scout, Nevada, and Rogers Hot Springs.
It is worth noting that in 2025, these three NPS sites together attracted 14.7 million visitors.
Researchers found no signs of N. fowleri in the samples from Olympic National Park or Newberry National Volcanic Monument.
There is no information available from any U.S. government agency about what concentrations of N. fowleri are safe or unsafe. However, the Public Health Ministry of France set the upper limit for water recreation areas at 100 cells/liter. Australia, meanwhile, set the standard for its drinking water at 2 cells/liter, which means that all samples that tested positive were above. A 2023 sample from Polecat Springs in Grand Teton tested at 115.7 cells/liter.
The Significance
From 1962 to 2024, there were 167 reported cases of PAM in the U.S. All but four of the infected individuals died, according to the Centers for Disease Control data. While it has a very high fatality rate, it is not widespread. Less than 10 people in the U.S. get infected each year.
The odds that visitors to these three sites get infected are extremely low. A fatal case of PAM has never been reported at any of these locations. The study’s authors explained that this data calls for more caution and public messaging.
“The concentrations of N. fowleri observed here are within the limits of known PAM infections, which highlights potential benefits of informational signs, as well as public warnings, at sites to help prevent a future fatality,” they wrote.
The study was the first to report concentrations at several locations, including Lewis Lake Hot Springs, Upper Polecat Hot Spring, Nevada Hot Spring, Boy Scout Hot Spring, Blue Point Hot Spring, and Rogers Hot Spring. Essentially, researchers have found that N. fowleri is more widespread than previously thought in these NPS sites.
Climate Change Connections
The research also confirms the well-researched theory that as the climate warms, N. fowleri will spread north. Its presence in Lake Mead confirms recent findings that the amoeba has become more widespread in Nevada.
The authors argue that their work demonstrates the importance of the continued observation and study of this issue. “These findings … underscore the use of enhanced monitoring, public awareness, and risk management strategies in thermally influenced recreational waters,” they stated.
“The high fatality rate and expanding geographic range of N. fowleri infections indicate that improved pathogen surveillance can progress research in the field. In Australia, regular monitoring of N. fowleri, specific protocols in response to detections, and increased public awareness programs have helped limit the number of fatalities to 19 since 1965.”
Expanding surveillance for N. fowleri across U.S. could help inform the public about the pathogen’s continually expanding geographic range, the study concludes.
Read the full article here