Hot spring lovers beware: Recent research shows these popular vacation spots can harbor a deadly species of amoeba that feasts on brains.
Scientists at the U.S. Geological Survey and Montana State University studied recreational water samples taken from major national parks in the western United States. They identified widespread levels of Naegleria fowleri in three of these parks, including in areas where it had not previously been detected. Though N. fowleri only rarely causes human illness, cases could become more common as the climate continues to warm, the researchers warn.
“These findings indicate that N. fowleri is present in thermally impacted areas across the western United States and underscore the use of enhanced monitoring, public awareness, and risk management strategies in thermally influenced recreational waters,” the researchers wrote in their paper, published this March in the journal ACS ES&T Water.
The ubiquitous brain-eating amoeba
N. fowleri is a shapeshifting amoeba that lives in soil and warm freshwater. It normally feeds on bacteria and isn’t dangerous to humans when it’s simply ingested. When the amoeba enters our body through the nose, however, it can migrate to the brain. Once inside, it can literally eat brain cells and spark massive inflammation, leading to a severe infection called primary amebic meningoencephalitis, or PAM.
The amoeba is widely distributed throughout the world. But the researchers wanted to extensively track its presence across warm recreational water sites at national parks in the Western U.S. Over the span of eight years (2016-2024), they collected and analyzed 185 samples from 40 sites in five national parks: Yellowstone National Park, Grand Teton National Park, Olympic National Park, Newberry National Volcanic Monument, and Lake Mead National Recreation Area.
All told, 34% of samples tested positive for N. fowleri. All the positive samples came from three parks in particular: Lake Mead, Yellowstone, and Grand Teton. And though the amoeba was found in places where it’s previously been identified, they also found it at some sites for the first time ever. These included Lewis Lake Hot Springs, upper Polecat Hot Spring, Nevada Hot Spring, Boy Scout Hot Spring, Blue Point Hot Spring, and Rogers Hot Spring.
What to do about N. fowleri
PAM is almost universally fatal, but it’s thankfully very rare. Since its discovery, there have only been around 150 cases reported in the U.S. That said, global cases have been increasing, while the distribution of cases in the U.S. has gradually moved north, according to the researchers. And since N. fowleri loves warm environments, it’s likely that climate change will allow it to continue expanding its territory, leading to more cases of PAM.
On the positive side, the researchers say their work highlights the value of improved surveillance for N. fowleri, and it might even teach us new things about the amoeba itself. In some sites, for instance, both N. fowleri and other related but non-infectious species of Naegleria were found. That suggests, the researchers say, that N. fowleri might occupy the same niches as these species in natural water systems.
“Broadening surveillance for N. fowleri in the U.S. could help inform the public of the expansive geographic range of this pathogen,” they wrote.
Wherever this amoeba is found, there are simple steps you can take to avoid infection. You should try to limit any water that goes up the nose while visiting warm freshwater sources, for example, such as by pinching your nose shut when jumping in. And you should always use distilled, sterilized, or boiled and cooled tap water for nasal irrigation.
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