Naegleria fowleri, also known as the “brain-eating amoeba,” was detected at Lake Mead National Recreation Area and Grand Teton and Yellowstone national parks, according to a 2026 study (attached). The free-living amoeba, which can cause amoebic meningitis, was found in hot springs and thermally impacted waters within the three parks.
Amoebic meningitis is a rare brain infection caused by N. fowleri when it enters the body via water flowing into the nasal cavity. While only a few people are diagnosed in the United States each year, the infection is fatal in 98 percent of cases, with death usually occurring in five to ten days.
The study notes that fatal N. fowleri cases have occurred in recreational waters with reported water temperatures starting at 71 degrees Fahrenheit and rising to temperatures above 86 degrees Fahrenheit. Cases have expanded northward since 1962, a trend that is predicted to continue as global temperatures increase, say the researchers.
The study looked at 40 thermally impacted recreational waters across five western United States national parks and recreation areas−Yellowstone, Grand Teton and Olympic national parks, Newberry National Volcanic Monument, and Lake Mead National Recreation Area−to check for the presence, concentration, and associated environmental conditions of N. fowleri. A total of 185 water samples were analyzed, and the amoeba was detected in 34 percent of the samples.
The highest concentration of N. fowleri was found at Polecat Hot Springs in Grand Teton National Park, a popular area with recreational hot spring soakers. Here, the researchers found concentrations of N. fowleri of 78, 16, and 115.7 cells/L. The study notes that the only published N. fowleri concentration limit for recreational water is from the Public Health Ministry of France, which has established a maximum of 100 N. fowleri cells/L.
The results “underscore the use of enhanced monitoring, public awareness, and risk management strategies in thermally influenced recreational waters,” states the study.
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