A central North Carolina teenager is critically ill with a disease that has long been associated with the hottest days of summer in the South: a brain infection caused by the amoeba Naegleria fowleri.
State health officials have not said where the teenager encountered the organism. But the case comes as scientists are documenting a broader change in the geography of this rare infection. The number of U.S. cases has not significantly increased. The places where people have been exposed, however, have moved north.
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That distinction is central to understanding the climate risk. Scientists cannot attribute an individual infection to climate change, and the odds of becoming infected remain exceedingly low. But Naegleria fowleri thrives in heat. As rising air temperatures warm lakes, rivers and ponds, they can create favorable conditions across a wider area and potentially for a longer portion of the year.
“It’s heat-loving. It’s a single-celled organism,” said Dr. Tyler Evans, an infectious disease physician and public health expert. “It’s found in the soil or warm freshwater.”
The clearest evidence of a geographic shift comes from a 2021 study by scientists at the Centers for Disease Control and Prevention. The researchers examined 85 U.S. cases tied to recreational water exposure between 1978 and 2018.
Seventy-four exposures occurred in the South. Six occurred in the Midwest, five of them after 2010. The northernmost exposure location moved about 13.3 kilometers, or 8.3 miles, north each year over the four decades studied.
The change did not amount to an increase in the national infection rate. The number of recreational water-associated cases ranged from zero to six each year, and researchers found no statistically significant upward trend.
They did find a temperature signal. During the two weeks before exposure, daily high and low temperatures near the sites averaged about 0.8 degrees Celsius, or 1.4 degrees Fahrenheit, above the 20-year average for the same place and time of year.
The study used air temperature because water-temperature records were unavailable for most locations. Its authors noted that air temperature is a major driver of lake-surface temperature but cautioned that the analysis could not prove higher temperatures caused the exposures. The disease may also be underdiagnosed because it is so rare and progresses so quickly.
Still, the pattern is consistent with the organism’s biology. Naegleria fowleri prefers temperatures up to 45 degrees Celsius, or 113 degrees Fahrenheit, according to the CDC study. When conditions turn cold, it can survive in a dormant cyst form and become active again when the water warms.
Most infections have historically occurred after swimming in warm, untreated freshwater in Southern states. Since 2010, infections have also been confirmed in Nebraska, Iowa, Minnesota, Indiana, Maryland and northern California, according to the CDC.
The changing climate may affect not only where exposure occurs but when. Most of the 157 U.S. cases with a known month of illness onset from 1937 through 2025 occurred in July and August. If waterways warm earlier in the spring or remain warm later in the fall, scientists say the period of favorable conditions could expand.
“We might see not necessarily just an increased risk of cases per se, but the times in which the cases are presenting,” Evans said.
Hotter weather may also change human behavior. Longer stretches of heat can send more people into lakes and rivers, creating additional opportunities for exposure even if the organism does not become more abundant. The CDC researchers identified increased swimming and water sports as one possible contributor to the changing geography of cases.
The infection, called primary amebic meningoencephalitis, or PAM, occurs when contaminated water enters the nose and the amoeba travels to the brain. It cannot be contracted by swallowing the water or from another person.
PAM is devastating but exceptionally rare. The CDC recorded 180 diagnosed U.S. cases from 1937 through 2025, with between zero and eight cases in any given year. The disease is nearly always fatal.
Because the concentration of amoebas can vary within the same body of water and there is no standardized rapid test to determine whether a lake is safe, the CDC advises swimmers to assume the organism could be present in warm freshwater. Nose clips, keeping the head above water and avoiding disturbed sediment in shallow areas can reduce exposure.
For public health officials, the emerging concern is not evidence of a sudden surge. It is that an organism adapted to heat may encounter suitable conditions in places and seasons where it was once less likely to pose a threat.
Evans said that changing patterns among infectious diseases will require sustained surveillance and a public health system prepared to respond to threats as they move.
“We will be seeing more of this,” he said.
