Brain-eating amoeba is spreading across US national parks — here’s what you need to know

A “brain-eating amoeba” called Naegleria fowleri has been found in recreational waters at several national parks across the U.S. West this year.

Scientists took 185 samples from Yellowstone National Park, Grand Teton National Park, Olympic National Park, Newberry National Volcanic Monument and Lake Mead National Recreation Area, finding the rare organism in about a one-third of them, according to the U.S. Geological Survey and its partner agencies.

Lake Mead, Grand Teton and Yellowstone had the most contaminated samples, the study says. Millions of people visited the parks last year.

But experts say people can keep their families safe from infections this summer by swimming with caution. The first step is knowing potential sources of exposure.

“Naegleria fowleri is a microscopic amoeba. It’s a free-living organism that lives in fresh water such as lakes and rivers. It can also live in poorly kept pools and water systems,” Dr. Lyssette Cardona, an infectious disease specialist with Cleveland Clinic Florida, explained in a statement Thursday.

Even if you’re unsure about the water’s safety, it’s best to take precautionary measures, she said. That includes avoiding dunking your head underwater in hot springs and holding your nose before jumping into fresh water.

Infections often occur when amoeba-infected fresh water enters the nose, traveling up to the brain.

This can lead to an infection known as “primary amebic meningoencephalitis,” or PAM.

Signs of the condition are varied and can be extremely serious, which is why early treatment is critical, according to Cardona.

“The symptoms of meningoencephalitis are usually fever, headaches, nausea, vomiting and confusion. If it progresses without awareness, you can have hallucinations and seizures with the increased brain swelling,” she said.

Other symptoms may include stiff neck, a lack of attention to people and surroundings and loss of balance.

While there are few cases of PAM each year in the U.S., nearly everyone – 97 percent – who develops PAM dies from it, according to the U.S. Centers for Disease Control and Prevention.

Most people die within 1-18 days after symptoms start, but PAM can lead to coma and death in just five days.

There were 167 reported cases – most who are young boys – between 1962 and 2024, but only four survived.

Less than 10 people get PAM in the U.S. each year and there are few exposures from sources other than bodies of water.

Still, human-caused climate change is making ripe conditions for infections, according to experts.

Infections typically occur in the warm summer months when water temperatures have been hot for a while and there are low water levels.

Warmer temperatures fueled by climate change have spread infections northward, with a 2021 CDC study showing that the amoeba is moving from the U.S. South and 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 says.

Fortunately, people cannot be infected after swallowing contaminated water or by exposure to others who are infected.

There’s also no evidence that Naegleria fowleri can spread through water vapor or aerosol droplets and you cannot get PAM from swimming or playing in a properly cleaned, maintained and disinfected pool.