California fish carry human-infecting parasites that can cause stroke or heart attack, researchers find

More than 90 percent of popular freshwater game fish in Southern California contain a parasite that is capable of inducing strokes or heart attacks in the humans who eat them.

Researchers said the parasites were two species of flatworms known as trematodes. At no more than a few centimeters in length, the flukes – a class of flatworms – can result in weight loss or lethargy. Although rare, infection has also resulted in more severe consequences.

“Americans don’t usually think about parasites when they eat freshwater fish because it hasn’t historically been an issue here,” Ryan Hechinger, an ecologist and parasitologist at UC San Diego’s Scripps Institution of Oceanography, said in a statement. “But these trematodes have now been widely introduced in the U.S. and that means that doctors and the public should be aware.”

He is the senior author of the study which was published Tuesday in the Journal of Infectious Diseases. It was funded by the National Institutes of Health.

To identify this issue, Hechinger, the University of Nebraska’s Daniel Metz, and former Scripps graduate student and Smithsonian Environmental Research Center marine scientist Emma Palmer examined 84 fish from seven different species in 2023. Those fish included largemouth bass and bluegill, which were collected from five frequented fishing locations in San Diego County.

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Of those, 93 percent were infected with the Haplorchis pumilio parasite, and some were the host to thousands. Two of the five locations yielded positive results for the second parasite known as Centrocestus formosanus, which occurred in 91 percent of the fish.

Historically, the trematodes have infected people in Southeast Asia and likely arrived in the U.S. more than a decade ago inside an invasive aquatic snail known as the red-rimmed melania. That snail is found in 17 states and Puerto Rico.

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Previous work led by Hechinger had found that the red-rimmed melania and its parasites are widespread in the Golden State.

So, what is the risk to Californians and Americans at large? Hechinger says there haven’t been any reported cases of parasites infecting Americans yets, but “nobody is looking for cases and doctors aren’t required to report them.” Californians should make sure to monitor fish advisories for any changes.

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The risks posed by them are easy to avoid as, if you fully cook your fish or freeze any that’s intended to be eaten raw for at least a week, the parasites will die. Although, the authors found that many Americans are likely consuming freshwater fish without taking the proper precautions. The greatest health threats come from repeated infection over many months or years. Foodborne trematode infections cause two million life years lost to disability and death worldwide every year, according to the World Health Organization.

They recommend that fish-borne trematode infection be added to the list of diseases doctors are required to report to public health officials.

“These parasites are here in the U.S., and they’re infecting fish that people are eating,” said Hechinger. “We hope this study can help make public health officials, doctors and the public more aware.”