Freshwater U.S. Fish Carry Parasitic Flatworm That Can Infect Humans

Key TakeawaysFreshwater game fish in the U.S. now carry an exotic flatworm that can infect humansMore than 90% of game fish like bluegill and largemouth bass tested carried the flatworms, called trematodesPeople can protect themselves by fully cooking fish or freezing it for a week.FRIDAY, June 6, 2025 (HealthDay News) — Freshwater game fish like bluegill, largemouth bass and black crappie in the U.S. commonly carry parasitic flatworms that can infect humans, a new study warns.More than 90% of popular game fish tested in Southern California contained illness-causing flatworms called trematodes, researchers reported recently in The Journal of Infectious Diseases.“Americans don’t usually think about parasites when they eat freshwater fish because it hasn’t historically been an issue here,” said senior researcher Ryan Hechinger, an ecologist and parasitologist at the University of California-San Diego Scripps Institution of Oceanography.“But these trematodes have now been widely introduced in the U.S. and that means that doctors and the public should be aware,” he added in a news release.There’s no need to panic, Hechinger said, as fully cooking fish will kill off the flatworms. Sushi lovers also can avoid infestation by freezing for at least one week any fish they plan to eat raw.Unfortunately, the research team found dozens of YouTube videos in which people were eating raw and unfrozen fish, which could lead to infection.“Nearly 5 million views shows there is widespread interest and possibly a widespread practice of folks eating freshwater fish raw,” lead researcher Emma Palmer said in a news release. She's a marine scientist at the Smithsonian Environmental Research Center in Edgewater, Md.Typically, these infectious worms cause gastrointestinal (GI) problems, weight loss or lethargy in humans, researchers said in background notes.These flatworms tend to lodge in the lungs or the bile ducts of the liver and gallbladder, according to the World Health Organization (WHO). Infection can result in severe liver and lung damage if people aren’t treated.For the study, researchers examined 84 fish from seven different species, mostly largemouth bass and bluegill, that were caught in five popular fishing spots in San Diego County.Results showed that 93% of the fish were infected with the Haplorchis pumilio parasite. Some individual fish harbored thousands of the flatworms.A second parasite, Centrocestus formosanus, was found at two of the five locations, where it was present in 91% of the fish there.These parasites have historically infected people in Southeast Asia. Researchers said they likely arrived in the U.S. more than a decade ago through an invasive snail species that has spread to 17 states and Puerto Rico.“These parasites are here in the U.S., and they’re infecting fish that people are eating,” Hechinger said. Researchers hope their findings will prompt health officials to remind people about the hazards of eating raw or undercooked fish.Doctors might also look to these parasites as a potential explanation for mysterious cases of GI illness, Hechinger said.“There haven’t been any reported cases of these parasites infecting Americans,” said Hechinger, “but nobody is looking for cases and doctors aren’t required to report them.”Hechinger gave a hat tip to the National Institutes of Health, which funded the study.“This is research a private company would never fund because it won’t make anyone rich, but might make the general public a little healthier,” he said. “If the federal government doesn’t fund this sort of study, who will?”More informationThe World Health Organization has more on foodborne trematode infections.SOURCE: University of California-San Diego, news release, May 30, 2025 .What This Means For YouPeople should make sure they fully cook any freshwater fish they catch, or freeze it for at least a week if they plan to consume it raw..Sign up for our weekly HealthDay newsletter

Freshwater U.S. Fish Carry Parasitic Flatworm That Can Infect Humans

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