Most Freshwater Fish in California Are Carrying Human-Infecting Parasites

More than 90% of popular freshwater game fish in Southern California contained an introduced parasite capable of infecting humans, according to a new study from researchers at UC San Diego’s Scripps Institution of Oceanography.

The parasites found in the study – two species of flatworms called trematodes – typically cause gastrointestinal problems, weight loss or lethargy when they infect humans. In some rare and severe cases, the parasites have caused strokes or heart attacks. The findings, published June 3 in the Journal of Infectious Diseases and funded by the National Institutes of Health (NIH), suggest that these parasites pose a previously unrecognized public health risk in the United States.

“Americans don’t usually think about parasites when they eat freshwater fish because it hasn’t historically been an issue here,” said Ryan Hechinger, an ecologist and parasitologist at Scripps and the study’s senior author. “But these trematodes have now been widely introduced in the US and that means that doctors and the public should be aware.”

Hechinger emphasized that there is “no need to panic” as the risks posed by these parasites are easy to mitigate: Fully cooking fish or freezing any intended to be eaten raw for at least one week should kill the trematodes, per Food and Drug Administration guidelines. But a social media survey included in the study suggested that people in the US are likely consuming freshwater fish without taking these precautions, which can dramatically increase the odds of infection.

The study identified two species of parasitic trematodes, Haplorchis pumilio and Centrocestus formosanus. These trematodes have historically infected people in Southeast Asia and likely arrived in the US more than a decade ago inside the bodies of one of their hosts: an invasive aquatic snail commonly known as the red-rimmed melania or Malaysian trumpet snail (Melanoides tuberculata). The invasive snail has spread to 17 American states and Puerto Rico.

The trematode’s life cycle involves parasitizing three hosts: first a red-rimmed melania snail, then a fish and then, finally, a warm-blooded vertebrate, like a bird or a human, that consumes the infected fish.

Previous work led by Hechinger showed that the red-rimmed melania and its associated trematode parasites are widespread in California. In the present study, Hechinger said he and his co-authors wanted to determine whether fish that Americans commonly catch and eat carry these infectious parasites, and whether people are consuming these fish in ways that increase their odds of infection.

In 2023, the researchers examined 84 fish from seven different species, including largemouth bass and bluegill, collected from five popular fishing locations in San Diego County. The researchers found that 93% of all the fish in the study were infected with the Haplorchis pumilio parasite, with some individual fish harboring thousands of the parasites. The second parasite, Centrocestus formosanus, was found at two of the five locations where it occurred in 91% of the fish.

“These parasites are here in the US, 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.”

Additionally, the study authors conducted a survey of 125 YouTube videos with a total of nearly 5 million views, and found 65% did not mention the proper cooking or freezing of the fish, which promotes the transmission of the parasites.

“Nearly 5 million views shows there is widespread interest and possibly a widespread practice of folks eating freshwater fish raw,” said Emma Palmer, a marine scientist at the Smithsonian Environmental Research Center who conducted the study during her graduate studies at Scripps.

Properly cooking or freezing fish before consumption kills the parasites, and minor infections shouldn’t cause serious harm. Greater health risks come from chronic, repeated infection over many months or years. The researchers emphasize that public education about the risk of infection and proper fish preparation is crucial, particularly among people who may rely on freshwater fish for food.

The study authors plan to share their results with public health officials in several Southern California counties to increase awareness. The researchers also hope the study will reach medical practitioners, who might not think of these trematodes as a possible cause of gastrointestinal complaints or other problems in their patients.

“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.”

To facilitate more accurate tracking, the study authors recommend that fish-borne trematode infection be added to the list of diseases doctors are required to report to public health officials.

“This kind of research is so important to identifying new public health threats, and it wouldn’t have been possible without NIH funding,” said Hechinger. “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. If the federal government doesn’t fund this sort of study, who will?”

Reference: Palmer EM, Metz DCG, Hechinger RF. Further evidence for plausible transmission of fishborne trematodiases in the United States: Game fish carry human-infectious trematodes and are eaten raw. J Infect Disease. 2025. doi:10.1093/infdis/jiaf180

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