UCSD study finds local freshwater fish carry human-infecting parasites

Last Updated on June 10, 2025

Though the study’s senior author states there’s no need for panic, precautions must be taken to avoid invasive worms.

Freshwater fish lovers, beware: more than 90 percent of popular freshwater game fish in Southern California contain an introduced parasite capable of infecting humans, according to a new study from researchers at UC San Diego’s Scripps Institution of Oceanography.

The study, published June 3 in the Journal of Infectious Diseases and funded by the National Institutes of Health, found two species of flatworms called trematodes that cause gastrointestinal problems, weight loss or lethargy in humans, and in rare cases, strokes or heart attacks.

There is “no need to panic,” study senior author and SIO ecologist and parasitologist Ryan Hechinger said, adding the risks can be avoided through fully cooking the fish or freezing any for raw consumption for at least a week to kill the parasites.

However, a social media survey included in the study suggested that people in the US are likely eating freshwater fish without taking these precautions, dramatically increasing the odds of infection.

“Americans don’t usually think about parasites when they eat freshwater fish because it hasn’t historically been an issue here,” Hechinger said. “But these trematodes have now been widely introduced in the U.S. and that means that doctors and the public should be aware.”

The two parasitic trematodes identified, Haplorchis pumilio and Centrocestus formosanus, have historically infected people in Southeast Asia and likely arrived in the US more than a decade ago via an invasive aquatic snail commonly known as the red-rimmed melania or Malaysian trumpet snail (Melanoides tuberculata).

Trematodes Centrocestus formosanus (left) and Haplorchis pumilio
UCSD/SIO

Following up on his 2022 study showing the red-rimmed melania and its associated trematode parasites are widespread in California, Hechinger looked 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.

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 percent 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 percent 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.”

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.

“There haven’t been any reported cases of these parasites infecting Americans,” Hechinger said, “but nobody is looking for cases and doctors aren’t required to report them.”