A UC San Diego study published Tuesday found that more than 90% of freshwater game fish in Southern California had parasites capable of infecting humans and causing health problems.
The parasites are introduced species from elsewhere, and Americans are perhaps not aware of the risk, the authors write 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 Ryan Hechinger, an ecologist and parasitologist at UCSD's Scripps Institution of Oceanography and the study's senior author. "But these trematodes have now been widely introduced in the U.S. and that means that doctors and the public should be aware."
Trematodes, a type of flatworm, made up the study's scope. The scientists studied two species which can cause gastrointestinal problems, weight loss or lethargy when they infect humans β even strokes or heart attacks in rare cases.
Despite the dire-sounding situation, the paper's authors said proper food handling will prevent illness: fully cooking freshwater fish or freezing any intended to be eaten raw for at least a week. However, a survey found many Americans who do consume freshwater fish are not taking these precautions.
The two species of trematodes β Haplorchis pumilio and Centrocestus formosanus β have historically infected people in Southeast Asia and likely arrived in the U.S. 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, the authors write. This invasive snail has spread to 17 American states and Puerto Rico.
The flatworms first infect the snail, then a fish, and finally a warm- blooded vertebrate, like a bird or a human, that consumes the infected fish.
Hechinger led previous work showing the red-rimmed melania and the parasites are widespread in California. The study published Tuesday wanted to find what sorts of fish carry the parasites and if it poses a health risk.
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, they write. 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 U.S., and they're infecting fish that people are eating," Hechinger said. "We hope this study can help make public health officials, doctors and the public more aware."
According to the authors, the risk is greater from chronic, repeated infection over many months or years. They said they hope the study will reach doctors, who might not think of these parasites as a possible cause of gastrointestinal complaints or other medical issues.
"There haven't been any reported cases of these parasites infecting Americans, but nobody is looking for cases and doctors aren't required to report them," Hechinger said.
The study was funded by the National Institutes of Health, which has had its funding sharply reduced under the Trump administration's efforts to cut down the size of the federal government.
"This kind of research is so important to identifying new public health threats, and it wouldn't have been possible without NIH funding," Hechinger said. "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?"
