California freshwater fish carrying invasive parasitic worms: Study
More than 90 percent of popular Southern California freshwater game fish sampled in a new study contained invasive worms capable of infecting humans, scientists warn.
The parasites — two species of flatworms called trematodes — may pose a previously unrecognized public health risk in the United States, according to the study, published Tuesday in the Journal of Infectious Diseases.
In Southern California, these trematodes are 'introduced parasites,' or parasites transported to a new place by an alien host. They often cause gastrointestinal problems, weight loss and lethargy in people — and in more severe cases, strokes or heart attacks, the authors noted.
'Americans don't usually think about parasites when they eat freshwater fish because it hasn't historically been an issue here,' said senior author Ryan Hechinger, an ecologist and parasitologist at the University of California San Diego's Scripps Institution of Oceanography, 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,' Hechinger added.
The two trematodes of focus in this study, Haplorchis pumilio and Centrocestus formosanus, likely arrived in the U.S. from Southeast Asia more than a decade ago, according to the study. They presumably did so via the bodies of one of their hosts: an invasive aquatic snail, the red-rimmed melania, which has now spread to 17 states and Puerto Rico.
The parasites first move into the snails, which are then consumed by fish — leading them finally to the dinner plate of a warm-blood vertebrate, such as a human or a bird, the authors explained.
While previously work led by Hechinger demonstrated the prevalence of red-rimmed melania and the trematode parasites across California, this research aimed to show whether fish that Americans enjoy consuming contains the parasites.
To answer this question, Hechinger and his colleagues examined 84 fish across seven species, including largemouth bass and bluegill, from five fishing spots in San Diego County.
Ultimately, they discovered that 93 percent of all fish included in the study were infected with the first trematode, Haplorchis pumilio. The second parasite, Centrocestus formosanus, was present at two of the five locations, occurring in 91 percent of those fish, per the study.
'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.'
At the same time, Hechinger emphasized that there is 'no need to panic,' as the dangers posed by the parasites can be prevented by fully cooking fish or freezing any fish intended to be consumed raw for at least a week.
However, the scientists noted that a social media survey they included in the study showed that Americans may be eating freshwater fish without taking such precautions. Doing so, they warned, can raise the odds of infection considerably.
In their survey, which included 125 YouTube videos with a total of nearly 5 million views, the scientists noted that 65 percent made no mention of proper cooking procedures.
Going forward, the study authors said they intend to share their results with local public health officials to raise awareness. They expressed hope that their findings would reach medical practitioners, who might not immediately think of trematodes as a possible cause of gastrointestinal infections and other illnesses.
The researchers also recommended that fish-borne trematode infection be included on the list of diseases doctors are required to report to public health agencies.
'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,' he added.
Copyright 2025 Nexstar Media, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.
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