A largemouth bass in a lake. Photo: iStock/FtLaudGirl

Freshwater Fish in U.S. Carry Introduced Human-Infecting Parasites

Fish species frequently caught and eaten by people were found carrying large numbers of invasive parasitic worms

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 U.S. 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 U.S. are likely consuming freshwater fish without taking these precautions, which can dramatically increase the odds of infection. 

A bkluegill fish specimen is examined in the lab
This bluegill collected during the study contained 16,973 Haplorchis pumilio and 8 Centrocestus formosanus infectious trematode parasite larval stages. Photo: Emma Palmer

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 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 (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 U.S., 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.  

A researcher in the field with a net to catch fish
Emma Palmer, first author of the new study, conducting fieldwork with the California Department of Fish and Wildlife at a popular fishing site.

“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?”

In addition to Hechinger and Palmer, the study was co-authored by Daniel Metz of the University of Nebraska.

About Scripps Oceanography

Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.

About UC San Diego

At the University of California San Diego, we embrace a culture of exploration and experimentation. Established in 1960, UC San Diego has been shaped by exceptional scholars who aren’t afraid to look deeper, challenge expectations and redefine conventional wisdom. As one of the top 15 research universities in the world, we are driving innovation and change to advance society, propel economic growth and make our world a better place. Learn more at ucsd.edu.

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