A recent study from UC San Diego’s Scripps Institution of Oceanography has revealed that over 90% of freshwater game fish in Southern California carry parasites known to be capable of infecting humans and posing health risks.
The study, published in the Journal of Infectious Diseases, highlights the emergence of trematodes—flatworms that can lead to various health issues—within the region.
Ryan Hechinger, an ecologist and parasitologist at UCSD’s Scripps Institution of Oceanography and the study’s senior author, emphasized the lack of awareness among Americans regarding parasites associated with freshwater fish.
“Americans don’t usually think about parasites when they eat freshwater fish because it hasn’t historically been an issue here,” Hechinger stated. “But these trematodes have now been widely introduced in the U.S., and that means that doctors and the public should be aware.”
The research focused primarily on two particular species of trematodes, which can lead to gastrointestinal problems, weight loss, lethargy, and, in rare cases, severe complications like strokes or heart attacks.
Despite the alarming findings, the authors reassured that proper food handling techniques—such as fully cooking the fish or freezing it for at least a week—can effectively prevent illness.
However, a survey indicated that many Americans consuming freshwater fish are not adhering to these safety practices.
The trematodes in question, Haplorchis pumilio and Centrocestus formosanus, have historically been prevalent in Southeast Asia.
The study suggests that these parasites likely entered the U.S. more than a decade ago via an invasive aquatic snail species known as the red-rimmed melania or Malaysian trumpet snail.
This invasive snail has managed to spread to 17 American states and Puerto Rico, carrying the parasites along with it.
The life cycle of these flatworms involves an initial infection of the snail, followed by an infection of the fish, and ultimately a warm-blooded vertebrate like a bird or human that consumes the infected fish.
Hechinger, who has previously demonstrated the widespread presence of both the red-rimmed melania and parasites in California, aimed to identify which fish species are infected and the potential health risks they present.
In their 2023 research, the team examined 84 fish specimens from seven different species, including popular catches like largemouth bass and bluegill, collected from five well-known fishing spots in San Diego County.
The results were concerning: 93% of the fish tested were found to be infected with the Haplorchis pumilio parasite, with individual fish hosting thousands of the parasites.
In addition, the second parasite, Centrocestus formosanus, was discovered at two of the five locations, infecting 91% of the sampled fish there.
Hechinger remarked, “These parasites are here in the U.S., and they’re infecting fish that people are eating. We hope this study can help make public health officials, doctors, and the public more aware.”
The study noted that the risk associated with these parasites escalates with chronic, repeated infections over extended periods.
Hechinger expressed hope that the research would reach medical professionals who might overlook these parasites as a potential cause of gastrointestinal issues or other health problems.
To date, there have been no reported cases of these parasites infecting Americans, largely because they are not actively being monitored or reported by healthcare providers.
The research received funding from the National Institutes of Health, which has faced significant budget cuts under the Trump administration’s budgetary policies.
Hechinger pointed out, “This kind of research is crucial for identifying new public health threats. It wouldn’t have been possible without NIH funding. Research like this isn’t profitable for private companies but can significantly contribute to public health. If the federal government doesn’t support this type of research, who will?”
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