A recent study conducted by UC San Diego has raised alarming concerns about the health risks posed by parasitic infections in freshwater game fish throughout Southern California.
Published in the Journal of Infectious Diseases, the research indicates that over 90% of these fish are infected with trematodes—parasites capable of causing human illness.
Ryan Hechinger, an ecologist and parasitologist at UCSD’s Scripps Institution of Oceanography, emphasized that it is a prevalent misconception among Americans that parasites are not a concern when consuming freshwater fish.
According to Hechinger, “Americans don’t usually think about parasites when they eat freshwater fish because it hasn’t historically been an issue here.”
The study focused on two trematode species: Haplorchis pumilio and Centrocestus formosanus, which may lead to gastrointestinal issues, weight loss, fatigue, and in rare cases, severe health problems such as strokes or heart attacks.
In spite of these risks, the study’s authors reassured the public that proper food handling practices, such as thoroughly cooking fish or freezing them for a week prior to consumption, can mitigate the chances of infection.
However, a survey indicated that many Americans consuming freshwater fish are not aware of these necessary precautions.
The researchers traced the origin of these invasive parasites to Southeast Asia, where they have a long history of infecting people. They likely made their way to the United States more than a decade ago, traveling within the bodies of invasive aquatic snails like the red-rimmed melania, also known as the Malaysian trumpet snail.
This invasive snail species has proliferated across 17 states and Puerto Rico, establishing itself in various aquatic ecosystems.
The transmission cycle begins when the flatworms infect snails, subsequently moving to fish, and ultimately infecting warm-blooded animals, including birds and humans, that consume the infected fish.
Hechinger’s team previously identified the widespread presence of the invasive snail and parasites in California, prompting the latest study to investigate which fish species carry these parasites and to assess the associated health risks.
In 2023, researchers examined 84 freshwater fish from seven species, including popular catches like largemouth bass and bluegill, collected across five notable fishing spots in San Diego County.
The results were striking, with 93% of the sampled fish infected with Haplorchis pumilio, some containing thousands of parasites.
Moreover, Centrocestus formosanus was detected in 91% of the fish at two of the five fishing locations under study.
Hechinger cautioned, “These parasites are here in the U.S., and they’re infecting fish that people are eating.”
The study serves as a crucial wake-up call for public health officials, medical practitioners, and the general populace to be aware of these potential health hazards.
Although there have been no documented cases of these parasites infecting Americans, Hechinger pointed out that the absence of reported cases might be due to a lack of awareness and screening among healthcare providers.
“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,” he noted.
This important research was financially supported by the National Institutes of Health, which has experienced drastic funding cuts in recent years as a part of broader government austerity measures.
Hechinger emphasized the vital role of federally funded research in identifying emerging public health threats, stating, “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?”
image source from：https://www.kpbs.org/news/environment/2025/06/03/vast-majority-of-freshwater-fish-caught-in-san-diego-county-contain-parasites
