Think Your Freshwater Fish Is Safe? 93% Carry Parasites That Can Infect Humans

More than 90% of popular freshwater game fish in Southern California harbor invasive parasites capable of infecting humans, according to new research from UC San Diego’s Scripps Institution of Oceanography. The study, published in the Journal of Infectious Diseases, reveals that fish species frequently caught and eaten by Americans carry large numbers of parasitic worms that can cause gastrointestinal problems, weight loss, and in severe cases, strokes or heart attacks.

The parasites—two species of flatworms called trematodes—arrived in the United States more than a decade ago inside invasive snails from Southeast Asia. Now they’ve spread to fish that millions of Americans catch and eat, creating what researchers call a previously unrecognized public health risk.

The Hidden Threat in Your Catch

“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.”

The researchers examined 84 fish from seven different species, including largemouth bass and bluegill, collected from five popular fishing locations in San Diego County during 2023. Their findings were stark: 93% of all fish contained Haplorchis pumilio parasites, with some individual fish harboring thousands of the microscopic invaders.

A second parasite, Centrocestus formosanus, infected 91% of fish at two of the five locations where it was present. Both parasites originally infected people in Southeast Asia before hitchhiking to America inside red-rimmed melania snails, which have now spread to 17 states and Puerto Rico.

How Fish Become Mobile Parasite Hotels

The trematodes follow a complex three-host lifecycle that transforms fish into unwitting carriers. Free-swimming parasite larvae exit infected snails and burrow into fish tissue, forming protective cysts called metacercariae. These infectious stages concentrate in different parts of the fish depending on the species—Haplorchis pumilio embeds primarily in fin bases and surrounding muscle tissue, while Centrocestus formosanus prefers gills.

What makes this particularly concerning is the parasite load. One bluegill collected during the study contained 16,973 Haplorchis pumilio and 8 Centrocestus formosanus larvae. Even fish that appear perfectly healthy can harbor hundreds or thousands of parasites without showing any visible signs of infection.

YouTube Videos Reveal Risky Eating Habits

The researchers didn’t stop at examining fish. They analyzed 125 YouTube videos with nearly 5 million combined views to understand how Americans actually prepare and eat freshwater fish. The results were troubling: 65% of videos showed people consuming raw, unfrozen fish—exactly the conditions that allow parasites to survive and infect humans.

“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.

Many videos promoted dangerous misconceptions about food safety. Common myths included claims that citrus marinades “cook” fish and kill parasites, that healthy-looking fish are safe to eat raw, or that visual inspection can detect parasites. None of these beliefs are scientifically accurate.

Key Study Findings:

• 93% of examined freshwater game fish carried human-infecting parasites
• Some fish harbored over 16,000 individual parasite larvae
• 65% of YouTube fishing videos showed unsafe raw fish consumption
• Videos collectively reached nearly 5 million viewers, suggesting widespread risky practices
• Parasites now present in fish across multiple states where invasive snails exist

Simple Prevention, Serious Consequences

Hechinger emphasizes there’s “no need to panic” because prevention is straightforward. Fully cooking fish or freezing any intended for raw consumption for at least one week kills the parasites, following Food and Drug Administration guidelines. The parasites cause relatively mild symptoms in most cases, but chronic infections over months or years pose greater health risks.

However, the gap between safe practices and reality concerns researchers. The study revealed that many people rely on freshwater fish for food, particularly in communities where proper preparation education may be lacking.

“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.”

A Growing But Hidden Problem

Perhaps most concerning is the invisible nature of this health threat. “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.”

The researchers believe many infections go undiagnosed because American physicians rarely consider these tropical parasites when treating gastrointestinal complaints. To address this blind spot, they recommend adding fish-borne trematode infections to the list of diseases doctors must report to public health officials.

The team plans to share their findings with Southern California county health departments and hopes to reach medical practitioners who might encounter these infections without recognizing them. With invasive snails continuing to spread across the United States, the geographic scope of this health risk will likely expand.

“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?”

As Americans increasingly embrace diverse culinary traditions including raw fish dishes, understanding and preventing these hidden health risks becomes ever more critical. The solution remains simple: cook your catch thoroughly, or freeze it first if you prefer it raw.