Scientists Find Parasitic Worms in 93% of Freshwater Fish Sampled in California

This bluegill collected during the study contained 16,973 H. pumilio and 8 C. formosanus infectious trematode parasite larval stages. (CREDIT: Emma Palmer)

Researchers found human-infecting parasitic worms in 93% of freshwater fish sampled from popular fishing spots in southern California, including bluegill and largemouth bass.

A review of 125 YouTube videos revealed widespread raw consumption of freshwater fish across the U.S., often without proper freezing or cooking, ideal conditions for parasite transmission.

These parasites are likely more widespread than reported, and public health experts warn that eating raw, unfrozen freshwater fish could pose an emerging health risk in the United States.

SAN DIEGO — Americans love their sushi, ceviche, and poke bowls. But a new study reveals that eating raw freshwater fish, particularly those caught in your local lake or river, could expose you to dangerous parasitic worms that are spreading rapidly across the United States.

The study, published in The Journal of Infectious Diseases, revealed that two species of parasitic trematodes, tiny worms that can cause serious intestinal illness, are now infecting common game fish throughout southern California and likely much of the country. Researchers also found hundreds of YouTube videos showing Americans eating these same infected fish species completely raw, often promoting dangerous myths about food safety.

“Americans don’t usually think about parasites when they eat freshwater fish because it hasn’t historically been an issue here,” says study author Ryan Hechinger from the University of California-San Diego, 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.”

The widespread introduction of an invasive snail called Melanoides tuberculata has brought two human-infecting parasites, Haplorchis pumilio and Centrocestus formosanus, to American waters. Locally transmitted fishborne trematodiasis has not previously been a public health concern in the United States.

The invasive freshwater snail (melanoides tuberculata) (Olga Chezhina/Shutterstock) Scientists from the University of California-San Diego examined 84 freshwater fish from five popular fishing spots in San Diego County. They found that virtually every fish was infected with these parasites, some harboring thousands of the microscopic worms.

Adult worms, typically 1-2 millimeters long, infect the small intestine and can cause symptoms ranging from mild stomach discomfort to severe diarrhea, weight loss, lethargy, and in extreme cases, death. In Asia, where these parasites are endemic, they’re recognized as globally important human pathogens affecting millions of people.

The invasion started with Melanoides tuberculata, a freshwater snail native to Asia and Africa that has spread to 17 U.S. states and Puerto Rico. This snail serves as the first host for at least 11 species of human-infecting trematodes, including the two that have now established themselves in American freshwater systems.

Parasites follow a complex life cycle. They start in infected snails, then release free-swimming larvae called cercariae that burrow into fish and form cysts called metacercariae, the infectious stage for humans. When people eat infected fish raw or undercooked, the parasites mature into adult worms in the intestine.

Researchers collected fish through routine electrofishing conducted by the California Department of Fish and Wildlife during summer and autumn 2023. They examined seven commonly caught and eaten species: bluegill, largemouth bass, green sunfish, redear sunfish, black crappie, common carp, and bluegill-green sunfish hybrids.

The team carefully dissected each fish and examined tissues under microscopes to count the encysted parasites. They confirmed the species identity using DNA analysis, comparing genetic sequences to known specimens from around the world.

Haplorchis pumilio infected all seven fish species examined, with overall infection rates of 93% across all fish tested. At individual locations, infection rates in bluegill and largemouth bass ranged from 80% to 100%. Some fish harbored massive parasite loads. Bluegill carried anywhere from 3 to over 5,000 individual parasites, while largemouth bass had 18 to 185 parasites each.

Centrocestus formosanus was less widespread but still concerning, infecting fish at two of the five locations with infection rates of 83-100% where present. DNA analysis confirmed these were indeed the same species causing human illness in Asia.

Most H. pumilio parasites concentrated in the connective tissue around fin bases, the areas where fins attach to the fish body. Since these areas are at the edges of fish fillets, parasites could easily contaminate the meat during cleaning. C. formosanus parasites primarily infected the gills but could spread to surrounding tissues.

When they searched YouTube for videos of Americans eating raw freshwater fish, they discovered 125 videos posted between 2008 and 2024, collectively viewed 4.7 million times. Some of the videos included the exact species they found to be infected.

Eighty-two percent of videos showed people preparing traditional raw fish dishes like ceviche, sashimi, or poke using freshly caught fish. Researchers classified 65% of these videos as showing consumption of raw, unfrozen fish, exactly the conditions needed for parasite transmission.

Ceviche Ceviche is a dish made with marinated raw fish or seafood. (Metamore Studio/Shutterstock)

Videos came from across the country, with 49% filmed in states where these parasites have been documented and another 27% in states where only the invasive snail has been found. Fish species known to harbor the parasites appeared in 31 videos, with 18 of those showing raw consumption.

Many videos spread dangerous misinformation about food safety. Sixteen videos claimed that marinating fish in citrus juice (as in ceviche) “cooks” the fish and kills parasites, which is false. Other videos incorrectly suggested that “healthy-looking” fish or those from “clean streams” are safe to eat raw, or that visual inspection can detect parasites.

Hidden Scope of the Problem “There haven’t been any reported cases of these parasites infecting Americans,” says Hechinger. “But nobody is looking for cases and doctors aren’t required to report them.”

Video creators often mentioned they had made similar dishes before, suggesting this wasn’t one-time behavior but regular practice. Some channels had hundreds of thousands of subscribers, amplifying the reach of potentially dangerous food preparation methods.

Twenty-two videos showed even riskier “shock factor” consumption, where people ate entire fish heads, organs, or whole fish—often on dares. These practices would expose people to the highest concentrations of parasites.

Researchers noted this likely represents just the tip of the iceberg, as most people eating raw fish probably don’t film themselves doing it.

Scientists suspect these parasites are already more widespread than documented. Bird migration can spread the parasites across vast distances, and the lack of surveillance in most states means infections could be going undetected. Currently, only four states—California, Utah, Texas, and Florida—have confirmed the parasites, but that likely reflects limited testing rather than limited spread.

Unlike marine fish used in professional sushi preparation, which undergo strict freezing protocols to kill parasites, freshwater fish face no such requirements. Home cooks catching and preparing their own fish are unlikely to freeze them properly before eating them raw.

“This kind of research is so important to identifying new public health threats, and it wouldn’t have been possible without NIH funding,” says 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?”

Prevention is straightforward but requires awareness. Freezing fish at -4°F (-20°C) for at least 72 hours kills the parasites, as does thorough cooking. Professional sushi-grade fish undergoes commercial freezing specifically to eliminate parasite risks.

Researchers recommend public health officials increase awareness about transmission risks and proper food safety measures wherever invasive snails are established. They also suggest adding fishborne trematodiases to reportable disease lists to track this emerging threat.

Millions of Americans enjoy freshwater fishing and eating their catch. Cook it thoroughly, freeze it first, or risk swallowing thousands of parasitic worms that your immune system was never designed to handle. Your Instagram-worthy ceviche isn’t worth a trip to the emergency room.

Researchers collected 84 freshwater fish of seven species from five popular fishing locations in San Diego County, California, during summer-autumn 2023. They used standard parasitological techniques to examine fish tissues under microscopes, counting and identifying parasitic cysts. DNA analysis confirmed parasite species identity. Separately, they searched YouTube for videos of Americans eating raw freshwater fish, analyzing 125 videos for fish species, preparation methods, geographic location, and food safety information.

Both parasite species infected fish at very high rates—Haplorchis pumilio was found in 93% of all fish examined, while Centrocestus formosanus infected 89% of fish at locations where it was present. Individual fish carried anywhere from a few parasites to over 5,000. YouTube analysis revealed widespread consumption of raw freshwater fish across the U.S., with 65% of relevant videos showing potentially dangerous preparation methods. Many videos spread food safety misinformation.

This study examined fish from only one geographic region (San Diego County) and focused on just two parasite species. YouTube videos may not represent typical consumption patterns and could overrepresent risky behaviors. Research didn’t directly measure human infection rates or health outcomes, only the potential for transmission based on fish infection rates and consumption behaviors.

This research was supported by the National Institutes of Health (grant number 1R03AI156569-01 to R.F.H.). Authors reported no conflicts of interest. Published as an open access article under Creative Commons licensing.

The paper “Further Evidence for Plausible Transmission of Fishborne Trematodiases in the United States: Game Fish Carry Human-Infectious Trematodes and Are Eaten Raw” is authored by Palmer, E.M., Metz, D.C.G., and Hechinger, R.F. It was published in The Journal of Infectious Diseases on June 3, 2025.

I only eat wild-caught Alaskan salmon.

Alaskan canned wild salmon is good too for a change of pace. No need to only eat the fresh flown in wild. I used to eat once each summer. Great cooked or raw in faked sushi. Which means eat with soy sauce and Dijon mustard instead of wasabi. White rice on the side. I do same with raw tuna.

“Salmon are infested with tapeworms. Bears can eat them raw because they hibernate and the worms starve to death over the winter.”

Grok3 says>>>
Yes, salmon can be infested with tapeworms, particularly a species called Diphyllobothrium, which is a type of fish tapeworm. These parasites can infect wild and farmed salmon, typically when they consume infected crustaceans or smaller fish during their life cycle. The tapeworm larvae can form cysts in the fish’s muscle tissue, which may be consumed by humans if the fish is eaten raw or undercooked.
However, the risk to humans is low if salmon is properly cooked, frozen, or sourced from reputable suppliers adhering to food safety standards. Cooking salmon to an internal temperature of at least 145°F (63°C) or freezing it at -4°F (-20°C) for at least 7 days kills the tapeworm larvae. Sushi-grade salmon is typically frozen to eliminate parasites.

“Get me some fresh caught yellowtail ...”

fish used in sushi must typically be flash-frozen to extremely low temperatures to be considered safe for consumption. This process helps kill parasites that may be present in the fish, making it safer to eat raw.

Elaboration:

Parasite Destruction:
.

Freezing fish at or below -4°F (-20°C) for at least 7 days or at -31°F (-35°C) for at least 15 hours is recommended by the FDA to effectively kill parasites.

Flash Freezing:
.
Flash freezing involves rapidly freezing the fish to prevent ice crystal formation that can damage the fish’s texture.
Sushi-Grade Fish:
.
Fish labeled as “sushi-grade” or “sashimi-grade” has typically undergone this freezing process.
Safety Concerns:
.
Raw fish can contain parasites, bacteria, and viruses, making it a food safety concern.
Restaurant Practices:
.
Many reputable sushi restaurants use frozen fish to ensure safety, though there is no strict oversight of chefs to ensure they follow freezing guidelines.

Thanks for the details. I was describing what my buddies and I would do when fishing in the Pacific years ago. It was common when the first tuna was landed the soy and wasabi appeared and we all had a little snack.

Now for restaurant grade sushi that’s all we eat today. No gas station sushi for us!

I make ceviche using bluegills that I catch from my pond. I only do it in the spring before the water temps get above 60° F.

It is absolutely delicious. No ill effects noted so far.

The Mrs. and I do a parasite cleanse a couple or three times a year as a part of our normal health regimen.

I am glad you have not had any problems with parasites. Have you had any perch yet?

My B.i.L. was a fisherman and sometime in his thirties he came down with something that resulted in a hole in his intestine, infection, and sepsis which led to his death. I have wondered whether he had a fish hookworm from an undercooked fish. (He drank and smoked tobacco and other stuff which probably did not help.)

The only raw fish I have ever had is lox, which IIRC, does have some thing done to it. (Grav-lox is sugar cured I think.)

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.