Worms in Fish Are Actually Quite Common — Here’s What to Know

There was plenty of attention on food at the Olympic Games, with reports circulating about egg shortages in the Olympic Village and an ongoing debate about how good those viral chocolate muffins actually were. But one athlete has made headlines after claiming some people in the Olympic Village found worms in their fish. 

“I like my fish and people are finding worms in the fish. It’s just not good enough,” British swimmer Adam Peaty said in an interview with iNews. “The standard, we’re looking at the best of the best in the world, and we’re feeding them not the best.”

Peaty’s comments have raised a lot of questions about worms in fish, as well as just how common this is. Unfortunately, it’s not rare. Here’s the deal. 

So, how do worms end up in fish?

There are a few different ways that worms can wind up in your fish. Fish can pick up intestinal parasites like a fluke or tapeworm by eating infected marine life, says food safety expert Ellen Shumaker, Ph.D., director of outreach for the Safe Plates program at North Carolina State University. “Parasites like these are found in freshwater and saltwater, and are fairly common,” she says. “When fish consume infected crustaceans, the parasite larvae migrate into the flesh of the fish and into the organs. If larvae have matured and developed into worms, these worms could be visible.” 

Freshwater fish may also get what’s known as trematodes, which are parasites that can enter through their skin, says Erin Arneson, seafood specialist at the University of Georgia Marine Extension and Georgia Sea Grant.

This is “pretty common” in cod, haddock, and salmon, says John Hawdon, Ph.D., professor of Microbiology, Immunology, and Tropical Medicine at the George Washington University School of Medicine & Health Sciences. Deep sea fish like tuna can also carry worms, but “they generally have fewer parasites,” Hawdon says. 

When worms are in fish, you may see them — or you may not. “The worms in fish are usually small and white or almost clear in color,” says Shannon Stover, M.S., R.D., food safety educator at Michigan State University Extension. “In some fish, like cod, they may be more prominent and darker—like stray pieces of seaweed. The tapeworm can be gray-white, with a soft, flabby appearance.”

Can parasitic worms in fish make you sick?

It depends. The big issue is how the fish is prepared. “Parasitic worms present the greatest health hazard when fish is consumed raw,” Arneson says. To control for parasites in raw fish, the meat must be frozen at or below –31°F and stored at that temperature for at least 15 hours, she explains. It can also be frozen and stored at –4°F for at least seven days. 

“The human health hazard from parasitic worms can be easily controlled by freezing at recommended times and temperatures or by cooking fully to an internal temperature of 140°F,” Arneson says. “As a note, most home freezers range from 0 to 10°F, and may not be cold enough to kill parasites.”

Still, “if the fish is frozen or cooked, the worms will die and be harmless,” Hawdon says. But if you happen to eat undercooked fish or sushi that wasn’t properly frozen first, Hawdon says there’s a risk of getting sick. “People can get parasites,” he points out. 

“Symptoms from this type of infection can vary,” Schumaker says. “Some infected people may not show any symptoms at all, while some will have gastrointestinal issues like diarrhea, vomiting, stomach pain, and weight loss.”

You could have a reaction to the worms, too, even when they’re no longer alive. “There are cases of consumers having allergic reactions to parasitic worms even though they have been killed through a verified freezing or cooking process,” Arneson says.

What to do if you spot worms in your fish

Again, worms can and do show up in fish. “I know personally of one case where somebody was served cod ceviche and a live worm came out that piece of fish,” Hawdon says. 

If you see worms in your fish, it’s best to avoid eating them. After all, if the fish is undercooked and the worms are still alive, you run the risk of getting a parasitic infection. That’s why the Food and Drug Administration (FDA) says that the safest route when eating fish is to cook it first. But if your fish is fully cooked and you realize halfway through that you ate a worm, you’ll likely be OK — just grossed out. “At that point, it’s just more protein,” Hawdon says. 

Experts say this just underscores the importance of following food safety with fish. “If the guidelines for properly cooking or freezing the fish are followed, all of this can be prevented,” Stover says.