Your next McDonald's Filet-O-Fish sandwich might have a stowaway
The Rocky Horror Picture Show. Tamagotchis. The McRib. As far as cult classics go, McDonald’s Filet-O-Fish is right up there with the best. And like any good cult favorite, this delightfully weird Mickey D’s menu item has a massive and dedicated fan base. But the crispy, golden-brown, square-pattied fish sandwich is also one of the chain’s more polarizing offerings, prompting endless head-scratchers like why does it only have a half-slice of cheese?
Love it or hate it (come on, have you even tried one?), the Filet-O-Fish has thrived for more than 60 years. And during Lent, the Filet-O-Fish is absolutely everywhere. But move over, Rocky: The Filet-O-Fish has a fishy horror lurking, and it might be enough to turn you off the beloved sandwich for good. (Just wait until you see the picture!)
It’s gross, yes. But we’re also here to make a different case. We spoke with Kimberly Baker, the food systems and safety program team director at the Clemson University extension, and Martin Bucknavage, the senior food safety extension specialist at Penn State University, to get to the bottom of this deep-sea McDonald’s mystery. Read on to learn what people are finding in their catch of the day … and whether you have anything to be worried about.
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What’s in a McDonald’s Filet-O-Fish, exactly?
The Filet-O-Fish has four simple ingredients: a crispy fried-fish-filet patty, tartar sauce and a half-slice of pasteurized American cheese on a plain steamed bun. The fish filet is made with wild-caught Alaskan pollock sourced from sustainable fisheries—and it’s cut right from the fish. No chopped and formed patty here!
Alaskan pollack is a mild fish, which make it a perfect backdrop for a dollop of tangy tartar sauce. And because it’s fresh-caught and sustainably sourced, you can feel good about choosing a sandwich that neither hurts the environment nor depletes the fish population.
What else might be in your Filet-O-Fish?
OK, deep breath. The McDonald’s subreddit tipped us off that people were finding tiny dead worms in their Filet-O-Fish! And you won’t be surprised to learn they were not happy about it.
- “Ugh gross. I think you’ve finally broken me of my soft spot for this sandwich,” wrote one user.
- “Thank you. I thought I’d have to get my stomach pumped but this did the trick,” wrote another.
- “That’s disgusting,” said a third, summing up the overall vibe.
Other Redditors chimed in and said there was no reason to panic:
- “Former fishmonger here – congrats on finding a nematode! If you’ve ever eaten wild-caught seafood either fresh/frozen, then congratulations as you’ve more than likely eaten a nematode prior!”
If a random Reddit commenter isn’t enough reassurance, here’s Clemson University’s Baker confirming the find: “This is likely a nematode, a type of roundworm commonly found in saltwater fish such as pollock, cod, flounder and halibut,” she says. “Small numbers of these worms are normal and commonly present in healthy, wild-caught fish.” Phew!
Why is this in McDonald’s fish?
It goes back to the fact that McDonald’s sources fresh fish right from the sea.
“Nematodes get into fish through the ocean food chain,” says Baker. She explains that fish like pollock, which McDonald’s uses, will eat small fish or crustaceans that have eaten the worms (or have been infected with them)—and then the worms will move into the larger fish.
“Fish pick up worms naturally through their normal activities,” adds Bucknavage. “If anything, it tells you that it is real fish.”
Does this have anything to do with McDonald’s food-prep techniques or cleanliness?
“Not at all,” says Bucknavage.
It doesn’t matter where you buy or order fish from or how expensive it is—if you’re eating wild-caught pollock, cod, flounder or halibut, there’s a chance you’ll come across a nematode. Baker says that sometimes nematodes are detected and removed before the fish makes it to your plate, but that’s not always possible.
“Some may be embedded deeper within the thicker layers of fish tissue, making them more difficult to identify and remove during routine processing,” she explains. In other words, if you’re eating these types of fish on a regular basis, there’s a chance you’ve ingested these tiny worms before.
Are these parasites dangerous?
No, they aren’t typically dangerous. And they’re almost always dead if you do eat them.
“Nematodes in fish are killed when the fish is either cooked to a minimum internal temperature of 145 degrees or commercially frozen to -4 degrees for at least seven days,” Baker says. McDonald’s uses deep fryers to cook its fish patties, and they typically contain oil heated to somewhere between 350 and 375 degrees Fahrenheit. So as long as your patty is cooked all the way through, you have nothing to worry about.
The only time these parasites can cause problems is if they are consumed while still alive, which can happen if you eat raw fish that was not meant to be eaten raw (meaning not frozen first, often called “sushi-grade”) or you eat undercooked fish. In these cases, Baker says they can cause an infection called anisakiasis, which typically causes temporary gastrointestinal symptoms like abdominal pain, nausea, vomiting and diarrhea.
But here’s some slightly positive news: “Because the human gastrointestinal tract is not an ideal environment for nematodes, they usually do not survive longer than seven to 10 days,” she adds.
Is there any way to ensure your fish doesn’t have a friend in it?
“If you are worried about worms in seafood, the only real way to avoid them is to not eat seafood,” Bucknavage says. Since nematodes occur naturally in fish, there’s no real way to guarantee that the fish you buy is entirely free of them.
But just because they occur naturally doesn’t mean they are in every piece of fish. While there’s a good chance you’ve consumed one in the past if you eat seafood regularly, you’ve also likely enjoyed countless pieces of fish that were totally nematode-free.
Plus, “you can rest assured that properly cooking the seafood will kill worms,” adds Bucknavage, so all you’re in danger of the next time you order a Filet-O-Fish is a slightly nasty visual.
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Sources:
- Kimberly Baker, food systems and safety program team director at Clemson University; email interview, March 2026
- Martin Bucknavage, senior food safety extension specialist at Penn State University; email interview, March 2026
- McDonald’s: “Filet-O-Fish”
- McDonald’s: “McDonald’s Filet-O-Fish Journey”
- Reddit: “r/McDonalds – Parasite in filet-o-fish?”
