A fatty tuna that is often used for sashimi or canning may be swimming with parasites more often than not.

When scientists at the Federal University of Santa Catarina in southern Brazil analyzed 53 locally caught fish, they found nearly all were contaminated by worms.

According to a brief communication published in Ocean and Coastal Research, the research team counted more than 1,600 little worms in total.

Using powerful microscopy tools as aids, they found that the kidney and heart were the only organs free of parasites in every fish.

Otherwise, the worms and their signs were everywhere.

Of the 53-fish haul, 96 percent were infected by parasites.

The intestines were the most heavily infected organ, followed by the muscle tissue – notably, the bit we eat – and then the stomach.

"These results highlight the importance of sanitary inspection," the study authors conclude, "and emphasize the importance of immediate evisceration to avoid the migration of larvae from the viscera to the musculature."

The study from Brazil is small and covers only a single point in time, but it suggests that tuna in this region commonly host two worm genera: Trypanorhyncha and Anisakis.

Parasites and fish have evolved to coexist in the world's oceans, so it's not altogether surprising to find them in tuna.

What is eyebrow-raising is just how many worms were in each sample and how widely they were spread among the organs.

The tuna species analyzed in the study is known as skipjack, oceanic bonito, or katsuo (Katsuwonus pelamis), depending on where you are in the world, and it has high commercial value.

It is the most abundant tuna species in Brazil and, in 2022, the third most-captured marine species globally. That year, 3.1 million tons of skipjack were caught and sold around the world.

Who knows how many parasites each one of those fish carried.

Tuna sold for consumption is usually carefully examined by seafood processors and sushi chefs, and visible parasites are thankfully removed. But the truth is that some tiny creatures inevitably sneak past our defenses.

Indeed, parasitologists in the United States are studying worms hidden in expired cans of fish to track parasite numbers.

Before anyone goes throwing their tinned fish away, let it be known that accidental human infection with Trypanorhyncha larvae is rare, and not all species of Anisakis are recognized as causative agents of human disease.

When killed during the canning process, most parasitic worms are harmless to humans.

That said, if eaten in raw or undercooked fish, some species can latch onto the human intestinal wall and cause gastrointestinal symptoms such as abdominal pain, vomiting, nausea, or diarrhea.

Even if these parasites don't make someone sick, though, finding a worm in a can of tuna is enough to turn some consumers away.

"Beyond their potential impact on human health," explain the researchers, "parasites can reduce the marketability and value of seafood products due to implications for food safety and quality, decreasing consumer confidence and causing economic losses for the fishing industry."

Parasites are often villified as agents of disease, but Chelsea Wood, a parasitologist at the University of Washington, says their presence can be a good omen, too.

"Everyone assumes that worms in your salmon is a sign that things have gone awry," said Wood in 2025.

"But the anisakid life cycle integrates many components of the food web. I see their presence as a signal that the fish on your plate came from a healthy ecosystem."

At this point, it's impossible to say whether this many parasitic worms in tuna is normal.

Studies on these fish parasites are scarce, despite their importance to fisheries worldwide.

In 2020, researchers at the University of Washington found evidence that Anisakis worms in raw seafood have increased 280 times since the 1970s.

On average, they explained, we have gone from finding less than one worm in every 100 caught fish to more than one worm in every single host caught.

But that isn't necessarily a bad thing for us or the marine ecosystem.

Parasites like Anisakis also infect fish, squid, whales, and dolphins. Their goal is not to end up in humans.

"My gut is that this is about the improvements we've made in marine mammal conservation," Wood told ScienceAlert in 2020.

"The time frame of our study directly overlaps with when a bunch of really important marine mammal legislation went into effect, like the Marine Mammal Protection Act in 1972 and the International Whaling Commission moratorium on commercial whaling, which came in the 1980s."

Related: Expired Cans of Salmon, Decades Old, Preserve a Huge Surprise

More research needs to be done tracking parasites, not only off the coast of Brazil, but in other key fishing regions around the world, to better understand how they are faring in a rapidly changing world.

For now, the average fish consumer should take their cue from Wood, a researcher well acquainted with the reality of eating fish.

"So if I'm being honest with you, there are worms in everything you eat," Wood told an audience at the Seattle Aquarium in 2020.

"But cooking kills the menagerie of parasites that we find in our food, and it's only when we eat raw or undercooked food that we wind up tangling with live and infectious parasites."

Besides, she added, "when you eat these parasites, you get to be part of a natural cycle that links plankton to fish to whales to you."

How… comforting.

The study is published as a brief communication in Ocean and Coastal Research.