Scientists at the Federal University of Santa Catarina in southern Brazil analyzed 53 locally caught fish and found nearly all were contaminated by worms. According to a brief communication published in Ocean and Coastal Research, the team counted more than 1,600 little worms in total. Using powerful microscopy tools, 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.

The study authors conclude that these results highlight the importance of sanitary inspection and emphasize the importance of immediate evisceration to avoid the migration of larvae from the viscera to the musculature. The tuna species analyzed is skipjack (Katsuwonus pelamis), the most abundant tuna species in Brazil and the third most-captured marine species globally. In 2022, 3.1 million tons of skipjack were caught and sold around the world.

While tuna sold for consumption is usually carefully examined and visible parasites removed, some tiny creatures inevitably sneak past defenses. Indeed, parasitologists in the United States are studying worms hidden in expired cans of fish to track parasite numbers. Parasites and fish have evolved to coexist in the world's oceans; it is not altogether surprising to find them in tuna.

Beyond their potential impact on human health, 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. 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, but the anisakid life cycle integrates many components of the food web.