For seafood lovers, there are few things more appealing than a simple slab of raw tuna — a fish whose fatty, savory, and tender muscle tissue has been a delicacy for thousands of years.

Unfortunately for those foodies, fresh raw fish comes with the distinct possibility of parasitic infection. Off the coasts of southern Brazil, that risk is rising, as hauls of the common skipjack tuna come with the near certainty of parasitic contamination.

That’s not an exaggeration. According to a new report published in the journal Ocean and Costal Research and flagged by ScienceAlert, a whopping 96 percent of a recent load of tuna was infested with gut-busting worms.

Out of 53 skipjack, marine researchers counted over 1,600 worms infesting nearly every organ of every fish hauled in from the ocean. Only the kidneys and hearts of the tuna remained worm-free, the researchers noted. Everything else, including the muscle tissue we humans eat, was festering with flatworms and roundworms.

Though most preparation methods will kill the parasites, the images are pretty off-putting. As the researchers explain, the sheer volume of parasitic infection is alarming.

“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 authors write.

Still, parasitologists say the worms aren’t all bad. Though increased presence of worms also comes with more risk of infection, University of Washington parasitologist Chelsea Wood told ScienceAlert in a prior story that they can actually be a sign that the fish is well-sourced.

“Everyone assumes that worms in your salmon is a sign that things have gone awry,” Wood said. “But the anisakid [roundworm] 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.”

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