Parasitic sushi worms have quietly been on the rise, study says

A lot of sashimi bites back.

“Sushi parasites” have been increasing exponentially over the past half-century, according to findings published Thursday in the journal Global Change Biology[1].

Led by the University of Washington researchers, the study — called “It’s a Wormy World” — found a dramatic increase in the number of worms transmittable to humans who consume raw or undercooked seafood.

Using findings from previous studies, the authors concluded that seafood lovers today have a much higher likelihood of biting into a parasite than their 1970s cohorts did. Scientists are not sure why, but there’s been a 283-fold spike in the amount of Anisakis, or herring worms, in raw fish between 1967 and 2017.

And the results of eating one aren’t pretty.

Humans may interpret the nausea, vomiting and diarrhea they experience after eating the worms to be food poisoning, but these symptoms are in fact caused by the parasite invading the intestinal wall. The worms cannot live or reproduce for longer than a few days within a human intestine (although they can persist in marine mammals). Symptoms let up once they have died.

Despite its name, the worm is not unique to herring and can be found in a wide variety of squid and fish species. Anisakis generally finds its way into its host by first infecting shrimp or copepods, which in turn are eaten by larger fish, which in turn are eaten by humans.

Industry experts have been good at removing worms from food before it is made available for consumption — but with the worms’ numbers growing, it’s becoming more difficult.

“At every stage of seafood processing and sushi preparation, people are good at finding worms and removing them from fish,” study author and assistant professor at the University of Washington School of Aquatic and Fishery Sciences Chelsea Wood said in a press release[2]. A precaution concerned sushi eaters can take, Wood said, is inspecting every bite for worms.

While gross, the worms do not pose much of a health risk to humans — but they do to other animals.

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