Sushi: The amount of parasitic worms in fish multiplied by 283 in 50 years

A new study led by the University of Washington reveals a dramatic increase in the number of worms that can be transmitted to humans who eat saltwater fish, raw or undercooked seafood. The number of worms transmitted to humans has multiplied by 283 since the 1970s, and could have consequences for human health.

In this study, thousands of papers have examined the presence of a particular parasitic worm, known as Anisakis or “herring worm”. This study is the first to combine the results of these papers to investigate how the overall abundance of these worms has changed over time.

Contamination by a marine worm: symptoms of food poisoning

This study shows how the risks to humans and marine mammals change over time. Despite their name, herring worms are found in a variety of marine fish and squid species. When people eat live herring worms, the parasite can invade the intestinal wall and cause symptoms that mimic those of food poisoning, such as nausea, vomiting, and diarrhea. In most cases, the worm dies after a few days and the symptoms disappear. This condition, called anisakiasis, is rarely diagnosed because most people assume they’ve just had a bad case of food poisoning.

A chain of transmission from small crustaceans to fish to humans

After the worms hatch in the ocean, they first infect small crustaceans, such as small shrimp. When the smaller fish eat the infected crustaceans, the worms then transfer into their bodies, and this continues as the larger fish eat the smaller infected fish. Humans and marine mammals are infected when they in turn eat fish that contain worms. The worms cannot reproduce or live for more than a few days in a human’s intestine, but they can persist and reproduce in marine mammals.

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For sushi eaters who are concerned about these worms, it is recommended to cut each piece in half and search for the worms, with a 2cm plunger, before eating.

The study authors aren’t sure what’s caused the sharp increase in Anisakis worms in recent decades, but climate change, increased nutrients from fertilizers and runoff, and increased marine mammal populations during the same period could all be potential reasons, they said.


Evan A. Fiorenza et al: “It’s a wormy world: Meta-analysis reveals several decades of change in the global abundance of the parasitic nematodes Anisakis spp. and Pseudoterranova spp. in marine fishes and invertebrates,”. Global Change Biology (2020). DOI: 10.1111/gcb.15048


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