The next time you eat sashimi, nigiri or other forms of raw fish, you should consider doing a quick check for worms, according to a team of researchers.

A new study led by scientists at the University of Washington found “dramatic increases in the abundance of a worm that can be transmitted to humans who eat raw or undercooked seafood.” 

“Its 283-fold increase in abundance since the 1970s could have implications for the health of humans and marine mammals, which both can inadvertently eat the worm,” according to a report on the scientists’ work. They looked at a parasitic worm, known as Anisakis or “herring worm.” 

Their’s is the first study to combine the results of previous research to investigate how the global abundance of these worms has changed through time. The findings were published March 19 in the journal Global Change Biology.

“This study harnesses the power of many studies together to show a global picture of change over a nearly four-decade period,” said corresponding author Chelsea Wood, an assistant professor in the UW School of Aquatic and Fishery Sciences. 

“It’s interesting because it shows how risks to both humans and marine mammals are changing over time. That’s important to know from a public health standpoint, and for understanding what’s going on with marine mammal populations that aren’t thriving.”

Anisakis worm life cycle courtesy of the University of Washington. Click to enlarge.

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 disease, called anisakiasis or anisakidosis, is rarely diagnosed because most people assume they merely suffered a bad case of food poisoning, according to the U.S> Centers for Disease Control and Prevention.

The worms can’t reproduce or live for more than a few days in a human’s intestine, but they can persist and reproduce in marine mammals. The worms actually reproduce in the intestines of these animals and are released into the ocean via the marine mammals’ feces. Then the worms infect other creatures.

The worms can be up to 2 centimeters in length, about as long as a nickel is wide.

The worms actually reproduce in the intestines of these animals and are released into the ocean via the marine mammals’ feces, then infecting more animals.

“One of the important implications of this study is that now we know there is this massive, rising health risk to marine mammals,” Wood said. “It’s not often considered that parasites might be the reason that some marine mammal populations are failing to bounce back. I hope this study encourages people to look at intestinal parasites as a potential cap on the population growth of endangered and threatened marine mammals.”

The authors aren’t sure what caused the large increase of Anisakis worms over the past several decades, but climate change, more nutrients from fertilizers and runoff, and an increase in marine mammal populations over the same period could all be potential reasons, they wrote.

Co-authors are Evan Fiorenza, who completed the work as a UW graduate student; Catrin Wendt, a graduate student in the UW School of Aquatic and Fishery Sciences; Katie Dobkowski of Bates College; Teri King of Washington Sea Grant; Marguerite Pappaioanou and Peter Rabinowitz of the UW School of Public Health’s Department of Environmental and Occupational Health Sciences; and Jameal Samhouri of NOAA’s Northwest Fisheries Science Center.

This study was funded by Washington Sea Grant, the National Science Foundation, the Alfred P. Sloan Foundation and the University of Washington.

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