Fish are infected with 283 times more parasitic worms than they were 40 years ago. Anisakis worms can infect a variety of marine fish and squid species, as well as marine mammals such as whales and dolphins – and can be present in fish used for sushi.

Chelsea Wood at the University of Washington in the US, and her colleagues analysed the abundance of Anisakis, or herring worm between 1978 and 2015. The team gathered data on the average number of parasites per fish from 123 studies – which included 56,778 fish across 215 species – and found a 283-fold increase over nearly 40 years.

Anasakis starts its life cycle in the intestines of marine mammals, is excreted into their faeces and then infects fish, small crustaceans or krill in the larval stage.

“If eaten by fish they go on to form a cyst in the muscle tissue of that fish,” says Wood. When the fish gets eaten by the marine mammal, the life cycle recommences.

Humans can also contract these parasitic worms by consuming infected fish that is raw, smoked, or improperly frozen.

“When they enter the intestine of a human, it’s a great disappointment to the worm. They’re not going to be able to complete their lifecycle there,” says Wood.

In humans, the worm dies, in the process initiating an immune response that can cause nausea, vomiting and diarrhoea.

Fish consumers need not worry unnecessarily. Food processors and sushi chefs are skilled at spotting and removing these worms, says Wood. “I still eat sushi all the time,” she says.

The reason for the increased abundance of these worms is unclear, but Wood says that it may be linked to the rise in marine mammal numbers from the 1970s onwards, after the introduction of protections against hunting. Warming seas could also increase the rate of Anasakis reproduction, she says.

“We want to try and minimise the number of worms that people are encountering in their sushi dinner,” says Wood. “That is going to become more challenging into the future as we get these increasing abundances.”

Journal reference: Global Change Biology , DOI: 10.1111/gcb.15048

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