This study will make you think twice about ordering sushi

Due to the ongoing COVID-19 pandemic, millions of people are ordering delivery for dinner a lot more often. If you fall into that category and were thinking about sushi tonight, a new study has a pretty important warning for you. 

Researchers at the University of Washington have discovered an almost incomprehensible increase in the number of parasitic worms in the ocean capable of entering the human body via raw or undercooked seafood. Hold the spicy tuna roll, please.

These parasites, referred to as Anisakis or “herring worms,” are 283 times more populous than they were in the 1970s. Yes, you read that right. The dramatic increase in these worms’ numbers has health implications for both humans and other marine mammals. First, the worm enters a fish’s body in the ocean, then the worm makes its way into a human stomach when that fish is consumed by someone for lunch.

The study’s authors came to their conclusions by combining all prior research performed on the proliferation of these parasites over the past decades. All of that information was used to create one absolute report on the worms’ population explosion all over the world.

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

Despite being called herring worms, these little guys can be found in a variety of different fish and squid. In the unfortunate event that a worm makes its way into a human stomach, it rudely enters our intestinal walls and causes not-so-fun symptoms like diarrhea, nausea, and vomiting. Most of the time, people have no idea they’ve eaten a parasitic worm and simply assume they’re dealing with a normal case of food poisoning. 

To be clear, eating one of these parasites doesn’t condemn an individual to a lifetime of stomach problems. In the majority of cases, the worm dies in our intestines after a few days and symptoms disappear. Even with that fact in mind, though, these findings are unsettling, to say the least. 

After hatching in the ocean, these parasites gradually make their way through larger and larger beings. First, they usually infect small bottom-dwelling shrimp or copepods. The infected crustaceans are then eaten by smaller fish who eventually end up getting eaten by large fish. All the while, the worms are along for the ride.

How about some good news? Both seafood processors and sushi chefs are well versed in detecting these parasites while preparing food. So, the vast majority are found and disposed of before seafood hits grocery store shelves or restaurant kitchens.

“At every stage of seafood processing and sushi preparation, people are good at finding worms and removing them from fish,” Wood says.

There’s always going to be at least some human error, and with the huge increase in these worms’ presence in oceans all over the world, at least a few are bound to sneak by seafood inspections. Wood recommends that concerned sushi diners cut each roll in half and look inside to ensure there are no worms hiding. The parasites would be pretty hard to miss; they can grow to about the size of a nickel.

While all of this is definitely gross for us humans, and will probably change a few dinner plans, these parasites are mostly harmless to people. For marine mammals, like dolphins, whales, and seals, the worms are actually much more harmful. While the parasites can’t survive for long in our bodies, they thrive and reproduce in these animals’ intestines.

“One of the important implications of this study is that now we know there is this massive, rising health risk to marine mammals,” Wood comments. “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.”

All of this still leaves one burning question. Why in the world are there so many more of these parasites today than just a few decades ago? The research team can’t be sure, but they hypothesize that climate change, an influx of nutrients from fertilizers and other runoff, or a general increase in marine mammal populations are all plausible explanations.

“It’s possible that the recovery of some marine mammal populations has allowed recovery of their Anisakis parasites.” Wood concludes. “So, the increase in parasitic worms actually could be a good thing, a sign that the ecosystem is doing well. But, ironically, if one marine mammal population increases in response to protection and its Anisakis parasites profit from that increase, it could put other, more vulnerable marine mammal populations at risk of increased infection, and that could make it even more difficult for these endangered populations to recover.”

The full study can be found here, published in Global Change Biology.