Studies show there are likely more ‘sushi worms’ in Alaska salmon and other fish than there used to be

Allison Little, a middle school assistant principal from Anchorage, has been fishing on the Kasilof River her entire adult life, so she’s used to seeing wormy parasites coiled up in fish on occasion. But last year, as she cleaned her dipnet catch, the worm situation edged into horror movie territory.

“There were so many in this one fish, I wasn’t quite sure what I was looking at,” she said. “I put salt on the fillet and they just started coming out everywhere. This was enough where I actually disposed of it.”

Lately, everybody’s got a gross-out story — wormy restaurant ceviche, live worms in grocery store fillets, backyard barbecues derailed by too many cooked worms coming off the grill. White “sushi worms” — in this case, species of nematodes in the family Anisakidae — are part of life in a place where filling freezers with fish is a cultural, economic and nutritional mainstay. But, in the last few years, fishermen, fish sellers, diners, chefs, scientists and home cooks say they are seeing more worms than they’re used to. It turns out, according to a couple of recent studies done by University of Washington researchers, their observations are right on.

Worms are on the rise. Anisakis worms in all saltwater fish and some cephalopods like squid increased 283-fold from 1978 to 2015, according to a 2020 study analyzing hundreds of papers.

“What we saw was an increase from an average of less than one worm per 100 fish — to more than one worm in every fish,” said Chelsea Wood, the Seattle-based parasite ecologist who oversaw the study.

Another study Wood supervised, published in April, looked only at Alaska salmon. It used long-expired canned fish that had been stored at the Seafood Products Association, a trade organization located in Seattle. The researchers dissected fish processed between 1979 and 2019 and found that worms increased in both chum and pink salmon. The study did not find increases in reds or silvers, she said, but that doesn’t rule out an increase, nor does it rule out a more recent uptick in parasites in those species. The reason for the spike in parasites isn’t definitive, she said.

“My suspicion is we have more now than we used to because we have more marine mammals than we used to,” she said.

The number of marine mammals — the primary hosts for the parasites — has steadily increased since the passage of the Marine Mammal Protection Act in 1972 and the moratorium on commercial whaling in 1986.

“Marine mammals were at a low ebb in the ‘60s and ‘70s and that’s exactly coincident with the rise in sushi worms that we’ve observed globally,” she said.

The sushi worm parasites start their lives in the intestinal tracts of marine mammals like seals and whales. Their eggs enter the water in feces, where they hatch. They then get eaten by crustaceans, like krill. Salmon and other fish eat those crustaceans. The parasites take hold in fish bodies and grow. Marine mammals eat the fish, starting the cycle over again. Humans who happen to swallow the parasites are not preferred hosts. The parasites can’t live for long in human bodies, but they can, sometimes, make people sick. Oftentimes they don’t cause problems at all.

“You’re probably eating quite a few worms without realizing it,” Wood said.

Should you eat fish if there are worms in it?

The parasites are present in lots of fish, including cod and halibut, but they’re easier to see in salmon because the white worms contrast with the flesh. (Farmed salmon don’t get sushi worm parasites. Their food is sterile and they don’t swim in the open sea.)

People sometimes ask Wood if they should throw a fillet away, she said. She says no. If the worms are well frozen or cooked, they don’t pose a threat.

“I tell them we’re lucky, the worms in that fish are telling them that the fish came from a healthy ecosystem, that it came from a place that’s being well managed and not overexploited,” she said. “I know that they’re super gross to see, but they’re actually a really good sign.”

Deenaalee Hodgdon, who has been subsistence fishing on the Nushagak River, said this year they are seeing worms, but not much more than usual.

“I wouldn’t say it’s anything over the average, but the average is different in my lifetime than if you asked my mom,” Hodgdon said.

They have been giving a lot of thought to how Alaska Native people coexisted with parasites for generations, eating fish with plants like wormwood, called “caiggluk” in Yup’ik, which has anti-parasitic properties.

“Traditionally we would have been intaking more of our traditional medicines and other berries that help with the natural cleansing of our system,” Hodgdon said

The freak-out factor

Alaskans are committed salmon eaters. Intellectually, many know that worms can be killed with proper preparation, but sometimes there’s a visceral aversion that’s a little hard to get over, said Matt Akers, an Anchorage pilot and sportfisherman. He’s haunted by a time when he was processing red salmon roe for bait a couple of seasons ago.

“I was pulling worms out by the handful — I was dry heaving,” he said. “I can get past it, it’s kind of part of it, but at the same time, it’s definitely something that’s kind of turned me away a bit on salmon.”

Daniel Buitrago, one of the hosts of the Alaska Wild Project podcast, which has a large audience of fishermen and hunters in Alaska and the world, said that he’s also heard about and observed the increase in worms. The non-Alaskan worm freak-out factor also seems higher, he said. A couple years ago he posted a video with friends, eating ceviche riverside, made from the first salmon catch of the season. It went semi-viral, with hundreds of grossed-out responses mostly from Outside.

“The comments were insane, literally every comment was like you’re gonna die, you’re gonna get worms,” he said. “I just kinda laughed it off — this is not the first time and it won’t be the last time we’re gonna do this. It’s never affected any of us or anyone we know.”

Last year, some of Alaska’s salmon also made a stir when people started posting videos of live worms in fillets purchased at places like Costco, Whole Foods and Publix — garnering millions of views and horrified comments. The Alaska Seafood Marketing Institute said in an email that worms are naturally occurring and that consumers are safe if they prepare fish according to safety guidelines. The organization wrote that the TikTok videos “highlight the need for science-based consumer education about where their seafood comes from and how to properly prepare it.”

Will the worms make you sick?

Even if there are more worms out there, Louisa Castrodale, acting infectious disease program manager with the state, said that reports of people getting infected by fish worms are “relatively rare” and that there has been no documented increase.

When the live parasites enter the human digestive system, they can stimulate the immune system, causing abdominal pain, vomiting and diarrhea, Wood said. By some estimates, sushi worm parasites may be responsible for a third of what people think is food poisoning caused by fish, she said. In very rare circumstances, the parasite can stay in the lining of the intestinal tract, causing symptoms similar to appendicitis, sometimes requiring medical attention.

People who handle a lot of raw fish can also become allergic to the parasites. This is sometimes perceived as being allergic to salmon, Wood said.

“There’s actually some evidence that people who work in seafood processing plants can wind up having really serious allergies because they get exposed so much,” she said.

Jayde Ferguson, fish pathologist with the Alaska Department of Fish and Game, said they have been getting some recent reports about worms in fish, mainly red salmon, though there’s no way to draw conclusions about whether that indicates an increase. Fishermen have also contacted him to see if there are ways to process the fish to reduce the worms, like removing the belly. Though some fish worms do migrate into the meat after the fish dies, studies have shown that’s not true here, he said.

“The best thing to do is keep them chilled and process them quickly,” he said.

What’s the best way to avoid salmon worms?

Ferguson said people can also inspect the meat before cooking it and remove worms. To really reduce the risk of infection, the government recommendation is to cook salmon to 145 degrees or freeze it in a regular residential freezer for a week, he said. Pickling or curing the fish isn’t effective. Studies have found that parasites have lived for almost a month in a brine with more than 20% salinity. (Some fishermen say salting the meat causes worms to be more active and easier to see.) Commercial businesses with powerful freezers are required to “flash freeze” fish being served raw, bringing it to a very low temperature to kill parasites. Wood has doubts about that.

“I will tell you I have bought a lot of sushi-grade fish in markets and found anisakids in all of it — I don’t know if there’s a lot of noncompliance or they are more resistant to freezing than we know,” Wood said. “They are hardy worms.”

Shane Moore, a longtime chef in Anchorage, late last month was working on a gillnetter in Bristol Bay, fishing for red salmon. He doesn’t cook his fresh salmon to 145 degrees at home, instead preferring to remove it from the heat at about 108 or 110 degrees. After that, the temperature usually rises to about 120 degrees, he said, which to him is about perfect. He’s handled countless fish in his career, both fishing and cooking, and thinks of the worms as pests like aphids. The risk of encountering them is worth the reward of eating fresh wild fish in the Alaska summertime.

“Relax,” he said. “It’s just part of it.”