Squeamish ceviche: chic raw fish dishes are tasty, but watch for worms

With raw seafood dishes — from poke to sushi to ceviche — growing in vogue, diners are increasing their odds of swallowing a parasite, experts say.

Most of the off-putting creatures are harmless, rare and killed by proper freezing of the fish by commercial sushi-grade fish suppliers.

But dozens of parasites are finding their way into peoples’ guts every year.

The B.C. Centre for Disease Control (BCCDC) which does not formally track parasite infections, put together available statistics for CBC. Reported cases of fish tapeworms in patients hit a 14-year high of 64 in 2015.

Experts say many more cases go undiagnosed. A growing taste for chic raw seafood dishes, and a desire to try to prepare them at home, potentially puts more British Columbians at risk of everything from parasites to food poisoning, if prepared wrong.

And experts say many diners don’t even realize it.

Tracking a tapeworm

Dr Linda Hoang had one patient who learned the hard way.

A few years ago, a Vancouver preschooler grabbed candy-pink, raw salmon slivers in a restaurant — meant for fondue — and gobbled them down, until her mother noticed.

Six months later she had a tapeworm, probably from the platter of freshly-cut fish, but the incubation period — up to six months long — often makes it difficult to determine the source, said Hoang, a BCCDC medical microbiologist.

That is one reason the issue often goes undiagnosed and underreported in B.C.

This metre-long tapeworm was found in the gut of a B.C. patient. (Dr. L. Hoang/BCCDC Public Health Lab)

“It’s hard to say ‘I acquired that from X restaurant because I ate sashimi there 6 months or a year ago,’ ” said Hoang.

The child’s parasite passed after two bouts of bitter medication, said Hoang, who can’t name the patient for privacy reasons.

“It wasn’t any fun for her.”

Raw fish risks

“Every time you eat raw fish it increases risk,” said Lorraine McIntyre, a food safety expert with the B.C. Centre for Disease Control.

Some of the worms found in fish remain hidden in a human host because they take months to incubate and require a gut biopsy to detect. The consumer may not even notice.

Many seafood lovers just keep on digesting the problem parasites. And people risk it because raw fish is yummy.

“People like to eat undercooked fish,” Hoang said, noting that salmon is often prepared rare.

“I was flabbergasted some of the cooking shows prepare salmon tartare or salmon tataki using fresh salmon and that’s a huge risk.”

Parasite experts urge home chefs to leave raw fish dishes to the experts.

Know your worms

It’s common to find parasites, especially anisakis, in fresh B.C. salmon, often turning up in supermarket fillets of cod or halibut, experts explain.

“Somewhere in the fillet there would be one of these organisms. They tend to be coiled,” said Dr. Gary Marty, a fish pathologist with the provincial ministry of agriculture.

A coiled herring worm or anisakis is seen in a Pacific herring caught near Alaska. The larva of this parasitic nematode is common in wild salmon. (Gary Marty/Ministry of Agriculture)

“They are just very common parasites. Studies show they occur in pretty close to 100 per cent of our wild salmon,” once the fish are big enough to harvest, he said.

If the worms are ingested alive, they can penetrate the human intestinal tract and cause an infection, even if the worms eventually die.

The effects can include severe stomach upset and,for some people, a serious allergic reaction.

And some creatures do not die.

Giant Japanese tapeworms — which can grow up to nine metres long — have been found in Alaska salmon.

So what can lurk in that delicious bite?

Watch the endoscopic removal of a fish parasite0:13

Tapeworm can cause an intestinal infection called diphyllobothriasis, which has afflicted 388 people in B.C. since 2002.

Anisakis worms or nematodes can also be picked up by eating raw or marinated fish. They can cause severe pain and vomiting. 

Deep freeze for safety

So how do sushi chefs make safe food?

If fish is uncooked, the only thing that will kill parasites is to freeze the fish in -20 C temperatures for at least a week. Most commercial sushi-grade seafood suppliers freeze at -40 C.

But many home freezers only go to -18 C.

Most commercial sushi-grade seafood suppliers freeze at -40 C. Most home freezers only go to -18 C. (Credit: iStock/Getty Images)

Vendors can check to see if there are worms in their fish by using a bright fluorescent light. Provincial inspectors allow for three “foreign objects” per 10 pounds of fish.

And what about ceviche? Marinating fish in citrus or other substances does not kill bacteria — or parasites, food safety experts explain.

Some commercially-made ceviche is flash-pasteurized to deal with this, but most is raw.

So enjoy that succulent citrus-tinged ceviche — just make sure to ask how the fish was prepared — and pass on anything too fresh, especially if it’s wiggling.

Does this look delicious? It is, but make sure it’s not moving. Parasites are not killed by the citrus juice that marinates the fish in ceviche. It looks cooked, but it’s not. (Stu_spivack/flickr)