Using fresh-off-boat salmon for your homemade sushi could land you in the emergency ward if you don’t know what you are doing.

“Some people buy the fish in Steveston and end up in the hospital,” said Taka Omi, sushi chef at the Fairmont Pacific Rim Hotel. “The fishermen say ‘sashimi grade’ and people think it’s so fresh there is no problem.”

The truth is that wild salmon and mackerel — two species widely used for sushi — have to be deeply frozen to kill harmful parasites before they can be eaten raw. And not just frozen in the freezer compartment of your fridge at home, but cooled to at least -20C for a week or more. That is likely far colder than you can hope for from your crowded home freezer.

“Please don’t freeze it at home,” said Omi, who runs classes for home sushi cooks. “Commercial suppliers freeze to -40C and that’s what we use.”

An Alberta man recently had the misfortune of hosting a rare parasitic worm from raw fish he bought at a grocery store.

Medical staff were stumped when the 50-year-old man showed up in the emergency room in August 2014 in extreme pain with perpetual vomiting, doctors reported in a paper published last month.

“This is such a rare, unusual etiology, I don’t think most people would put it too high on their list,” said Dr. Stephen Vaughan, an infectious disease consultant with a special interest in tropical medicine.

An X-ray and CT scan showed irregularities in the man’s stomach just hours after he made himself sushi at home with raw wild salmon he bought at a Calgary Superstore.

When a gastrointestinal specialist sent a small camera down his throat and into his stomach, what he found was the stuff of nightmares.

Worms, about a centimetre long, were chomping their way through the man’s stomach lining. Doctors plucked a few of the larva out using endoscopic forceps, Vaughan said.

A microbiologist identified the worms as anisakis, which can infect people who eat raw or undercooked seafood, the doctors report in the Canadian Journal of Infectious Diseases and Medical Microbiology.

Trout, salmon, pike and sea bass may also be infested with fish tapeworm, which can cause an intestinal infection called diphyllobothriasis. More than two dozen people have been diagnosed with diphyllobothriasis in B.C. during the past decade.

The value of freshness for sushi varies from species to species and few fish are served in their natural raw state, said Omi.

Only large species of tuna are exempt from freezing requirements in B.C., according to the B.C. Centre for Disease Control. Yellowfin and Bluefin (called ahi and toro on sushi menus) are highly prized and perfectly safe to eat when fresh.

For other species, sushi chefs serve previously frozen fish or use a variety of techniques to ensure that their fish has the right flavour and texture, while controlling pathogens that can be harmful to humans.

“Some fish we have to cure with salt and sugar for three days and that ensures the parasites are going to die,” said Omi. Other traditional curing techniques use kelp, which has a natural antimicrobial effect.

rshore@postmedia.com

with file from Janet French, Edmonton Journal