Calgary doctors say they've treated what's believed to be the first case of stomach worms resulting from the eating of raw fish purchased at a Canadian grocery store.

Dr. Stephen Vaughan, who works out of the South Health Campus in Calgary, reports the findings as lead author of a "clinical vignette" in the Canadian Journal of Infectious Diseases and Medical Microbiology.

The report says a 50-year-old man arrived at the emergency department complaining of vomiting and severe stomach pain, one hour after eating raw salmon.

"He made his own sushi at home and obviously it wasn't prepared properly," Vaughan tells CBC News.

The fish the man purchased was not sushi-grade, a process that eliminates parasites through freezing and cold storage.

"Within six hours after eating it he developed severe, intense abdominal pain, some of the worst that he could describe."


An Alberta man is the first known person in Canada to be infected by a rare, parasitic worm after he ate raw salmon purchased at a grocery store. (Shutterstock)

As time went on, the man developed a fever of 39 C and continued to experience severe abdominal pain.

After running numerous tests, doctors eventually diagnosed the problem by sending a small camera down the man's throat and spotting the worms at the centre of small ulcers in his stomach.

Some of the worms were removed and identified as anisakis, a type of parasite that lives in fish and aquatic mammals and leads to a condition known as anisakiasis.

1st case of its kind

"To our knowledge, this is the first case of anisakiasis acquired from raw 'wild salmon' purchased from a Canadian supermarket," the doctors wrote in their report.

"Humans become infected by eating raw seafood in dishes such as sushi, sashimi, ceviche, lomilomi, or other undercooked fish and squid dishes," the report said.

"Although a skilled sushi chef will recognize the distinctive 'watch coil' appearance of the larval worms (approximately one centimetre to two centimetres) in raw fish, individuals preparing their own sushi may not, and may, inadvertently, become infected after ingestion of the larval nematodes."

Freeze your fish

To prevent the parasitic infection, amateur sushi chefs are advised to freeze raw fish for seven days at –20 C, or at a lower temperature for a shorter period of time.

The doctors note that sushi prepared in Canadian restaurants and supermarkets is "very unlikely" to spread parasites because it is typically flash-frozen to –35 C for at least 15 hours.

Vaughan says the risk, which exists only in wild salmon, is relatively low.

He says while some surveys have suggested up to 20 per cent of wild salmon can be infected in certain areas, he would speculate the risk of buying one from a grocery store is much lower, in the one to five per cent range.

"Increasing reports of acute anisakiasis will likely occur in the next few decades given the growing consumption of sushi and sashimi worldwide," the doctors conclude in the report.

Vaughan says his patient had no lasting effects.

"He had almost immediate relief of the pain after extraction of the worms and he has been perfectly fine since that time."