What is the food safety risk of eating raw or undercooked seafood?

Jim Chan.

Eating raw or undercooked seafood can post food safety risks

Raw seafood dishes have been popular and trendy in North America for the past few years. Celebirty chefs on cooking shows often prepare raw seafood dishes such as salmon tartare, poke, sushi, sashimi, ceviche, raw oysters and clams, tuna carpaccio, just to name a few. People with weakened immunity, elderly, pregnant women and young children are at higher risk for foodborne illness and should not eat raw or partially cooked food.


It is common to find parasites such as Anisakis, a parasitic nematode in fresh salmon. This parasite is pathogenic to humans and can cause Anisakiasis. Sometimes, parasites can turn up in supermarket fish counter on fillets of salmon, cod or halibut causing customer complaints to public health inspectors.

Most of the fish parasites are harmless and can be killed by proper freezing at commercial seafood processing plants as well as sushi-grade fish suppliers for fish products that are intended to eat raw. A commercial plant has the ability to freeze the fish at -20°C/-4°F temperatures for at least a week, and in most commercial sushi-grade seafood suppliers, they have equipment that can freeze fish to -40°C/-40°F in order to kill all the parasites in fish. Most of the home freezers at home can only freeze food to -18°C/0°F and should not be used as a process to kill parasites in fish.

Recently, British Columbia Centre for Disease Control (BCCDC) in Canada put together available statistics to show the number of parasite infections is hitting 11-year high in the West Coast of Canada, especially Diphyllobothrium or Fish Tapeworm. Cases investigated have been reported to eating raw or undercooked seafood in restaurants and homemade raw fish or seafood dish. Some of the parasites (flukes, worms) can remain hidden in a human host for a long time as they take months to incubate and require a gut biopsy to detect. Fish tapeworm infection can be acquired by eating both freshwater and ocean fish, especially ocean fish that spawn in freshwater rivers, such as salmon. Marinated and smoked fish can also transmit the worm. Therefor, cooking fish to a safe temperature or use fish that have been commercially treated to eliminate parasites is the best way to avoid infection.


Parasites such as roundworms, tapeworms and flukes can be carried by shellfish such as crab and crayfish. Fresh water crab may carry a nasty fluke parasite called Paragonimus that can infect human.

Video showing live parasites such as flukes in fresh water crabs

The parasites can also travel to other organs (lungs, heart) and cause an infection. The incubation period is usually long and it can take up to six to ten weeks to become sick after eating infected crabs. Symptoms can include coughing, diarrhea, breathing problems, abdominal pain, fever and hives. In some cases, the parasites carried by infected crabs can migrate from the intestines to the lungs and can cause lung infection. Avoid eating raw or undercooked seafood, especially when travelling oversea.

Among the different disease-causing viruses found in oysters, Norovirus is known to cause foodborne disease outbreaks worldwide, making raw oysters a high risk food item. Recently, a Norovirus outbreak linked to eating raw and undercooked oysters in Canada (Media article: CBC News Norovirus outbreak from B.C. oysters makes dozens sick in Canada). Public Health issued a warning about the risk of eating oysters from Canada’s West Coast as over 100 Norovirus cases have been reported that linked to eating raw or undercooked oysters. The majority of people affected have frequent vomiting and diarrhea.

Food safety tips: Shellfish such as oysters can become contaminated from the water before they are harvested. Avoid raw oysters from affected areas or cook oysters to an internal temperature of 90C/194F for at least 90 seconds to kill Norovirus or any other harmful pathogens. For more food safety information on seafood, please visit Canadian Food Inspection Agency – Fish and Seafood .