The Public Health Agency of Canada (PHAC) is investigating gastrointestinal illnesses in British Columbia, Alberta and Ontario.
Public health partners in these provinces have linked cases to eating oysters from British Columbia.
Norovirus confirmation
Although not all cases have been tested, norovirus infection has been confirmed in some of them.
Shellfish like oysters may be contaminated by sewage in water before they are harvested.
Cases are from British Columbia (143), Alberta (35) and Ontario (24) and individuals became sick between December and January.
Illnesses can be avoided if oysters are cooked to an internal temperature of 90° Celsius/194° Fahrenheit for a minimum of 90 seconds and hand washing and food safety practices are followed.
Oysters that do not open when cooked should be discarded.
Noroviruses are very contagious and can spread easily from person to person.
People usually develop symptoms of gastroenteritis within 24 to 48 hours but they can start 12 hours after exposure.
It is the first outbreak investigation into illness linked to raw oysters in Alberta since 2015, when 19 cases of Vibrio parahaemolyticus infection were linked to them.
Ontario investigation
Dr David McKeown, Ontario's acting chief medical officer of health, said a rise in illness may be associated with oysters from British Columbia.
"Since January 2017, 24 cases of gastrointestinal illness consistent with norovirus have been reported in Ontario from individuals who reported eating raw or undercooked oysters,” he said.
“Foodborne outbreaks of norovirus illness can occur when food is contaminated with the virus. Shellfish such as oysters can become contaminated from the water before they are harvested.
“The common symptoms of gastrointestinal illness are nausea, vomiting, diarrhea and stomach cramps. In most healthy people, acute diarrhea and vomiting should resolve in one to two days.
“Symptoms may last longer in some people. Anyone experiencing these symptoms after eating oysters should seek the advice of their health care provider."
An outbreak of norovirus in oysters is also ongoing in New Zealand with 11 people ill.
Earlier this year, France reported an outbreak from oysters in the south of the country and late last year Netherlands noted cases from oysters imported from a different region in France.