As more people continue to get sick, Canadians are being warned yet again about an outbreak of norovirus and other gastrointestinal illnesses linked to undercooked and raw oysters from B.C.

The Public Health Agency of Canada (PHAC) said 22 more people have contracted the illness since Feb. 24, bringing the total number of cases since December to 289.

The vast majority of the norovirus cases have been in B.C., but some have been reported in Alberta and Ontario.

The agency is still looking for the cause of the contamination, according to a statement.

Farms closed

Last month, Mark Samadhin — director of PHAC's outbreak management division — said oysters are usually contaminated wth norovirus after coming into contact with untreated sewage in the water.

Four B.C. farms where oysters are harvested have been shut down as the investigation continues.

To avoid sickness, officials say consumers should cook oysters to an internal temperature of 90 degrees Celsius for at least 90 seconds before serving them.

The main symptoms of norovirus are vomiting, diarrhea and nausea. The illness usually resolves itself within two days without medical attention, but anyone with severe symptoms after eating shellfish is asked to call their healthcare provider.