Oysters from B.C. have been linked to 22 additional cases of highly contagious norovirus or other stomach ailments in three provinces.

The Public Health Agency of Canada says 289 cases of gastrointestinal illnesses were under investigation as of Monday.

A public health notice issued by the agency says that includes 201 cases in B.C., 48 in Ontario and 40 in Alberta.

The illnesses are linked to raw and undercooked B.C. oysters but the agency says the cause of the contamination remains unidentified.

It says four oyster and shellfish farms have been closed but an investigation is underway in other harvest areas because illnesses continue to be reported.

Officials say the risk is low and illnesses can be avoided if oysters are cooked to an internal temperature of 90 C for at least 90 seconds, and proper hand washing and food safety practices are followed.

The Canadian Press