Some brands of B.C. oysters shouldn't be consumed because they may be contaminated with foodborne pathogens, the Canadian Food Inspection Agency (CFIA) warned Friday.

The Effingham extra small raw oysters, harvested from Effingham Inlet on Vancouver Island's west coast, have been distributed in British Columbia, Alberta and Ontario – but may have also been shipped out to other provinces.

It is believed the shellfish were contaminated with norovirus from the water. Norovirus is commonly transmitted by faecally contaminated water and person-to-person contact.

The CFIA said Friday there have been upwards of 20 illnesses already reported. Two or three of the people ate the oysters at a "community-type" event, and the remainder were consumed at restaurants.

The oysters were processed and packaged by Albion Fisheries of Vancouver (lot 172688 and 172929) or Sea World Fisheries (lot NY-OY-10091401). All of the packages contain five dozen oysters in each bag.

Guy Dean, Vice-President of Albion Fisheries, told that the recall affects about 300 dozen oysters in two of its lots.

"Most of that has been sold over the past two weeks so the vast majority would have been consumed already," he said.

As a part of the Ocean Wise sustainable seafood program, the company supplies a large quantity of shellfish and other products to local Vancouver restaurants.

Dean says there is no risk to the public going forward, adding that its lotting system allows them to pin down exactly where the oysters originated.

"It's important to understand we carry 30 different oysters so it's a very small number," he said.

The CFIA has taken samples of the oysters for analysis.

The oysters may also have been sold at some retail seafood counters. The CFIA says consumers who are unsure if they have the affected product should check with their retailer or supplier.

Vancouver Coastal Health, British Columbia Centre for Disease Control, Public Health Agency of Canada (PHAC) and Health Canada are also investigating.

The CFIA warns that the product may not look or smell spoiled but eating food with these organisms can cause food poisoning.

Symptoms can include nausea, vomiting, abdominal pain, fever, headache, dizziness and neck stiffness. Pregnant women, young children and the elderly are particularly vulnerable.

For more information consumers and industry can call the CFIA at 1-800-442-2342 / TTY 1-800-465-7735 (8:00 a.m. to 8:00 p.m. Eastern time, Monday to Friday).

Related: More information