Norovirus outbreak linked to BC oysters: What you need to know

B.C. oysters have been linked to 221 illnesses in Canada in the past two months.
B.C. oysters have been linked to 221 illnesses in Canada in the past two months. Phil Sears / File / AP Photo

British Columbia oysters have been linked to more than 200 cases of norovirus since late December, with illnesses reported in multiple provinces.

Investigation of the contamination is now being led by the Public Health Agency of Canada but the main question remains: how did the oysters become contaminated.

READ MORE: 5 things you need to know about norovirus

The exact source of contamination has not yet been pinpointed but experts said there’s a good chance sewage was the culprit.

“The vehicle is usually contaminated water …. discharge of household waste, municipal waste, untreated into a water source,” said food safety expert Rick Holley, distinguished professor emeritus in food science at the University of Manitoba.

Oysters, along with clams and mussels, are filter feeders, meaning they filter their food out of the water around them.

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“They pick up a lot of particulate nutrient material out of the water — including algae, as well as these noroviruses that impact humans — and concentrate them in their bodies. And then when we eat the mussels and the oysters without cooking them, we get sick.”

B.C. is Canada’s leading oyster producer, producing an average of 5,600 tonnes annually.

Considering the number of illnesses, the current norovirus outbreak would be considered large scale, said Mark Samadhin, director of the outbreak management division, Public Health Agency of Canada.

“There’s no clarity yet on what’s been introducing norovirus to the oysters,” said Samadhin.

“Norovirus is an intestinal pathogen in humans, so when we see it in something like oysters, for instance, the general consideration there is somehow, some sort of sewage contamination may have happened.”

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While Samadhin was careful to stress the link is not confirmed, past cases show that “when we see norovirus in oysters it’s usually some kind of a sign that the oysters have been contaminated with sewage.”

The suspect oysters came from multiple harvest sites; illnesses in B.C., Alberta and Ontario have been linked to the outbreak.

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Holley, who is not involved with the investigation, suspects cross-contamination in holding tanks contributed to the widespread outbreak.

“You’ve got oysters in several different areas in the east and west coast of Vancouver Island causing the problems,” said Holley. “Now we know that the oysters are held in bulk in large communal tanks and it’s perhaps during distribution, after harvest, that provides an opportunity for the oysters to cross-contaminate each other.”

A problem first detected in December, Holley said it’s “frustrating” the outbreak has been going on for so long.

The Department of Fisheries and Oceans temporarily shut down four B.C. harvest sites, Samadhin said.

Moving forward, changes to filtration systems or water temperature of holding tanks could be looked at, Holley said. More importantly, he wants to see the contamination be prevented in the first place, by way of proper maintenance and upgrades to municipal water treatment facilities.

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“The priorities need to be slightly changed in order to make sure these problems are stopped and they don’t recur in the future,” said Holley. “Because it will recur if we don’t make the proper investment in the systems to prevent discharge of sewage into water systems.”

Samadhin agreed these situations can provide learning opportunities.

“Any sort of foodborne outbreak, any sort of safety challenge or issue that we deal with I think is a good vehicle for prompting change,” said Samadhin.

Consumer caution

Consumers should always take caution when it comes to consuming any raw shellfish, Holley said, as risks certainly exist. But you don’t necessarily need to ban it from your diet.

“When you take a look at the number of folks that consume raw oysters in North America and the number of cases [of illness] that occur in proportion is not all that large,” said Holley.
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READ MORE: 5 things a Canadian food safety expert will never eat

Both experts said cooking oysters will greatly reduce the risk of illness, along with proper food handling.

Holley said those with underlying medical conditions should think twice before consuming raw shellfish.

“For other healthy individuals, you are increasing your risk of getting ill somewhat, but I think that for someone who really likes raw oysters it shouldn’t really stop you from eating them.”

Holley said people are taking just as much of a risk when eating pre-cut bagged salads.

Food contaminated with noroviruses often look, smell and taste normal. Norovirus rarely results in death, Holley said, but it is very “uncomfortable” and spreads rapidly.

Typical symptoms include nausea, diarrhea, vomiting and stomach cramps; symptoms can also include low-grade fever, chills, muscle aches, headache and fatigue. Illness usually last for one to three days.

You are contagious while sick, and for as long as a few days to two weeks after recovery.

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