An outbreak of norovirus linked to B.C.-harvested oysters is now under federal investigation.

The Public Health Agency of Canada (PHAC) says it has taken on a leadership role in the investigation, now that cases have been reported in Alberta and Ontario, as well as B.C.

As of Feb. 14, the agency says it's aware of 221 reported cases of norovirus connected to B.C. oysters.

"We knew in November-December that there were cases popping up in B.C., but it wasn't until the middle of January or so ... that we started seeing or hearing about other cases in Ontario and Alberta," said Mark Samadhin, director of PHAC's outbreak management division.

"We know that it's oysters from B.C., but beyond that, we don't know what's contaminated the oysters."

Samadhin said local investigations are still being carried out by provincial health authorities, the Department of Fisheries and Oceans (DFO) and the Canadian Food Inspection Agency (CFIA), but PHAC has taken on a coordinating role in the investigations now that the outbreak is multi-jurisdictional.

Source hard to pinpoint

Samadhin said oyster-related norovrius outbreaks of this scale are rare in Canada.

He said this outbreak does not seem to come from one particular source. He said the DFO and CFIA have shut down multiple farms for the short term after testing positive for norovirus.

Samadhin said that when oysters are contaminated with norovirus, it's usually due to contact with untreated sewage, but the investigation has yet to prove the exact cause of the current outbreak.

Norovirus is a relatively common virus that can be contracted from many sources, Samadhin said — including those already infected, which can make it hard to find the original source.

"Those secondary transmission cases would've contracted the illness from somebody who was ill and not necessarily from direct contact with the food," Samadhin said.

Samadhin said the nature of norovirus can make it hard to pin it to any one source, but health officials have shut down several farms that have tested positive. (Getty Images)

Farmers concerned

Keith Reid, owner of Odyssey/Stellar Bay Shellfish in Deep Bay, B.C., said his farm was closed for about a week-and-a -half while officials investigated before giving him the all clear.

Though his sales remain strong, Reid says outbreaks like this are troubling due to the difficulty in tracking down the source.

"There are so many things shellfish are exposed to before they're finally eaten, so it's really hard to trace exactly where this is coming from," He said.

"It's important as an industry that we put product into the marketplace where we're confident that, no, this isn't going to make you sick."

What to do

Samadhin said the best way to avoid contracting norovirus from shellfish is to follow proper food safety practices.

This includes ensuring shellfish is cooked all the way through before eating it, keeping raw food separate from cooked food, and to wash your hands thoroughly — particularly if you've had contact with someone who is ill themselves.

Symptoms of norovirus include nausea, vomiting and diarrhea. (It's the same virus that causes the "winter vomiting bug.")

Samadhin said people generally recover from the illness in a day or two and often don't seek medical attention.

But he said that if anyone gets sick after eating oysters, they should contact a health care provider and ensure that the case is reported to their local health authority.

PHAC's full public health notice regarding the outbreak can be found on its website.

With files from Greg Rasmussen.