VANCOUVER - Oysters contaminated with norovirus in British Columbia have become a costly problem and the issue magnifies a broader failure to keep oceans clean, says the provincial shellfish growers association.

Darlene Winterburn, the executive director with the B.C. Shellfish Growers Association, said the latest outbreak this spring will likely take a "significant" toll on the province's industry that is still trying to absorb $9.1 million in losses from last year.

"It's significant because a lot of people haven't recovered from last time, it's significant because branding is being affected and it's significant because the uncertainty is making particularly small guys very hesitant to reinvest," she said.

The Public Health Agency of Canada said 172 people reported getting sick in B.C., Alberta and Ontario in March and April after eating raw oysters. It follows an outbreak that started in November 2016 and sickened 400 Canadians over a five-month period, ending in March 2017.

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Norovirus, a gastrointestinal illness, causes diarrhea and vomiting and is easily spread from person to person with symptoms beginning as early as 12 hours after exposure to the virus, the agency said. Most people feel better after a day or two, although those with compromised immune systems can suffer from more severe symptoms.

The latest outbreak led the United States Food and Drug Administration to release a warning this week for consumers and retailers to avoid potentially contaminated oysters from Canada.

The U.S. warning said the raw oysters were harvested in the south and central parts of Baynes Sound, an area between Denman Island and Vancouver Island, and could be distributed in a number of states including Washington, New York, and Massachusetts.

The California Department of Public Health said about 100 people became ill with norovirus after consuming raw B.C. oysters.

The Canadian agency said the number of people reporting illnesses has been on the decline since April 27 and the investigation into the specific source of the contamination is ongoing.

Marsha Taylor, an epidemiologist with the B.C. Centre for Disease Control, said norovirus can contaminate various foods and oysters are typically affected when taking in sewage from water.

"(Oysters) filter the water that can be contaminated with norovirus and then they become contaminated with norovirus and then we unfortunately consume them and make ourselves sick that way," she said.

Sickness can be prevented by cooking oysters for at least 90 seconds at 90 degrees Celsius, Taylor said, adding practising good hygiene, such as hand washing, can help prevent the spread of the virus.

Winterburn said although the specific source of the contamination wasn't pinpointed in last year's outbreak, the current investigation is likely to be more conclusive, which can help prevent future incidents.

Farms in the area of Baynes Sound have been closed as a precaution while the investigation is underway, and Winterburn said oysters from other areas of B.C. remain safe for consumption.

"The vast majority of the industry is functioning as normal," she said, adding that unaffected farms are still seeing a decline in orders because of fears the problem is more widespread.

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Winterburn said these incidents point to a broader issue of polluted oceans.

"The oyster is telling us something," she said. "We need to be very cognizant of what it is we're putting into our water."

Sewage treatment practices, the quality of septic systems and illegal dumping from vessels are all areas Winterburn said improvements could be made that would prevent contamination and better preserve the marine environment.

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