Should a Rise in Food Poisoning Worry Oyster Lovers?

Credit: Eugene Mymrin / Getty Images

In the tiny seaside Canadian town of Tofino, hundreds gathered in November for the annual Clayoquot Oyster Festival. They celebrated the start of the harvest by slurping down 11,000 bivalves over two days of revelry. But for around 100 festival-goers, food poisoning from tainted raw oysters made the next 48 hours decidedly less fun. Since then, 221 more people across Canada have fallen ill from consuming British Columbian Oysters, leading the Canadian government to issue a public health notice last week telling citizens to cook their oysters as a precaution.

Meanwhile, across the continent, researchers from the University of New Hampshire announced they’d discovered a new strain of bacteria responsible for rising incidences of food poisoning in Atlantic oysters, and that global warming is contributing to the increase.

These are edgy times for raw-bar lovers. Yet these two causes of food poisoning are pretty different. And understanding the facts behind them both will provide you with guidelines for when it’s safe to eat oysters raw and when it’s necessary to cook them.

The Difference Between the Outbreaks

While the end result may look very similar (vomiting, diarrhea), the causes of oyster-related food poisoning are quite different: One is a virus and the other comes from bacteria.

Canadian health officials are currently dealing with an outbreak of norovirus, traced to polluted water. “We haven’t concluded our investigation yet, but our best guess is raw sewage contamination,” says Mark Samadhin, director of outbreak management for the Public Health Agency of Canada.

On the other hand, professors Cheryl Whistler and Stephen Jones at the University of New Hampshire are studying a newly discovered bacteria strain, vibrio parahaemolyticus, that’s responsible for rising incidences of people getting sick from oysters during the last few years.

So, with the B.C. oysters, it’s a discreet outbreak connected to a contamination, and once that contamination is cleared up, the oysters will be good to go again. Since the bacteria is naturally occurring in the water, the health risks surrounding it will be managed through good harvesting and handling practices by oystermen and purveyors.

Is Global Warming a Culprit?

Historically, the cold waters of the Northeast aren’t an ideal locale for vibrios to thrive. They’re more common in warmer water. But with sea surface temperatures on the rise, these bacteria can now gain a foothold where they couldn’t before. “No matter what you think about global warming, 2012 was the warmest year on record in the Northeast going back 150 years in terms of sea surface temperature,” Jones says. “And that coincided with the first outbreak.”

Knowing the importance of keeping the oysters cold to keep vibrio at bay has helped change harvesting habits to make shellfish safer for consumers. Most of the time, that means making sure the oysters never warm up after harvesting. “Most shellfishmen put them in a cold slurry immediately, so it arrests the growth of vibrios,” Jones says. “In Massachusetts, they move them into colder waters for a week before harvesting, and that really drops the incidences.” If the oysters are pulled from cold waters, then stay cold all the way until you eat them, vibrios have less opportunity to multiply. But those conditions are riskier when the oceans are warmer than usual.

What about what’s happening in Canada? Some articles have mentioned a loose connection between the norovirus outbreak and global warming, but that’s less likely in the B.C. incident. “Norovirus [is] not restricted to warm water, so norovirus can end up in the water and survive there whether it’s warm or cold,” Whistler says.

Should You Stop Eating Raw Oysters?

In Canada, because of the norovirus outbreak occurring right now, it’s best to cook oysters from B.C. waters until public health officials give the all-clear. You won’t be able to rely on your senses to determine if an oyster is off. The norovirus won’t cause it to smell bad or look weird.

With oysters from the Northeast, properly harvested and handled oysters will be safe to continue eating raw. Part of the reason we’re seeing increased incidences of food poisoning (the CDC estimates 45,000 annually from vibrio in oysters) is not just warming waters allowing for a harmful bacterium to grow. It’s that we’re just eating so many more oysters. In 2007, $81.5 million worth of oysters were harvested in America, and by 2013 that jumped to $157.3 million, according to the National Oceanic and Atmospheric Association. “You’re getting this higher rate of consumption,” Jones says. “Maybe the rate of disease is the same as the past, but there’s another factor amplifying that signal.”

That doesn’t mean you should throw caution to the wind. Whistler has some advice for her fellow mollusk lovers. “I’d suggest people enjoy eating oysters during cooler months. Definitely don’t buy oysters from some guy on the side of the road with just a cooler,” she says. “If you have an underlying illness, like liver disease or are on medication that compromises your immune system, you should consider oysters to be very risky. And if you’re really freaked out about it, just throw them on the grill until they pop open and you’re good to go.”

What You Should Do If You Get Sick

Listen, getting food poisoning happens, sometimes even from the best of places. In most cases you’re going to have a rough couple of days and that’s about it. Samadhin says you’ll start to feel the onset within 10 to 48 hours. “The illness can last one to three days," he says. "If it persists, visit a qualified healthcare practitioner. Make sure you stay hydrated. Food poisoning can really dehydrate you.”

Whistler would like to see more people report their food poisoning, because it could help prevent future outbreaks. “If you do become ill, go see your doctor, because if we know what strains are making people sick,” she says, “it helps us develop the best strategies to monitor the strains in the harvest areas before they get into the food chain.”