As the holiday season is in full swing, raw oysters are often favorites on party menus.
But after dozens of people in British Columbia recently got sick after eating raw oysters, many may be wondering if they should avoid shellfish this year for safety reasons.
The BC Center for Disease Control Thursday posted a warning that between November 1 and December 18, 64 cases of “norovirus” gastrointestinal illnesses were reported in the province, after people ate raw oysters from local restaurants and retailers.
Some people went to emergency rooms with symptoms of diarrhea, vomiting or stomach pain, but no hospitalizations were reported, health authorities said.
“We remind you of the risks associated with consuming raw or insufficiently cooked oysters. Cooking destroys viruses and bacteria and reduces the risk of illness,” the British Columbia Centers for Disease Control said in a social media post.
Noroviruses are a group of viruses that cause stomach problems like diarrhea and vomiting. They are spread easily through contact with the stool or vomit of infected people.
Some foods can also be contaminated from the start, such as oysters, which can contract the virus in sewage before being harvested, according to the Public Health Agency of Canada (PHAC).
Sylvain Charlebois, director of the agri-food analysis laboratory at Dalhousie University, said that because oysters can be a holiday tradition for some, it is important to remain vigilant when eating them.
“The problem with the holidays is people treat themselves to oysters and their bodies may not be used to dealing with some of the viruses they ingest,” Charlebois said.
“During the holidays, we’re often more tired and drink more than usual, so getting sick from a virus tends to hit harder at this time of year,” he said, adding that when it comes to norovirus, children, the elderly and pregnant people are particularly vulnerable.
So before you grab that oyster, here’s a little more of what you need to know about oysters, bacteria and viruses.
Raw oysters can be a risky holiday treat because they can contain harmful bacteria, viruses and toxins that can lead to foodborne illness, said Keith Warriner, a food safety professor at the University of Guelph.
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Oysters feed by filtering water and microorganisms, which can lead to a buildup of toxins in their tissues, he explained, adding that it’s not just oysters: all bivalve shellfish (like mussels and clams) can absorb harmful substances from the water they filter.
“What happens is when you get polluted water, like sewage discharge, septic tanks…the oysters concentrate these pathogens. Norovirus is one, and the other famous one is vibrio,” Warruner said.
“Once they get in there, it’s hard to get out. There’s a process called depuration when you put these shellfish in these tanks and they try to flush them with water. Sometimes they use disinfectants, but they have to keep the oysters alive, because usually you don’t cook the oysters.
So, Warriner said, what happens in the sea can end up on your plate and make you sick, especially when it comes to norovirus, which is hard to kill and very contagious.
The good news is that you can eliminate norovirus by cooking oysters if you choose not to eat them raw.
Although British Columbia health officials have linked the latest outbreak to “norovirus” gastroenteritis, Warriner said he wouldn’t be surprised if vibrio was also involved.
Vibrio is a type of bacteria commonly found in warm coastal waters and is often associated with shellfish, such as oysters. It thrives in warmer waters, especially when temperatures exceed 15°C.
According to PHAC, with rising ocean temperatures due to climate change, vibrios can multiply more quickly in the water where oysters are harvested.
“Vibrio used to be this kind of thing that you found in the Gulf of Mexico, where the waters are warming, now you’re picking it up on the coasts of British Columbia and Nova Scotia,” Warriner said .
If someone ingests the bacteria, symptoms can include watery or bloody diarrhea, stomach cramps, nausea, vomiting, fever and headache, he said.
Oysters with some caution
If you plan to enjoy oysters or any other raw shellfish during the holidays, Charlebois advises finding out where they come from.
“The most important thing is to know the source…is to deal with a trustworthy source. Unfortunately, some oyster farms are from time to time exposed to certain contaminants. So it’s important that consumers know where their oysters come from,” he said.
The risk is lower if the oysters are part of a short distribution chain, as opposed to those that have traveled long distances, which can increase the risk to ensure their freshness, Charlebois explained.
British Columbia, New Brunswick, Prince Edward Island and Nova Scotia are prime locations for oyster farming in Canada, although they are also imported from countries like Thailand.
Charlebois added that while contamination of oysters can occur, it is not “too common.”
“But you have to be careful. Norovirus is obviously a very contagious virus. Extreme caution should be used, especially with people with weakened immune systems, the elderly, pregnant women and children.
Since eating oysters can still pose a contamination risk, Warriner suggests you can skip them altogether if you prefer.
“If you told me which region produces the safest oysters, I couldn’t even tell you, because every place I know that produces oysters has a water quality problem,” he added. .