About 12,000 people in Briton are poisoned by contaminated oysters each year; 11,800 of which are due to norovirus, according to the researchers at the Centre for Applied Marine Sciences on Anglesey.

According to recent findings from two studies “more than two-thirds of the shellfish on sale is infected with the contagious norovirus.” Scientists at the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) checked more than 600 samples sold in supermarkets over a 12-month period.

In the U.S., officials are concerned with an unusual strain of norovirus involved in an outbreak traced to raw oysters in British Columbia because people in California are now sick.

The California Department of Public Health said that about 100 people in the state have reported becoming sick with symptoms of norovirus after eating raw oysters from British Columbia. It was during this same time in which 172 people in Canada became ill after eating raw oysters from four oyster farms in Baynes Sound, British Columbia, according to the Public Health Agency of Canada.

“Laboratory testing has confirmed norovirus infection in several patients from both California and Canada,” the California health department reported.

Earlier this week, public health officials off of Canada’s Pacific Coast suspected that the likely source of a norovirus outbreak associated with the March herring run was from untreated sewage associated with marine operations.

In the United Kingdom, after analyzing 630 oysters bought online, at retailers and in restaurants, James Lowther of the CEFAS and his team found norovirus in 69 percent of samples.

“Contamination of bivalve shellfish, particularly oysters, with norovirus is recognized as a food safety risk,” Lowther wrote.

Additionally, when grouping the oysters by where they were caught, those off the coast of Holland carried fewest viruses, while ones from the U.K. showed more than a 60 percent contamination rate.

While the cleanliness of oysters has been recognized in recent years through testing of E coli bacteria, “Most of them pass this – but the greater risk of norovirus is currently ignored,” according to the report.

According to the studies, there were a total of 145 norovirus outbreaks across England between January and March of this year, and 4,100 additional cases were reported between last autumn and March. “… the overall number of cases is likely to much higher, given that most sufferers do not bother to visit their doctor,” according to the research report.

Advice to consumers
Public health officials in the United States, Canada and other countries warn that all raw shellfish can harbor dangerous viruses, parasites and bacteria. Oysters are particularly prone to such pathogens.

Quick steaming oysters will not kill norovirus and other pathogens, according to the U.S. Centers for Disease and Prevention. To be safe, seafood must be cooked to an internal temperature of at least 145 degrees.

Raw oysters can cause illness in anyone, but they are particularly dangerous for young children, older adults, pregnant women and people with compromised immune systems. Oysters and other food contaminated with noroviruses usually looks, smells and tastes normal.

Some foodborne illnesses caused by oysters, such as listeriosis, can take up to 70 days after exposure for symptoms to develop. Norovirus, however, usually causes symptoms within 12 to 48 hours after consuming contaminated foods or beverages.

Most people infected with norovirus develop diarrhea, vomiting, nausea and stomach pain. Diarrhea tends to be watery and non-bloody. Diarrhea is more common in adults and vomiting is more common in children. Regardless, symptoms can easily result in dehydration requiring intravenous fluids.

People who think they might have become ill from eating possibly contaminated oysters should talk to their healthcare providers. Contact your healthcare provider if you have diarrhea that lasts for more than three days, or is accompanied by high fever, blood in the stool, or so much vomiting that you cannot keep liquids down and you pass very little urine.

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