A norovirus outbreak traced to raw oysters from British Columbia has crossed the Canadian border, and U.S. officials say an unusual strain of the virus is involved.

About 100 people in California have reported becoming sick with symptoms of norovirus after eating raw oysters from British Columbia, according to the California Department of Public Health. The ill people said the oysters were sold by restaurants and retail stores.

“Laboratory testing has confirmed norovirus infection in several patients from both California and Canada. Although the number of reported new illnesses has decreased during the last week, the investigation is ongoing,” the California health department reported Tuesday.

In Canada, 172 people have become ill after eating raw oysters from four oyster farms in Baynes Sound, British Columbia, according to the Public Health Agency of Canada. The farms were closed between March 23 and April 13. They will remain closed until further notice. 

The outbreak involves an unusual strain of the highly contagious norovirus, according to a statement from the Center for Infectious Disease Research and Policy (CIDRAP).

“The U.S. Centers for Disease Control and Prevention said yesterday (Monday) that multiple states are currently affected by outbreaks related to raw oysters from British Columbia. It added that lab analysis has identified that an uncommon type of norovirus is involved,” according to the CIDRAP statement.

As of Tuesday night, the CDC had not posted any information about the outbreak. A spokeswoman at CDC did not comment on the outbreak Tuesday. She referred Food Safety News to the agency’s general information page on norovirus and the FDA notice.

However, the FDA reported Monday night that it was investigating the outbreak “along with the Centers for Disease Control and Prevention, the Public Health Agency of Canada, the Canadian Food Inspection Agency, and state and local partners.

The FDA is working on tracing the oysters in the United States. The agency knows the implicated oysters were distributed to California, Illinois, Massachusetts, New York, and Washington state. However, other states may have received the oysters directly from Canada or from other distributors in the United States.

Restaurants and retailers in the United States can identify the implicated oysters by looking for the following landfile numbers located on the shellfish tags: CLF #1402060, CLF #1411206, CLF #1400483, and CLF #278757. 

Advice to consumers
Public health officials in the United States and Canada 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 CDC. 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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