34 people in California have tested positive for norovirus, the state’s department of health says, and the CDC has counted 91 people in total who have fallen ill across the country, but the real number may be much higher.

While many local oysters and East Coast oysters are still fair game, it's a bad time right now to be ordering oysters that come from British Columbia. California's health department issued an official warning Saturday against consuming raw oysters from B.C., as the Associated Press reported.

Each of those 34 California cases in this norovirus outbreak have been linked to raw oysters that were imported from B.C., and happened between March 11 and March 19. There's no word on specific oyster varieties that may be the culprit, bu it appears the contaminated oysters are connected to multiple oyster farms. The waters off British Columbia are known to be home to upwards of 20 varieties of oysters.

Canada’s Public Health Agency says there could be more contaminated oysters for sale in Alberta, Saskatchewan, and Ontario as well.

Monday, the FDA issued warnings for thirteen states: CA, CO, FL, HI, IL, MA, MN, NJ, NV, NY, OR, TX and WA. You can see a map of the nationwide outbreak here.

The CDPH says several oyster farms have now been closed for sanitary contamination, but not before the norovirus hit the area where those oyster growing regions are situated. The Public Health Agency of Canada reports there have been 279 cases of norovirus and other gastrointestinal illnesses linked to the British Columbian oysters in Canada alone.

“The FDA and the states conducted a trace forward investigation to determine where the raw oysters were distributed and to ensure they’re removed from the food supply. Retailers should not serve raw oysters harvested from the following harvest locations... within Baynes Sound: #1407063, #1411206, #278737 in BC 14-8 and #1400036, in BC 14-15,” the FDA warned.

The CDC says this is the peak season for norovirus, with outbreaks happening most commonly between November and April. That agency also says norovirus is extremely contagious and spreads quickly, both through contact with contaminated foods and surfaces, and from infected people.

The most common symptoms are diarrhea, vomiting, nausea, and stomach pain, so it’s often confused with food poisoning or stomach bugs. Another problem is, those symptoms often don’t set in until 12 to 48 hours after the infection, so it’s often difficult for people to pinpoint why the fell ill.

There are ways to prevent against the illness:

  • Wash your hands often
  • Rinse fresh fruits and vegetables before consuming them, even if you aren’t eating them raw
  • Cook shellfish thoroughly, until it reaches an internal temperature of 145°F or higher.
  • If you’re sick, stay home until your symptoms have been gone for two days

Photo by Thomas John on Unsplash