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The California Department of Public Health has closed Tomales Bay to oyster harvesting because a norovirus outbreak that has sickened at least 44 people is associated with oysters from that area.

The CDPH posted a recall and listed 40 restaurants that may have sold the Hog Island oysters. The restaurants are located in San Francisco, Sonoma, Petaluma, Bolinas, Sacramento, Sebastopol, Napa, and Santa Rosa.

The closure was issued on January 3, 2019, according to Dr. Matt Willis, Marin County’s public health officer. The state of California has recalled Tomales Bay oysters that were served in 34 restaurants by Hog Island Oyster Company. The illnesses happened between December 29, 2018 and January 5, 2019.

The illnesses were in San Francisco. Public health officials interviewed patients and found that most of them had eaten oysters provided by Hog Island Oyster Company. There has been no confirmation of norovirus in oysters harvested and sold by that company.

Officials are sampling the oysters in Tomales Bay and have not found any norovirus or other pathogens in the shellfish. Willis said that “it’s likely this represents contamination of the water itself,” rather than contamination during harvesting transport, or processing. The area has received a large amount of rain. This can cause sewage to be swept into the ocean. The Tomales Bay harvesting area may be closed until sometime next week.

There have been 17 norovirus outbreaks associated with or linked to raw oysters in the past few years, including four just in 2018. Oysters are filter feeders, which mean they take in sea water and filter out nutrients. Unfortunately, that filter system also traps pathogens.

The symptoms of a norovirus infection include nausea, stomach cramps, diarrhea, and vomiting. The illness can start within a few hours of eating food or drinking beverages that are contaminated. And the virus is very contagious. Most people recover within a few days, and most people do not require medical care or need to be hospitalized.