The California Department of Public Health has closed Tomales Bay to oyster harvesting due to a norovirus outbreak associated with the oysters.

Dr. Matt Willis, Marin County’s public health officer, said the closure was issued on Jan. 3. and on the following day the state health department issued a recall order on Tomales Bay oysters that had been sold to 34 restaurants by Hog Island Oyster Co., based in Marshall.

“There were 44 confirmed cases of norovirus between Dec. 29 and Jan. 5 across the Bay Area,” Willis said. “Only seven of those 44 cases were Marin cases.”

Willis said there have been few new norovirus cases reported since Jan. 5.

Terry Sawyer, one of the owners of Hog Island, said his company voluntarily recalled its oysters before the state issued its recall order.

Norovirus, also known as Norwalk virus, is a gastrointestinal illness that causes nausea, vomiting, diarrhea and stomach cramps. Noroviruses are found in the stool or vomit of infected people. Most people who are infected recover in two to three days without hospitalization and with no long-term health effects.

“The outbreak was detected from a cluster of cases in San Francisco, among San Francisco residents,” Willis said.

The San Francisco residents who became sick were interviewed by health workers.

“That process led to Hog Island Oyster Co. as the likely common source,” Willis said.

The declaration of an outbreak by state health officials was based on epidemiological evidence and clinical symptoms rather than laboratory confirmation of contaminated oysters.

Willis said state health officials are now continuously sampling oysters in Tomales Bay and haven’t found any evidence of either norovirus or Vibrio parahaemolyticus, which is the most common cause of oyster-related food-borne illness.

“The decision to allow Tomales Bay oysters to be harvested and sold will be made by the California Department of Public Health based on testing results of oysters,” Willis said.

Willis said there is no concern in this case that the outbreak is related to food handling, cultivation or harvesting practices.

“It’s likely this represents contamination of the water itself,” he said. “The water testing showed high levels of bacterial and viral contamination, which is normal following high rainfall over a long interval.”

Sawyer said Tomales Bay would be closed to oyster harvesting at the present time regardless of the norovirus outbreak due to the amount of rain the area has received recently.

Because sewage and other contaminants are more likely to be swept into the ocean during periods of heavy rainfall, it is standard practice to close oyster farms at certain times, Sawyer said.

“It depends on the watershed and amount of rain falling in a 24-hour period,” he said. “It’s done around the nation.”

Sawyer said due to the rainfall alone oyster harvesting on Tomales Bay could remain closed until sometime next week.

In the meantime, Hog Island remains open.

“We have product from other growers in other areas that we feel are high quality,” Sawyer said. “We’re selling them here at the farm, and we’re also selling them at our restaurants.”

Willis said it is unusual for a norovirus outbreak to be linked to oysters. He said there are typically about 20 norovirus outbreaks every year in Marin County, and they usually occur in places where people congregate in close quarters such as schools or nursing homes.