Gastrointestinal illnesses potentially linked to raw oysters sickened nearly 200 people, according to health officials in Southern California, who urged residents to take extra precautions with shellfish.

The illnesses, recorded in Los Angeles County and San Diego, may be associated with oysters imported from a specific harvest in northwest Mexico.

The Los Angeles County Department of Public Health said in a statement on Wednesday that there were “more than 150 suspected local cases of gastrointestinal illness linked to the consumption of raw oysters, likely caused by norovirus.”

Officials there warned people to ask restaurants about where they sourced their oysters from, and to avoid eating oysters from Laguna De Guerrero Negro and Laguna Manuela in Baja California, Mexico, and from Bahia Salina in Sonora, Mexico. The department said it was still working to confirm the source of the illness.

In San Diego County, officials said they had linked 41 confirmed and probable cases of norovirus illness to raw oysters from northwest Mexico.

Officials said in a news release on Jan. 11 that the cases were reported in residents who had dined out and consumed raw oysters from Sonora, Mexico, starting in mid-December. “These implicated oysters have only been available through restaurants and wholesale locations,” the county said, adding that health officials had asked restaurants to set aside “Rocky Point oysters” packed by Golpac in Bahia Salina, in Sonora, Mexico.

The agency especially urged caution among vulnerable populations, such as young children, older adults and people who are immunocompromised.

The U.S. Food and Drug Administration on Thursday advised restaurants and retailers in California not to serve or sell oysters distributed by Sociedad Acuicola GolPac that were harvested on Dec. 18, Dec. 19 or Dec. 27 because of potential norovirus contamination. The F.D.A. also warned consumers in the area against eating those oysters.

The agency said the Mexican authorities had closed the Bahia Salina area to harvests on Jan. 12, and had initiated an investigation into the source of the illnesses.

Symptoms of norovirus include diarrhea, stomach pain or cramps, nausea, vomiting and fever. The virus can also cause dehydration. People experiencing those symptoms who may have consumed affected oysters should reach out to their health care provider, who should then report their symptoms to their local health department, the F.D.A. said.

Symptoms usually begin 12 to 48 hours after being exposed to norovirus, and most people with norovirus illness get better within 1 to 3 days.

“Until the source is confirmed, consumers should be cautious before eating raw oysters due to the potential risk of food borne illness,” Dr. Muntu Davis, a Los Angeles County health officer, said in a statement.

Dr. Davis added, “If you are sick, avoid spreading illness by washing your hands frequently and cleaning frequently touched surfaces such as doorknobs, light switches and kitchen countertops.”

Orange County officials had not reported likely cases of the virus, but were also warning residents to be careful about consuming raw oysters amid illnesses in some surrounding counties.

“Food contaminated with pathogens may look, smell and taste normal,” the F.D.A. said.

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