Two Norovirus Outbreaks From Mexican Oysters in California, Winter 2023–24

Norovirus is the most common cause of foodborne illness outbreaks in the United States, and raw oysters are a frequent source. In the winter of 2023–24, Southern California saw what looked like one large oyster outbreak. It turned out to be two.

How it unfolded

• December 31, 2023: a restaurant told the California Department of Public Health (CDPH) and San Diego County that 37 customers who ate oysters there on December 27–29 had fallen ill. The county lab confirmed norovirus on January 2.
• By January 9: 16 more complaints came from customers of three locations of an unrelated San Diego restaurant chain.
• January 12: reports arrived from Los Angeles County and surrounding jurisdictions.
• By January 16: 140 illness reports were linked to 51 food service facilities across greater Los Angeles and San Diego.

Because cases overlapped in time and place, investigators first treated them as a single outbreak. Norovirus cases aren't individually reportable in California, but outbreaks are, and CDPH and local health departments built a case definition, surveyed ill people, collected lab samples and traced the oysters.

Illness onset dates in the two outbreaks. Credit: Centers for Disease Control and Prevention.

Two outbreaks, not one

About 400 people in eight local health jurisdictions reported illness after eating oysters. Of those interviewed, 179 met the case definition — illness within 12–48 hours of eating oysters in Southern California — including 24 with laboratory-confirmed norovirus.

Tracing the federally required shellfish tags — records of harvest, dealer and certification — showed the two sets of oysters had no harvest or storage areas in common and were different types. Lab work agreed: the norovirus strains were genetically related within each outbreak but different between them. Other intestinal viruses — sapovirus, astrovirus, rotavirus and adenovirus — also turned up.

Across both outbreaks, 89.4% of patients had diarrhea, 81.0% vomiting and 81.6% stomach pain; two people were hospitalized.

What is wet storage?

Wet storage means keeping live, market-ready oysters in natural waters or seawater tanks, sometimes far from where they grew and mixed with lots from other harvest areas. It is common worldwide and lengthens the time between harvest and eating.

The outbreak 2 oysters came from two growing areas about 25 miles apart, but all had been held in wet storage at Rincón de Ballenas from November 22, 2023, to January 5, 2024. That shared stop — found through the investigation by Mexican authorities — points to wet storage as the likely source. Wet storage had been blamed before for bacterial oyster outbreaks in Hong Kong, but no viral outbreak had previously been linked to it.

The response

• The Food and Drug Administration issued advisories on oysters from certain areas of Sonora in late December 2023 and of Baja California on January 17, 2024, leading to recalls; distributor A recalled its oysters on January 12.
• Mexican authorities closed the growing areas for 21 days after norovirus was confirmed in patients.
• California and local agencies told consumers, restaurants and retailers not to eat, serve or sell oysters from the implicated regions.
• Inspections found mislabeled shellfish tags, missing harvest and storage information, and dealers distributing oysters without certification; CDPH issued notices of violation.

Limitations

• Outbreak 1's source was suspected but not confirmed.
• Because of wet storage, outbreak 2's exact source couldn't be confirmed.
• Cases were reported passively, so some were likely missed.

Lessons

• Concurrent outbreaks can be unrelated. Keep tracing every source, even after one is found.
• Watch wet storage. Growers and distributors should prevent contamination of oysters, including across mixed lots in storage.
• Cook oysters. The risks of raw oysters can be avoided by cooking them to 145°F (62.8°C).

Sources

Based on Zhu S, Grant C, Pan CY, et al., "Concurrent Norovirus Outbreaks Associated with Consumption of Oysters Harvested in Mexico — California, December 2023–January 2024," Morbidity and Mortality Weekly Report 74(13), Centers for Disease Control and Prevention; a work of the United States government in the public domain. Original report.