Concurrent Norovirus Outbreaks Associated with Consumption of Oysters Harvested in Mexico — California, December 2023–January 2024

Norovirus is the most common cause of foodborne illness outbreaks in the United States. In January 2024, local health jurisdictions and the California Department of Public Health (CDPH) identified two concurrent norovirus outbreaks across eight Southern California local health jurisdictions. CDPH was notified in late December 2023 and early January 2024 of gastrointestinal illnesses in persons who consumed raw oysters from food service facilities in San Diego County (outbreak 1). Additional illness reports came from multiple jurisdictions that included Los Angeles County and other areas in Southern California (outbreak 2). In total, approximately 400 persons across eight local health jurisdictions reported gastrointestinal illness after raw oyster consumption. A multiagency investigation confirmed that outbreaks 1 and 2 were unrelated, and implicated oysters were traced to two separate, nonoverlapping harvest regions in Mexico. A total of 179 outbreak-associated cases, including 24 laboratory-confirmed norovirus cases, were identified. Patient samples from both outbreaks identified norovirus genogroups I and II; other enteric viruses (sapovirus, astrovirus, rotavirus, and adenovirus) were also identified from one or both outbreaks. Noroviruses were genetically related by genotype within each outbreak but dissimilar between outbreaks. In outbreak 2, oysters might have been contaminated at a location separate from the original growing area, also known as wet storage. Concurrent outbreaks with similar modes of transmission can be unrelated, and the source for each should be confirmed through traceback. Proper storage and handling of shellfish is essential to maintaining safety of food products to consumers. Cooking oysters to 145°F (62.8°C) is recommended before consumption.