County of San Diego, California, investigates Salmonella Typhimurium outbreak linked to unpasteurized milk - September 2023-January 2024.

On 17 October 2023, the County of San Diego (CoSD), California's Epidemiology and Immunization Services Branch received reports of eight salmonellosis patients with illness onsets beginning in September 2023. CoSD disease investigators identified common exposure to Farm A-produced unpasteurized dairy products and performed epidemiologic and environmental investigations. Isolates were submitted for whole-genome sequencing. Cases were defined as persons with Salmonella infection with an isolation date on or after 15 September 2023. Twenty-five cases, including 23 confirmed and two probable, were identified among county residents, with 19 (76%) reporting exposure to Farm A unpasteurized dairy products. Median patient age was 12 years (range: 1-47 years). Three children (12%) were hospitalized. Environmental samples of unpasteurized milk collected from Farm A tested positive for S. Typhimurium, matching the outbreak strain by WGS. After Salmonella identification, CoSD released early advisories warning of Farm A-associated risks, likely preventing additional infections.