Salmonella in Commercially Sold Raw Milk Sickens 171 — California, 2023–2024

Summary

• Known before: unpasteurized (raw) milk has caused outbreaks of illness from E. coli and certain species of Brucella, Campylobacter, Cryptosporidium and Salmonella.
• New here: from October 2023 to March 2024, California investigated an outbreak of Salmonella Typhimurium linked to raw milk from one dairy farm. Of 171 cases in California and four other states, 70% were in children and adolescents under 18. Whole-genome sequencing found the outbreak strain in the dairy's raw milk and in raw milk cheese aged 60 days.
• What it means: commercially distributed raw dairy products can cause large, widespread outbreaks. Public health messaging should explain the risks, especially to people at risk of severe disease, including children.

Raw milk in California

In California, raw milk is regulated by the California Department of Food and Agriculture (CDFA). Raw milk dairies need a permit and must pass sanitation inspections of the farm and bottling; animals are tested every year for brucellosis and tuberculosis; the milk must meet strict bacterial and cell count limits and be kept at 45°F (7.2°C) or below. It may be sold in stores, with warning labels about possible contamination. Providers and laboratories must report Salmonella infections to local health departments within 1 working day.

How the outbreak was found

On October 18, 2023, the County of San Diego told the California Department of Public Health (CDPH) of eight salmonellosis cases, with onsets from September 21 to October 12, in people who drank brand A raw milk — made only by dairy farm A and sold across California. With another county's report of a similar patient, this set off a statewide investigation.

• Confirmed cases: a laboratory-confirmed infection with the outbreak strain, closely related by whole-genome sequencing (WGS), and illness beginning between September 15, 2023, and May 4, 2024.
• Probable cases: laboratory-confirmed Salmonella without serotype or WGS results, in someone who drank brand A raw milk within 7 days before falling ill.
• CDPH reviewed case report forms and searched PulseNet, CDC's national molecular subtyping network, for related isolates, then asked health departments to re-interview patients with a questionnaire about raw dairy products.

Outbreak cases by reported or estimated illness onset date, September 2023–March 2024. Chart: CDC.

Who got sick

171 cases: 159 (93%) confirmed and 12 (7%) probable. Illnesses began from September 21, 2023, to March 11, 2024; 140 (82%) were in September and October 2023 (median onset October 11) — the pattern of a continuous common-source outbreak.

Raw milk exposure

• Exposure information was available for 91 patients (53%); the rest could not recall or were lost to follow-up.
• 72 (79%) reported drinking raw milk (71) or raw heavy cream (1); 67 (93%) of them had brand A in the 7 days before falling ill. One more patient had eaten only brand A raw milk cheese.
• Among 79 confirmed patients with exposure data, 55 (70%) had brand A raw milk or cream — far above the 1.9% of Californians who drink raw milk in the 2018–2019 FoodNet Population Survey (p<0.001).
• Of patients with information, 24 (49%) drank brand A products daily and 17 (35%) weekly.

Tracing the milk

• On October 19, 2023, CDPH and CDFA notified dairy farm A of the first nine cases, and in October officials collected 40 brand A products — raw milk, cheese, heavy cream and kefir — from the farm, stores and patients' homes.
• Three of the 40 carried S. Typhimurium indistinguishable by WGS from patients' isolates: two bottles of raw whole milk from the farm's bottling facility (October 19 and 25) and one store sample with a best-by date of October 27. The farm was told of the first positive on October 23.
• The outbreak strain also turned up in January 2024 in raw cheese aged 60 days, made from the contaminated milk; no raw cheese sampled earlier tested positive.

The response

• On October 24, 2023, the farm halted production and voluntarily recalled raw milk and heavy cream with best-by dates of October 11–November 6 — one week after CDPH began investigating. Recalled lots were destroyed or held for aged cheese under CDFA impound; the cheese that tested positive never reached stores.
• CDFA inspected the bottling and cheese plant on October 25 and, with county inspectors, the farm on October 27. The farm's own testing had found Salmonella in milk from a recently purchased cow, which was removed; later herd testing found none. Production resumed on October 31.
• Health departments issued press releases and social media posts telling people to discard recalled milk and cream, to see a doctor for symptoms, and to choose pasteurized dairy.

Why it matters

This is one of the largest foodborne outbreaks linked to raw milk in recent U.S. history, and it fell hardest on young children. From 2009 to 2021, 143 enteric disease outbreaks confirmed or suspected to come from raw milk were reported to CDC; the 16 caused by Salmonella were all small (median 10 cases, range 2–33). In California, four confirmed and three suspected outbreaks have been linked to raw milk since 2012 — and none to pasteurized milk. The investigation also showed that raw milk cheese aged 60 days, which the Food and Drug Administration allows to be sold, can stay contaminated.

How the four out-of-state patients were infected is unknown: federal law bans selling raw milk for human consumption across state lines, but not raw milk sold for pets or raw cheese aged 60 days or more. One bought the farm's raw milk (and possibly cheese) in their own state, one ate only its raw cheese, one recalled no raw dairy, and one was lost to follow-up.

Limitations

1. Cases continued after the recall, and not every confirmed patient reported raw milk — some may have been infected person to person, or used products already at home.
2. Re-interviews came later than the first interviews, so memories may have been poorer.
3. Some patients may not have disclosed drinking raw milk.

For public health

Raw dairy products — even from a licensed producer — remain a risk for enteric and other infections, especially for children. Messages should reach parents, pregnant women and people with weakened immune systems, and collaboration and surveillance testing among local, state and federal partners should be strengthened.

Sources

Based on Weinstein E, Lamba K, Bond C, et al., "Outbreak of Salmonella Typhimurium Infections Linked to Commercially Distributed Raw Milk — California, 2023–2024," MMWR Morbidity and Mortality Weekly Report, volume 74, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's summary counts "four of 40 samples" positive; its results give three of the 40 collected in October, plus the aged cheese sampled in January, as stated here. Its "CFDA" is corrected to CDFA.