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H5N1 was first detected in U.S. dairy cattle in March 2024; California’s dairy-cow outbreak came later that year. On February 27, 2025, the California Department of Food and Agriculture (CDFA) reported 748 infected dairies since the state’s first detection, with 329 recovered dairies released from quarantine. Those are historical figures, not current totals. California produced 18.2% of U.S. milk in 2022, according to the State of California, making the outbreak a significant event for the national dairy industry.
How bird flu reached California dairy cattle
H5N1 had been circulating in the U.S. poultry outbreak that began in 2022 before it was detected in dairy cattle in March 2024. California was not the first state to report infected dairy cows: its dairy-herd outbreak developed later in 2024.
In its February 27, 2025 update, CDFA said 14 California dairy farms had been confirmed in the preceding 30 days. The department reported 748 infected dairies in total since California’s first dairy detection, and said 329 dairies that had recovered had been released from quarantine. Released dairies remained subject to surveillance testing. These numbers describe the situation on that date; CDFA points readers to the California Department of Public Health’s current situation dashboard and the USDA APHIS confirmed livestock cases page for newer counts.
California’s importance to the dairy supply is also date-specific: the State of California reported that the state produced 18.2% of U.S. milk in 2022. That figure should not be read as a current production share.
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Is pasteurized milk safe to drink?
California’s published consumer guidance says pasteurized milk and dairy products remain safe to consume. CDFA states that “Pasteurization of milk is fully effective at inactivating the virus.” The department also says milk from healthy cows has been cleared to ship for pasteurization. This assurance applies to pasteurized products; it is not an assurance that raw or unpasteurized milk is safe.
A CDC study helps explain why reports of bird-flu material in retail milk do not by themselves show that infectious virus is present. Researchers detected influenza A genetic material in some pasteurized milk samples, but found no evidence of viable virus in the influenza-A-positive pasteurized samples they tested. A test that detects viral nucleic acid establishes that genetic material was present; it does not establish that the virus can infect someone.
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What the CDC milk study measured
| Sampling period | Finding | What it means |
|---|---|---|
| April 13–May 3, 2024 | Influenza A genetic material was detected in 61 of 168 pasteurized retail milk samples (36.3%). | The study found no evidence of viable virus in the positive pasteurized samples it tested. |
| December 27, 2024–January 29, 2025 | Influenza A was detected in 33 of 477 retail milk samples (6.9%); all were processed in California. | This is a finding from that sample set and period, not an estimate of unsafe milk across the population. |
The study confirmed nearly all positive samples as influenza A(H5). Its sample percentages describe the tested products in those collection periods, not the proportion of all milk that was unsafe. The study authors wrote, “We found no evidence of viable virus in IAV-positive retail milk samples.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the outbreak meant for people exposed at work
California’s human cases reported in the CDC surveillance report were identified during September–December 2024, not as current totals. Among 171 people tested, CDPH identified 36 confirmed cases and one probable case among adult dairy farm workers, as well as one confirmed case in a child under 18 who had no known dairy-cow or poultry exposure. The report said 37 positive results were confirmed at CDC and all patients had mild illness.
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The worker cases show why risk differs by exposure. The CDC report documented infections among people working on dairy farms; it does not establish the same level of risk for someone drinking pasteurized milk. For people who handle infected or potentially infected animals or materials, CDFA relays California Department of Public Health advice to use eye protection (face shields or safety goggles), N95 respirators, and gloves. Workers should follow current CDPH workplace guidance and their employer’s protocols; this equipment list is not a substitute for workplace training.
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