California’s 2024 dairy-cow H5N1 outbreak was followed by human infections identified through monitoring of people working around infected cattle. A CDC report counted 38 positive H5N1 test results from September 30 through December 24, 2024; 37 involved dairy workers exposed to sick cows, while one child’s exposure was undetermined.
Those are historical outbreak-period findings, not a current California case count. CDC has described the risk to the general public as low and the risk as greater for people with occupational or recreational exposure to infected animals.
Key takeaways
- California detected H5N1 in dairy cattle in the Central Valley in August 2024.
- CDC counted 38 positive human H5N1 test results in California from September 30 through December 24, 2024. All patients had mild illness in that report.
- Thirty-seven cases involved dairy workers exposed to sick cows across 29 farms; one child’s exposure was undetermined.
- CDC’s March 2025 report said no human-to-human transmission had been identified in the United States as of that report. That dated finding does not prove transmission is impossible.
- CDC says the general-public risk is low, while people with occupational or recreational exposure to infected animals have greater risk.
How many human H5N1 cases were reported in California?
In its report on the period from September 30 through December 24, 2024, CDC counted 38 California residents with positive H5N1 test results. This is a completed historical reporting period, not a current surveillance total.
| Measure | Finding | How to interpret it |
|---|---|---|
| Reporting period | September 30–December 24, 2024 | The dates covered by the CDC investigation |
| Positive test results | 38 California residents | A historical period total, not a current count |
| People with occupational exposure | 37 dairy workers associated with 29 farms | Workers had occupational exposure to sick cows |
| One other case | A child with undetermined exposure | The investigation did not establish the source of exposure |
CDC’s March 13, 2025 report provides the detailed historical tally. The research reviewed for this article did not establish California’s latest human case total or dairy herd total as of October 7, 2026. Check live California Department of Public Health and USDA APHIS surveillance before citing a present-day total.
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How did H5N1 reach California dairy workers?
California detected H5N1 in cows from three Central Valley dairy farms on August 30, 2024, later confirmed by the National Veterinary Services Laboratories. In September, the California Department of Public Health was notified of the first human case through monitoring of workers on farms with infected cows. The first two confirmed California cases, reported October 3, were workers at separate farms; both had conjunctivitis, and one also had fever.
CDC’s investigation found that 37 of the 38 people in the September–December case series were dairy workers with occupational exposure to sick cows, associated with 29 farms. CDC states that people with occupational exposure to H5N1-infected dairy cattle are at increased risk for infection.
The child’s exposure was undetermined. The investigation did not establish that the child became infected at a dairy farm or from another person.
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Is bird flu spreading between people?
CDC’s March 2025 California report said no human-to-human transmission had been identified in the United States as of its publication. This is a dated finding; it should not be recast as proof that person-to-person transmission is biologically impossible or as a current update.
The child’s exposure was undetermined, so that case alone does not establish how infection occurred. Avoid inferring a transmission route when investigators did not identify one.
Is the risk to the public high?
CDC’s current-situation page, dated July 7, 2025 in the research reviewed, said it believed the risk to the general public from bird flu viruses was low. CDC also says people with job-related or recreational exposure to infected animals, including cows, have greater risk.
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Risk depends on exposure. The California case series was dominated by dairy workers with occupational exposure; it does not show that ordinary pet ownership caused those cases. Recheck CDC’s current risk language before publication, since the cited page is dated.
How can farm workers protect themselves?
CDC’s California report describes a coordinated response that included PPE guidance, worker training and PPE distribution, monitoring workers with farm managers, testing through local health departments, and laboratory testing to distinguish H5 from seasonal influenza. The report notes that recommended PPE included N95 respirators and that access to PPE was a practical concern.
- Follow current employer and public-health guidance for the specific task and workplace.
- Ask the employer about required PPE, training, and how to obtain and use the equipment correctly. CDC’s report described N95 respirators in its worker-protection response; a retail listing alone does not establish that a product is suitable for a particular job.
- Cooperate with workplace or public-health monitoring after a relevant exposure.
- If symptoms develop after animal exposure, promptly tell a healthcare provider about the work and exposure history so testing and care can be coordinated through appropriate public-health channels.
CDC’s report also describes oseltamivir treatment for cases and prophylaxis for close contacts. Treatment or preventive medication should be guided by healthcare and public-health professionals, not self-prescribed.
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FAQ
How many human H5N1 cases were there in California?
CDC counted 38 positive H5N1 test results among California residents from September 30 through December 24, 2024. That is a historical total for the period, not a current 2026 count.
Did dairy workers get bird flu from cows?
CDC’s investigation found 37 cases among dairy workers with occupational exposure to sick cows. The report describes the exposure association; it does not mean every individual route of infection was established.
Was the child’s infection linked to a dairy farm?
The child’s exposure was undetermined in the CDC report. The investigation did not establish whether the exposure came from a dairy farm, another person, or another source.
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Is H5N1 spreading between people?
CDC’s March 2025 report said no human-to-human transmission had been identified in the United States as of publication. That dated statement is not proof that transmission is impossible.
Who is at greater risk?
CDC says the general-public risk is low and people with occupational or recreational exposure to infected animals have greater risk. Check CDC’s current guidance for the latest assessment.
Sources and reporting limits
The historical case totals, exposure findings, chronology, and worker-response details in this article come from the CDC MMWR report on California human cases from September–December 2024. Risk context comes from CDC’s current situation information for bird flu in dairy cows, which was dated July 7, 2025 in the research reviewed. The dossier did not establish a California-specific current human or herd total as of October 7, 2026.
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