3rd human case of H5 bird flu confirmed in California dairy worker

The Centers for Disease Control and Prevention has confirmed a third human case of H5 bird flu in California, marking the latest in a series of similar cases linked to infected dairy cows. Like the two previous cases in the state, the affected individual is a dairy worker who had occupational exposure to infected livestock. This case is part of an ongoing multi-state outbreak, with 18 human cases reported in the U.S. since 2022, 17 of which have occurred in 2024.

All three California cases were reported from different dairy farms, with no known contact between the affected individuals. This suggests animal-to-human transmission rather than person-to-person spread, which is consistent with the CDC’s current risk assessment. The general public’s risk remains low, according to the agency.

As with the earlier cases, the third California patient experienced mild symptoms, primarily eye redness or discharge (conjunctivitis). None of the three individuals has required hospitalization. The mild nature of these symptoms aligns with the CDC’s expectations for people exposed to infected animals.

The CDC also provided an update on the first two confirmed cases in California, revealing that the viruses involved are clade 2.3.4.4b A(H5N1) strains closely related to those detected in dairy cattle. Whole genome sequencing of one case (A/California/135/2024) identified it as a B3.13 genotype virus. Importantly, there were no genetic changes indicating an increased ability for human-to-human transmission or reduced susceptibility to antiviral treatments. These sequences have been publicly posted in GISAID and GenBank, with sequencing for the third case currently underway.

While the discovery of H5 in individuals exposed to infected animals is not unexpected, the CDC emphasizes the importance of following recommended precautions for those with close or prolonged exposure to infected animals or contaminated environments. This includes dairy workers, farmers, and anyone handling livestock or wild birds. The CDC continues to stress that the current public health risk for the general population remains low.

The ongoing multi-state outbreak has primarily affected dairy cattle, with sporadic infections in wild birds, poultry flocks, and mammals. To date, there has been no evidence of person-to-person transmission of H5 bird flu in the U.S.

Across the U.S., a total of 18 human cases of H5 bird flu have been confirmed since 2022. California leads with four cases, while Colorado has reported 10, Michigan two, Missouri one, and Texas one. The CDC is currently awaiting confirmatory testing for two additional presumptive positive cases from California.