CDC Confirms 3rd Human Case of H5 Bird Flu in California

The Centers for Disease Control and Prevention (CDC) has confirmed a third human case of H5 bird flu in California. Like the two cases in that state confirmed last week, the person reported occupational exposure to infected dairy cows. To date, all three California cases occurred in dairy workers from three different affected farms with no known contact with each other, suggesting animal-to-human spread. Also consistent with the two previous California cases, the person experienced mild symptoms, including eye redness or discharge (conjunctivitis). None of the three cases has been hospitalized. The identification of H5 in people with exposure to infected animals is not unexpected and does not change CDC’s risk assessment for the general public, which continues to be low.

Including this most recent case, 18 human cases of H5 bird flu have now been reported in the United States since 2022; 17 cases during 2024. CDC is awaiting 2 additional presumptive positive specimens from California for confirmatory testing.

CDC Lab Update on First Two California Cases

Sequences from the first two cases in California confirmed these are clade 2.3.4.4b A(H5N1) viruses closely related to viruses detected in dairy cattle. The whole genome was sequenced from one of the two cases (A/California/135/2024) and was confirmed to be a B3.13 genotype virus. There were no genetic changes observed that are known to be associated with an increased ability to infect or spread between people or known to reduce susceptibility to antiviral medications. These sequences have been publicly posted in GISAID and have been submitted to GenBank. Sequencing from the third confirmed case is underway.

CDC continues to underscore the importance of recommended precautions for people with exposure to infected or potentially infected animals. People with close or prolonged, unprotected exposures to infected birds or other animals (including livestock), or to environments contaminated by infected birds or other animals, are at greater risk of infection.