What to Know
In this week´s spotlight, CDC reports six confirmed human cases of H5 bird flu in Washington state as well as three more cases in California for a total of 36 cases in the United States, and shares updated results of ongoing outreach efforts.

Situation Update
October 29, 2024 – CDC continues to respond to the public health challenge posed by a multistate outbreak of avian influenza A(H5N1) virus, or "H5N1 bird flu," in dairy cows, poultry and other animals in the United States. CDC is working in collaboration with the U.S. Department of Agriculture (USDA), the Food and Drug Administration (FDA), Administration for Strategic Preparedness and Response (ASPR), state public health and animal health officials, and other partners using a One Health approach.

Since April 2024, CDC, working with state public health departments, has confirmed H5 bird flu infections in 36 people in the United States. Fifteen of these cases were associated with exposure to H5N1 bird flu-infected poultry and 20 were associated with exposure to sick or infected dairy cows. The source of the exposure in one case, which was reported by Missouri on September 6, could not be determined. Serological testing of the contacts of the Missouri case have been reported, and that investigation has concluded.

The 36 cases include 16 cases in California, three of which were confirmed last week. CDC also confirmed six human cases of H5 bird flu in poultry farm workers in Washington state last week. These are the first cases of H5 bird flu in Washington. Testing of additional specimens from Washington is ongoing. Cases in California and Washington have occurred in workers on affected farms. All available data so far suggest sporadic instances of animal-to-human spread. The farm workers in California and Washington state all described mild symptoms, many with eye redness or discharge (conjunctivitis). Some cases in Washington reported some mild upper respiratory symptoms. None of the workers were hospitalized. Updated case counts, including by state and source of exposure, are reflected in a table on CDC´s website. To date, person-to-person spread of H5 bird flu has not been identified in the United States. CDC believes the immediate risk to the general public from H5 bird flu remains low, but people with exposure to infected animals are at higher risk of infection.

On the animal health side, since March 2024, USDA has confirmed infected cattle in 387 dairy herds in 14 U.S. states. The number of affected herds continues to grow nationally, with almost all new infections identified in herds in California. USDA reports that, since April 2024, there have been H5 detections in 39 commercial poultry flocks and 28 backyard flocks, for a total of 22.23 million birds affected.

Among other activities reported in previous spotlights and ongoing, recent highlights of CDC´s response to this include:

Laboratory Update
To date, CDC has confirmed infection of 16 farm workers with H5N1 bird flu in California. Genetic sequencing of nine of these cases confirms that all are H5N1 viruses from clade 2.3.4.4b and that all are closely related genetically to the virus causing infections in domestic dairy cattle. CDC has sequenced the full genomes of viruses detected in four cases from California (A/California/135/2024, A/California/147/2024, A/California/152/2024, and A/California/153/2024) and confirmed that all are B3.13 genotype viruses. Various gene sequences were successfully obtained from specimens from five other California cases. That sequencing information showed no changes in the hemagglutinin (HA) associated with increased infectivity or transmissibility among people. Additionally, there were no mutations associated with reduced susceptibility to available antiviral treatments (e.g., neuraminidase inhibitors or polymerase acid endonuclease inhibitors) and no mutations identified in other genes indicating additional mammalian adaptation. Genetic data have been posted in GISAID and GenBank. Additional data will be posted as they become available. CDC has successfully isolated virus from specimens from nine of the 16 cases. Attempts to isolate virus from additional specimens are ongoing. Antigenic characterization and antiviral susceptibility testing are underway. Antigenic characterization will inform whether existing H5 bird flu candidate vaccine viruses (CVVs) would provide good inhibition of these viruses.

On Thursday, October 24, CDC posted a spotlight describing the results of serology testing for evidence of prior infection with H5N1 bird flu in blood samples provided by seven people in Missouri.

Efforts to isolate and sequence viruses from specimens from the Washington cases are in progress, and sequencing information will be reported and posted to public databases when available.

Epidemiology Update
A multilingual CDC field team continues to assist the California Department of Public Health in its efforts to learn more about how this outbreak in California began and how to lower the risk to farm workers with exposure to infected cows. Two staff are on the ground, and additional staff are ready to deploy if needed. CDC staff are assisting with active surveillance efforts, including field assessments of suspected cases and household contacts; testing and treatment; and dissemination of information to farm workers and the community. CDC also deployed a separate multilingual field team to Washington state to assist the local health district with monitoring activities related to depopulation of an infected poultry farm, including on-site symptom screening, testing of workers coming on shift, and provision of oseltamivir. The three CDC staff worked with local epidemiologists on data management, epidemiological summaries, and case and contact investigations. There is no evidence of any person-to-person spread in either of the two states or anywhere in the United States.

Surveillance Update
On Wednesday, October 23, CDC received specimens from multiple presumptive positive cases of H5 bird flu in poultry farm workers in Washington state. As noted above, CDC has so far confirmed six cases in Washington. Testing of additional specimens is pending.

CDC continues to support states that are monitoring people with exposure to cows, birds, or other domestic or wild animals infected, or potentially infected, with H5N1 bird flu. To date, more than 6,100 people have been monitored as a result of their exposure to infected or potentially infected animals, and at least 300 people who have developed flu-like symptoms have been tested as part of this targeted, situation-specific testing. More information on monitoring can be found at Symptom Monitoring Among Persons Exposed to HPAI. In addition, since February 25, 2024, more than 55,000 specimens have been tested for avian influenza A(H5) virus or other novel influenza viruses at public health labs. One of the specimens, collected as a part of routine surveillance, was identified as presumptive positive for avian influenza A(H5) virus and was confirmed as H5N1 bird flu positive by CDC.

CDC also continues to monitor flu surveillance data using CDC´s 2024-2025 influenza surveillance strategy, especially in areas where avian influenza A(H5N1) viruses have been detected in dairy cows or other animals, for any unusual trends, including in flu-like illness, conjunctivitis, or influenza virus activity. Overall, for the most recent week of data, CDC flu surveillance systems show no indicators of unusual flu activity in people.