October 18, 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 avian influenza A(H5) infections in 27 people in the United States. Nine of these cases were associated with exposure to H5N1 bird flu-infected poultry and 17 were associated with exposure to sick or infected dairy cows 12. This includes 13 cases in California, seven of which were confirmed by CDC during the week of October 13, two of them on Friday, October 18. All California cases have occurred in dairy workers on affected farms. All available data so far suggests sporadic instances of animal-to-human spread. The farm workers who were diagnosed with avian flu infections in California all described mild symptoms, many with eye redness or discharge (conjunctivitis). None of the workers were hospitalized. CDPH is monitoring hundreds of workers in affected counties, and any who develop symptoms are being tested; if the test is positive in the state lab, the sample is sent to CDC for confirmatory testing. CDC is reporting confirmed cases, by state and source of exposure, in a table on its website, which is being updated three times weekly. 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 are pending. To date, human-to-human transmission of avian influenza A(H5) virus has not been identified in the United States. CDC believes the immediate risk to the general public from H5N1 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 320 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 38 commercial poultry flocks and 26 backyard flocks, for a total of 21.43 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 13 farm workers have been infected with H5N1 bird flu in California. Genetic sequencing of six 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 been able to sequence the full genomes of viruses detected in two cases from California (A/California/135/2024 and A/California/147/2024) and to confirm that both are B3.13 genotype viruses. Specimens from four other California cases were partially sequenced. That sequencing information showed no changes in the hemagglutinin (HA) associated with increased infectivity or transmissibility among people. Additionally, there are no mutations associated with reduced susceptibility to available treatments (e.g., neuraminidase inhibitors or polymerase inhibitors) and no mutations identified in other genes indicating additional mammalian adaptation. Genetic sequencing for those six cases has been posted in GISAID and GenBank. Additional sequencing data will be posted as it becomes available. CDC has successfully isolated virus from specimens from nine of the thirteen 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) provide good inhibition of these viruses.

Epidemiology Update
At the invitation of the California Department of Public Health, CDC is deploying a multilingual epidemiological field team to assist efforts to learn more about how this outbreak began in California and how to lower 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.

Surveillance Update
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 5,100 people have been monitored as a result of their exposure to infected or potentially infected animals, and at least 260 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 54,000 specimens have been tested for avian A(H5) or other novel influenza viruses at public health labs. One of the specimens, collected as a part of routine surveillance, was identified as presumptive A(H5) positive and was confirmed as H5N1 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.

Outreach Update
CDC has posted a Spanish-language collection of videos aimed at informing the U.S. public in general, and U.S. farm workers in particular, about H5N1 bird flu – what it is, how to prevent it, and what to do if they develop symptoms. English-language versions of the videos were first posted in September. The videos, each under a minute long and available on YouTube, answer commonly asked questions, such as "Who is at risk for getting H5N1 bird flu?", "How can I protect myself from getting H5N1 bird flu?", and "What do I do if I get H5N1 bird flu?"

CDC continues to support outreach to farm workers around the topic of H5N1 bird flu. This includes targeted outreach to farm workers in affected counties through Meta (Facebook and Instagram) and digital display and audio (Pandora). CDC also is running advertisements on local radio stations about 30 times each week to reach people who may not be on social media platforms. These resources provide information in English and Spanish about potential risks of H5N1 bird flu infection, recommended preventive actions, symptoms to be on the lookout for, and what to do if people develop symptoms. Outreach metrics are summarized in Table 1 (below). CDC also is developing an outreach strategy to raise awareness about the free seasonal flu vaccinations being offered to farm workers in affected states this season.