The Centers for Disease Control and Prevention (CDC) continues to respond to the public health challenge posed by a multi-state outbreak of avian influenza A(H5N1) virus, or "H5N1 bird flu," in dairy cows, poultry and other animals in the United States. The 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, 14 human cases of avian influenza A(H5) virus infection have been reported in the United States. Four of these cases were associated with exposure to sick dairy cows and nine were associated with exposure to avian influenza A(H5N1) virus-infected poultry. A BThe source of the exposure in the most recent case, which was reported by Missouri on September 6, has not been determined. The immediate risk to the general public from H5 bird flu remains low.

On the animal health side, the USDA is reporting that 238 dairy herds in 14 U.S. states have confirmed cases of avian influenza A(H5N1) virus infections in dairy cows as the number of infected herds continues to grow. The USDA reports that since April 2024, there have been A(H5) detections in 36 commercial flocks and 23 backyard flocks, for a total of 18.75 million birds affected.

The Missouri Department of Health and Senior Services has not, to date, identified ongoing transmission among close contacts of the case, first reported on Sept. 6, 2024. Missouri identified two healthcare workers who were exposed to the hospitalized case before droplet precautions were instituted (i.e., higher risk exposure) and subsequently developed mild respiratory symptoms (among 18 workers with this higher risk exposure); one tested negative for influenza by PCR, as previously reported, and the second provided a blood specimen for testing by CDC for potential influenza A(H5N1) antibodies. Missouri has since identified four additional healthcare workers who later developed mild respiratory symptoms. One of these workers was in the higher risk category and provided a blood specimen for H5 antibody testing. Three of these workers are among 94 workers who were exposed to the hospitalized case of avian influenza A(H5) after droplet precautions were instituted (i.e., lower risk exposure); blood specimens for those who became symptomatic have been collected for H5 antibody testing at CDC.

Aside from the one healthcare worker reported to have tested negative for influenza by PCR, the five remaining exposed healthcare workers had only mild symptoms and were not tested by PCR for respiratory infections. PCR testing would have been unreliable at the time of discovery of these individuals' prior symptoms. The healthcare worker monitoring effort has been part of the ongoing investigation as previously reported. Results of serology testing at CDC on the positive case and their previously identified household contact are still pending. To date, only one case of influenza A(H5N1) has been detected in Missouri. No contacts of that case have tested positive for influenza A(H5N1). The CDC continues to closely monitor available data from influenza surveillance systems, particularly in affected states, and there has been no sign of unusual influenza activity in people, including in Missouri.

The CDC has developed a validated protocol for subtyping of wastewater C samples that test positive for influenza A. These assays include primers and probes that target various subtypes, including influenza A, influenza A(H1N1), influenza A(H3), and influenza A(H5). Partner laboratories can use this protocol to bring these assays online and report influenza A virus subtyping data to CDC. Providing partner laboratories with a standardized approach to test wastewater for influenza A subtypes, including influenza A(H5N1), can support a more robust wastewater surveillance system.

The CDC has issued updated guidance for employers and workers to reduce the risk of Novel Influenza A for people working with or exposed to animals. The updated guidance identifies occupational groups at increased risk of exposure and actions employers can take to protect workers, including additional engineering and administrative control measures, how to identify personal protective equipment (PPE) that meets the safety specifications, and how to mitigate heat stress when wearing PPE in hot environments. Employers can use these updated materials to help prevent workplace exposures to H5N1 bird flu. A new hazard assessment worksheet can help dairy facility employers determine when and how to implement protective measures. Employers can also print, post, and share new fact sheets to help workers safely use PPE and protect themselves from H5N1 bird flu.

The CDC continues to support outreach to farm workers around the topic of H5N1 bird flu. This includes target 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 avian influenza A(H5N1) infection, recommended preventive actions, symptoms to be on the lookout for, and what to do if they develop symptoms. Outreach metrics are summarized in Table 1 (below). CDC is developing an outreach strategy to raise awareness about the free seasonal flu vaccinations being offered to livestock workers in affected states this season.

The 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 avian influenza A(H5N1) viruses. To date, more than 5,000 people have been monitored as a result of their exposure to infected or potentially infected animals, and at least 240 people who have developed flu-like symptoms have been tested as part of this targeted, situation-specific testing. Testing of exposed people who develop symptoms is happening at the state or local level, and CDC conducts confirmatory testing. Since February 25, 2024, more than 50,000 specimens have been tested at public health labs that would have detected avian influenza A(H5) or other novel influenza viruses. One of the specimens tested at a public health laboratory as a part of routine surveillance was identified as presumptive A(H5) positive and was confirmed as A(H5) positive by the CDC.

The CDC also continues to monitor flu surveillance data using CDC's enhanced, nationwide summer 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, including avian influenza A(H5N1) viruses.

As a reminder, the CDC recommends that:

• People should avoid exposures to sick or dead animals, including wild birds, poultry, other domesticated birds, and other wild or domesticated animals (including cows), if possible.
• People should also avoid exposures to animal poop, bedding (litter), unpasteurized ("raw") milk, or materials that have been touched by, or close to, birds or other animals with suspected or confirmed avian influenza A(H5N1) virus, if possible.
• People should not drink raw milk. Pasteurization kills avian influenza A(H5N1) viruses, and pasteurized milk is safe to drink.
• People who have job-related contact with infected or potentially infected birds or other animals should be aware of the risk of exposure to avian influenza viruses and should take proper precautions. People should wear appropriate and recommended personal protective equipment when exposed to an infected or potentially infected animal(s). CDC has recommendations for worker protection and use of personal protective equipment (PPE).

CDC has interim recommendations for prevention, monitoring, and public health investigations of avian influenza A(H5N1) virus infections in people.
Following these recommendations is central to reducing a person's risk and containing the overall public health risk.

Society for Healthcare Epidemiology of America (SHEA) president Tom Talbot, MD, MPH, chief hospital epidemiologist at Vanderbilt University Medical Center, issued a statement on the investigation following the exposure to H5N1 flu by six symptomatic healthcare workers reported late last week in Missouri:

"The report from the Centers for Disease Control and Prevention suggesting the potential transmission of H5N1 bird flu to healthcare workers in Missouri shows the response is too slow, and we need to do a better job coordinating during an emerging public health emergency. CDC received reports weeks too late of cases of healthcare personnel who developed symptoms after exposure to a patient with H5N1. These healthcare personnel did not receive rapid PCR tests, which created a gap in knowledge that is slowing the ongoing investigation. H5N1 human-to-human transmission is an indicator of pandemic potential. Prompt detection through surveillance and a coordinated response among federal agencies, state public health, and healthcare are critical. Effective surveillance, data sharing, and transparency would help avoid the mistakes that hindered the response to COVID-19 in the early stages of the pandemic."