May 17, 2024 – CDC continues to respond to the public health challenge posed by a multistate outbreak of avian influenza A(H5N1) virus, or “A(H5N1) virus,” in dairy cows 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), state public health and animal health officials, and other partners using a One Health approach. USDA is now reporting that 51 dairy cattle herds in nine U.S. states have confirmed cases of A(H5N1) virus infections in cattle. There have been no additional human cases detected since the one recent case from Texas was reported on April 1, 2024. [1][2]

Among other activities previously reported in past spotlights and still ongoing, recent highlights of CDC’s response to this outbreak of influenza A(H5N1) virus in dairy cattle and other animals include:

Continuing 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 300 people have been monitored as a result of their exposure to infected or potentially infected animals, and at least 37 people who have developed flu-like symptoms have been tested as part of this targeted, situation-specific testing. All except the one case in Texas have tested negative. Testing of exposed people who develop symptoms is at the state or local level, and CDC conducts confirmatory testing as needed.
Working on a plan for enhanced, nationwide summer monitoring to help ensure that even rare cases of A(H5N1) virus infection in the community would be detected. This plan includes increasing the number of influenza virus specimens that are tested and then subtyped in public health laboratories, which can detect A(H5N1). Nationally, since March 24, 2024, almost 11,000 specimens have been tested for influenza in public health laboratories as part of this surveillance stream. While influenza testing typically declines over the summer, this approach would maintain an increased level of testing. More information on this will be forthcoming.
Beginning the process of conducting a pandemic risk assessment on the virus from the human infection in Texas (A/Texas/37/2024) using the Influenza Risk Assessment Tool (IRAT). The IRAT is an evaluation tool developed by CDC and external flu experts that assesses the potential pandemic risk posed by novel influenza A viruses. The IRAT uses 10 risk elements to measure the potential of an influenza virus to emerge to cause a pandemic as well as the potential public health impact that a pandemic caused by that virus would have. An IRAT is a multi-step process that can take months to complete. (More on IRAT below.) The current assessment of the risk level to the general public, which is based on available epidemiologic and laboratory data, remains low.
Continuing to monitor flu surveillance data, especially in areas where A(H5N1) viruses have been detected in dairy cattle or other animals, for any unusual trends including in flu-like illness, conjunctivitis, or influenza virus activity.
CDC posted influenza A wastewater surveillance data for this first time this week. Wastewater surveillance complements other existing human flu surveillance systems to monitor trends in influenza viruses. Current wastewater monitoring methods detect influenza A viruses but do not distinguish the subtype or source of the influenza A virus (whether it is from humans or animals). Reported levels of influenza A virus from each site are compared against levels reported by the same site during the prior flu season. When influenza A levels in wastewater are high (at the 80th percentile or higher), that triggers follow up by CDC, including outreach to the relevant jurisdiction to determine the source of the signal and intensive surveillance review. The data are presented in the format of an interactive map and updated weekly.
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.
More on the CDC IRAT Process
CDC’s current public health risk assessment has been informed by initial epidemiology and supporting laboratory assessments and is outlined in an April 26, 2023, Technical Report. As mentioned above, CDC is beginning the process of implementing a full influenza risk assessment on A/Texas/37/2024 using the Influenza Risk Assessment Tool. The IRAT is an evaluation tool designed for use by public health professionals for pandemic preparedness purposes to help prioritize decision-making, including decisions related to the development of countermeasures. It is a multi-step process that takes several months to complete and involves a panel of experts completing complex analysis and scoring. The IRAT assesses potential pandemic risk based on two different questions related to “emergence” and “public health impact.”

“Emergence:” What is the risk that a virus not currently circulating in the human population has potential for sustained human-to-human transmission?
“Public health impact:” If the virus were to achieve sustained human-to-human transmission, what is the risk for significant impact on public health?
Previously, CDC has used the IRAT to assess many novel influenza A viruses, including four different A(H5N1) viruses. The most recent IRAT on A(H5N1) viruses was conducted on A/mink/Spain/3691-8_22VIR10586-10/2022, a contemporary A(H5N1) virus that caused an outbreak among mink at a mink farm in Spain during 2022. While that assessment resulted in a slightly higher score in some risk elements compared with a previously assessed A(H5N1) virus isolated from an American wigeon duck in 2022, the mean-high and mean-low acceptable score ranges for these viruses overlapped, indicating that these viruses remained similar, and their overall risk scores remained “moderate.” The average weighted risk score for the estimated potential “emergence” of the mink virus was 5.13, which is within the range of the moderate risk category (moderate scores range from 4.0 to 7.9). The average weighted risk score for the mink virus to potentially “impact public health” was 6.24, which also is within the range of the moderate risk category. The decision to conduct an IRAT on the A/Texas/37/2024 virus is based largely on the fact that the virus has emerged in a new host in the United States. While CDC believes that the immediate threat to the public posed by this virus remains low, the outcome of the IRAT will address the potential risk that this virus will change to cause human-to-human spread as well as the potential public health impact if that were to happen. Ongoing studies, including the ferret transmissibility and pathogenicity studies currently ongoing at CDC, will be used by experts to inform the IRAT. It’s important to gather as much information as possible on this virus. In the meantime, preventing opportunities for this virus to spread and further evolve is imperative.

CDC Recommendations
People should avoid close, long, or unprotected exposures to sick or dead animals, including wild birds, poultry, other domesticated birds, and other wild or domesticated animals (including cows).
People should also avoid unprotected 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 A(H5N1) virus.
CDC has interim recommendations for prevention, monitoring, and public health investigations of A(H5N1) virus infections in people. CDC also has updated recommendations for worker protection and use of personal protective equipment (PPE). Following these recommendations is central to reducing a person’s risk and containing the overall public health risk.
In addition to limiting interactions between infected animals and people, containing the outbreak among animals also is important, which underscores the urgency of the work being done by USDA and animal health and industry partners.

Additionally, high levels of A(H5N1) virus have been found in unpasteurized milk from infected cows. CDC and FDA recommend against the consumption of raw milk or raw milk products. The risk of human infection from drinking raw milk containing live A(H5N1) virus specifically is unknown. A(H5N1) virus binds to receptors that are primarily located in the lower respiratory tract of people. To date, A(H5N1) viruses have not acquired the ability to bind to virus receptors that are most prevalent in the upper respiratory tract of people. If a person consumed raw milk with live A(H5N1) virus, the person could become infected, theoretically, by the virus binding to a limited amount of virus receptors in the upper respiratory tract or by aspiration of virus into the lower respiratory tract where receptors that A(H5N1) viruses can bind to are more widely distributed. There are unpublished anecdotal reports of raw duck blood consumption as a potential source of A(H5N1) virus infection in a small number of human cases in the past. As a reminder, while the two U.S. cases of A(H5N1) had relatively mild illness, globally, A(H5N1) virus infections in people have ranged in severity from mild to deadly, with about a 50% fatality rate. CDC specifically recommends against consuming raw milk infected with A(H5N1) virus as a way to develop antibodies against A(H5N1) virus to protect against future disease. Consumption of contaminated raw milk could be dangerous. A recent report in Emerging Infectious Diseases documented the death of domestic cats that had been fed raw milk from sick cows. The cats quickly developed neurologic signs and blindness and high levels of A(H5N1) virus were found in their brains. Neurologic signs and high mortality are not uncommon among other mammals with A(H5N1) virus infection.

This is a rapidly changing situation, and CDC is committed to providing frequent and timely updates.

Footnotes
[1] The first human case of A(H5N1) bird flu in the United States was reported in 2022 in a person in Colorado who had direct exposure to poultry and was involved in the depopulating of poultry with presumptive A(H5N1) bird flu. The 2022 human case was not related to dairy cattle. The person only reported fatigue without any other symptoms and recovered. Learn more at U.S. Case of Human Avian Influenza A(H5) Virus Reported.

[2] The second human case of A(H5N1) bird flu in the United States was reported in 2024 and linked with dairy cattle and reported eye redness as their only symptom, consistent with conjunctivitis, and has recovered. Learn more about this case in a letter published in the New England Journal of Medicine titled Highly Pathogenic Avian Influenza A(H5N1) Virus Infection in a Dairy Farm Worker.