August 9, 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, 13 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 poultry12. Based on the information available at this time, CDC´s current assessment is that the immediate risk to the general public from H5 bird flu remains low. On the animal health side, USDA is reporting that 189 dairy cow herds in 13 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. USDA reports that since April 2024, there have been A(H5) detections in 35 commercial flocks and 21 backyard flocks, for a total of 18.68 million birds affected.

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

Risk Assessment
A new CDC report published today summarizes the findings of an assessment of the potential pandemic risk associated with an avian influenza A(H5N1) virus (clade 2.3.4.4b [A/Texas/37/2024]) isolated from a human case in Texas. The assessment was performed using the Influenza Risk Assessment Tool, an instrument developed by CDC and external influenza experts to evaluate the pandemic potential of influenza A viruses that currently circulate in animals but not in humans. The future pandemic potential of a virus is different from the immediate population or individual health risk, which is unchanged and remains low. Findings of IRATs are used to inform preparedness planning. This most recent IRAT score validates the proactive, coordinated U.S. government response. Systematic comparisons of data related to this avian influenza A(H5N1) virus using the IRAT to data from other influenza A viruses has scored this virus´s future pandemic potential as "moderate" based on information through June 26, 2024. This is similar to previous assessments of earlier avian influenza A(H5N1) viruses.

This analysis, which includes data through June 26, 2024, did not include the 10 human infections that occurred in Colorado in July following exposure to infected poultry (9 cases) and dairy cows (1 case). All were associated with mild illness and a majority experienced conjunctivitis. CDC completed the report to inform ongoing preparedness efforts.

The IRAT uses expert opinion to evaluate the potential for a representative novel influenza virus to gain the ability for person-to-person spread and the resulting potential public health impact if that were to happen, compared to that of other viruses evaluated in past IRAT reports. The IRAT does not predict future pandemics.

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 that a virus not currently circulating in the human population has the potential for significant impact on public health?
Earlier IRATs were conducted in March 2022 using A/American wigeon/South Carolina/AH0195145/2021 and again in April 2023 using A/mink/Spain/3691-8_22VIR10586-10/2022. This IRAT assessment indicates that while the virus that caused an H5N1 bird flu infection in a person in Texas [A/Texas/37/2024 virus] in 2024 scored slightly lower in some risk elements and slightly higher in others compared with two previously assessed avian influenza A(H5N1) clade 2.3.4.4b viruses, the mean-high and mean-low acceptable score ranges for these viruses overlap. This means that the future pandemic potential risk from these viruses remains similar and is classified as "moderate" risk.

Laboratory Update
CDC is posting summary results of a new laboratory study using a ferret animal model. The study looked at disease severity and spread of an avian influenza A(H5N1) virus from a human case in Michigan in ferrets. Ferrets are commonly used as a model for humans in the study of influenza viruses because they are susceptible to influenza virus infection and display clinical signs of infection shared by humans. The study showed that A/Michigan/90/2024 caused less severe disease in ferrets than an avian influenza A(H5N1) virus from the human case in Texas (A/Texas/37/2024).

While the A/Texas virus was deadly in all infected ferrets, none of the ferrets infected with the A/Michigan virus died or had to be euthanized because of severe illness. This finding is important because A/Michigan/90/2024 is more similar to currently circulating viruses than A/Texas/372024. The two viruses showed similar, less-than-efficient capacity to spread from ferret to ferret by respiratory droplets. CDC scientists used two models to assess the ability of A/Michigan/90/2024 to spread between ferrets: a "direct contact model" and a "respiratory droplet model." In the direct contact model, infected and healthy ferrets are placed in the same enclosure. In the respiratory droplet model, infected and healthy ferrets are placed in adjacent enclosures that share the same air. The results were similar to those of a previous CDC study in ferrets of the A/Texas/372024 virus from the human case in Texas that was reported on April 1, 2024.

CDC also continues to meet with commercial laboratories to discuss H5 assay licensing agreements and interest in development of commercial H5 tests. CDC´s Technology Transfer Office and the Influenza Division are actively pursuing licensing agreements with multiple companies, and several have been completed.

Outreach Update
CDC continues to support outreach to farm workers. This includes specific 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).

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 avian influenza A(H5N1) viruses. To date, more than 4,500 people have been monitored as a result of their exposure to infected or potentially infected animals, and at least 230 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 39,500 specimens have been tested at public health labs that would have detected avian influenza A(H5) or other novel influenza viruses. More information on monitoring can be found at Symptom Monitoring Among Persons Exposed to HPAI.

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.

Publication Highlights
An August 7, 2024, Letter to the Editor co-authored by CDC and Michigan local public health and state health officials in the New England Journal of Medicine provides a detailed description of avian influenza A(H5N1) virus infection in two dairy farm workers in Michigan, including their symptoms, course of illness, treatment, and genetic analysis of the avian influenza A(H5N1) viruses from the two farm workers. One of the workers developed eye soreness consistent with conjunctivitis and the other experienced more typical symptoms of acute respiratory illness associated with influenza virus infection, including avian influenza A(H5N1) viruses. The authors note that the infections, which occurred on separate farms, highlight the ongoing risk of transmission through direct and close exposure to raw milk and secretions from infected dairy cows.