August 2, 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) 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)-infected poultry. A B Based on the information available at this time, CDC´s current assessment is that the risk to the general public from H5 bird flu remains low. On the animal health side, USDA is reporting that 172 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 20 backyard flocks, for a total of 18.37 million birds affected.

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

Laboratory Update
CDC successfully sequenced full genomes of the viruses from the three human cases reported from the second poultry farm in Colorado. These cases occurred in workers exposed to H5N1-infected poultry. Sequencing of these viruses confirms the neuraminidase (the N in the subtype) for each of these three viruses is an N1, and the viruses are a HPAI A(H5N1) virus from clade 2.3.4.4b. The sequences are closely related to viruses detected in recent poultry outbreaks and infected dairy cattle herds and made available from USDA. The sequences maintain primarily avian genetic characteristics and lack changes that would make them better adapted to spread among humans. They also do not contain any changes suggesting an increase in disease severity or adaptation to human infection or spread. Furthermore, there are no markers known to be associated with influenza antiviral resistance. The genetic sequences for the viruses were publicly posted. C Overall, the genetic analysis of this HPAI A(H5N1) virus in Colorado supports CDC´s conclusion that the human health risk currently remains low.

On the laboratory front, 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 establishing licensing agreements with multiple companies, and several have been completed.

Partnership and Support Update
CDC announced two new investments to protect the health of farmworkers at heightened risk of exposure to H5N1 bird flu. First, $5 million will go to partner organizations, including the National Center for Farmworker Health, to support overall outreach and education efforts focused on farmworker safety and health. Second, a separate $5 million investment will be used to offer seasonal flu vaccines to U.S. livestock, dairy, and poultry workers to control and prevent seasonal flu among this group. Preventing seasonal flu infection and severe disease outcomes is good public health practice and something that CDC focuses on each year. Reducing barriers to seasonal flu vaccine and increasing uptake in livestock, dairy, and poultry workers protects people from severe illness, which can also help reduce strain on rural health care systems in the fall and winter when more people are likely to have respiratory illnesses. Fewer people who get the seasonal flu in the fall and winter also will help public health better detect if a person is sick with avian flu. Although the seasonal flu vaccine will not prevent infection from bird flu, it can reduce the prevalence and severity of seasonal flu and might reduce the very rare risk of coinfection with a human seasonal virus and an avian virus at the same time, and the theoretical risk that reassortment between the two could result in a new virus. Such dual infections, while very rare, could theoretically result in genetic reassortment of the two different influenza A viruses and lead to a new influenza A virus that has a different combination of genes, and which could pose a significant public health concern.

CDC also continues to partner with states that are conducting seroprevalence studies. CDC is supporting a Colorado Department of Public Health and Environment seroprevalence study of dairy workers exposed to H5N1 infected cattle that began this week. CDC will support interviews, specimen collection and testing from multiple farms through early August. The preliminary Michigan results were already shared on July 19 (CDC A(H5N1) Bird Flu Response Update, July 19, 2024), but they are continuing to enroll additional nearby farms in Michigan.

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. Since May 30, when English assets launched, Meta outreach generated about 16.6 million impressions. Spanish Meta assets launched on June 6, and since then garnered about 3.2 million impressions. (Table 1)

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 4,300 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 38,000 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.