July 19, 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, 10 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 six were associated with exposure to H5N1-infected poultry.* [A][B] This includes two additional cases in Colorado that were confirmed by CDC this week and not previously reported. The two new cases were in poultry workers with exposure to infected poultry during depopulation and disposal activities. Similar to previous cases, illness was mild. Based on the information available at this time, CDC’s current H5N1 bird flu human health risk assessment for the U.S. general public remains low. On the animal health side, USDA is reporting that 157 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 34 commercial flocks and 16 backyard flocks, for a total of 18.32 million birds affected.

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

Reporting the preliminary results of the Michigan-led seroprevalence investigation. CDC analyzed sera (blood) collected from people who were exposed to dairy cattle infected with highly pathogenic avian influenza (HPAI) A(H5N1) clade 2.3.4.4b viruses causing outbreaks among animals in the United States. These blood samples were collected as part of a type of study called a seroprevalence study. Seroprevalence studies test people’s blood for antibodies (an immune response) specific to a pathogen of interest, in this case HPAI A(H5N1). These studies can tell us whether someone has been previously infected.
Blood samples were collected in June 2024 from 35 people who work on dairies in Michigan with herds that were confirmed positive for HPAI A(H5N1) virus.
Study participants were from multiple counties and had different roles on affected farms, but most worked with sick cows directly and fewer than half reported using masks or goggles.
These samples were tested for antibodies against an avian influenza A(H5N1) clade 2.3.4.4b virus and a seasonal influenza virus (control virus) to measure antibodies.
Specimens were tested by two methods: microneutralization and hemagglutination inhibition (HI) assays.
None of these 35 people showed neutralizing or HI antibodies (a sign of prior infection) specific to avian influenza A(H5N1) virus.
Many of the people had neutralizing antibodies to seasonal flu.
The detection of antibodies to seasonal flu suggests that, not unexpectedly, participants in the study had been previously infected or vaccinated with seasonal influenza viruses and were able to generate an immune response.
The lack of antibodies to avian influenza A(H5N1) virus suggests these people were not previously infected with an avian influenza A(H5N1) influenza virus.
These data are consistent with other data demonstrating the seroprevalence to HPAI A(H5N1), even among workers with known exposures, is low.
Additional data are necessary to fully understand the occupational risks of exposure to the currently circulating avian influenza A(H5N1) viruses.
The Michigan Department of Health and Human Services is continuing the study and working with CDC to analyze the data and prepare a manuscript for peer-review publication once complete
Keeping in close contact with a CDC multidisciplinary bilingual field team of 10 people deployed to support Colorado’s response to ongoing poultry outbreaks. The team includes epidemiologists, veterinarians, clinicians and an industrial hygienist. Support includes accompanying Colorado health officials during daily visits to the affected farm to monitor workers for symptoms, test symptomatic persons, dispense antiviral treatment, follow-up with persons testing positive to collect additional information, and assess use of personal protective equipment.
Posting the sequence of the influenza virus genome from a positive specimen collected from one patient in Colorado participating in an A(H5N1) infected poultry farm depopulation (A/Colorado/109/2024) to GISAID (EPI_ISL_19263923) and GenBank (PQ032835). CDC sequenced the influenza virus genome, confirming the neuraminidase (the N in the subtype) is an N1 and the virus is a HPAI A(H5N1) virus from clade 2.3.4.4b. An analysis of the sequence indicates:
This virus is genotype B3.13 clade 2.3.4.4b HPAI A(H5N1) with each individual gene segment closely related to viruses detected in recent poultry outbreaks and infected dairy cattle herds made available from USDA. Among human virus sequences, A/Colorado/109/2024 is most similar to the genome of the human case in Michigan.
The sequence maintains primarily avian genetic characteristics and lacks changes that would make the virus better adapted to infect or spread among humans.
The sequence contains a change at PB2 M631L – which is the same marker of mammalian adaptation identified in >99% dairy cow sequences and that was also identified in the first Michigan human case.
The genome does NOT have the PB2 E627K change that was seen in the virus from the case in Texas, but not subsequent human viruses. (That change is known to be associated with viral adaptation to mammalian hosts and has been found before in people and other mammals, but with no evidence of onward spread among people.)
Furthermore, there are no markers known to be associated with influenza antiviral resistance found in the virus sequences from the patient´s specimen.
This is the only virus that CDC has been able to fully sequence out of the cases in Colorado so far. Partial gene segments were obtained from two of the other confirmed human cases associated with the poultry farm depopulation. Only internal genes (not HA and NA) generated sufficient data for analysis to date. This data confirmed identical gene sequences among the CO cases based on available data.
The HA sequence of the Colorado poultry worker virus is very closely related (only 2 or 4 amino acid changes) to two existing HPAI A(H5) candidate vaccine viruses [IDCDC-RG78A (A/American Wigeon/South Carolina/22-000345-001/2021-like) and IDCDC-RG71A (A/Astrakhan/3212/2020-like), respectively] that are already available to manufacturers, and which could be used to make vaccine if needed.
There are no changes to the virus that would suggest the risk to human health has increased.
Overall, the genetic analysis of the HPAI A(H5N1) virus in Colorado supports CDC´s conclusion that the human health risk currently remains low.
Continuing 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. Ten of those licenses are currently in place, and several more are in progress.
Held a Clinician Outreach and Communication Activity (COCA) Call on July 16, 2024, that gave an update on the current outbreak in the United States and current CDC surveillance and monitoring efforts. More than 4,600 people attended. The recording is available online.
Working in partnership with the American Association of Bovine Practitioners to implement a bovine practitioner veterinarian seroprevalence study that will take place in two months.
Continuing to support strategies to maximize protection of farm workers, who are at higher risk of infection based on their exposures. This includes targeted outreach to farm workers in affected counties through Meta (Facebook and Instagram), digital display, and audio (Pandora). These resources provide information in English and Spanish about potential risks of A(H5N1) infection, recommended preventive actions, symptoms to be on the look-out for, and what to do if they develop symptoms. Since May 30, when English assets launched, Meta outreach has generated nearly 7.8 million Spanish Meta assets launched on June 6, and since then have garnered 2.7 million impressions. (Table 1)
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 1,570 people have been monitored as a result of their exposure to infected or potentially infected animals, and at least 62 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 35,500 specimens have been tested at public health labs using a methodology that would have detected influenza A(H5) or other novel influenza viruses. More information on monitoring can be found at Symptom Monitoring Among Persons Exposed to HPAI.
Continuing 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.