September 13, 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, 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, USDA is reporting that 202 dairy cow 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. Most recently, outbreaks in cows on eight California dairy farms were confirmed. USDA reports that since April 2024, there have been A(H5) detections in 35 commercial flocks and 22 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:

Missouri Case Update
Missouri continues to lead the investigation into the H5 case reported last week with technical assistance from CDC in Atlanta. The case was in a person who was hospitalized as a result of significant underlying medical conditions. They presented with chest pain, nausea, vomiting, diarrhea, and weakness. The person was not severely ill, nor were they in the intensive care unit. They were treated with influenza antiviral medications, subsequently discharged, and have since recovered. One household contact of the patient became ill with similar symptoms on the same day as the case, was not tested, and has since recovered. The simultaneous development of symptoms does not support person-to-person spread but suggests a common exposure. Also shared by Missouri, subsequently, a second close contact of the case – a health care worker – developed mild symptoms and tested negative for flu. A 10-day follow-up period has since passed, and no additional cases have been found. There is no epidemiologic evidence to support person-to-person transmission of H5 at this time. CDC´s original report about the case in Missouri is available: CDC Confirms Human H5 Bird Flu Case in Missouri | CDC Newsroom.

CDC has attempted to sequence the full genome of the virus from the most recent case of H5 reported by Missouri. Because of low amounts of genetic material (viral RNA) in the clinical specimen, sequencing produced limited data for analyses. Full-length gene sequences were obtained for the matrix gene (M) and non-structural (NS) genes and partial gene sequences were obtained for the hemagglutinin (HA) and neuraminidase (NA) genes. The available gene sequences are all closely related to U.S. dairy cow viruses, and similar sequences have been found in birds and other animals around dairy farms, raw milk, and poultry.

The HA gene sequence confirms that the virus is clade 2.3.4.4b, and the NA sequence was confirmed as N1. There are two amino acid differences in the HA that have not been seen in sequences from previous human cases. These amino acid differences are not known to be associated with changes to the virus´s ability to infect and spread among people. However, both differences are in locations that may impact the cross-reactivity of clade 2.3.4.4b candidate vaccine viruses (CVVs). Additional antigenic testing is planned. One of the two amino acid differences (HA A156T) has been identified in fewer than 1 percent of viruses detected in dairy cows. The other amino acid difference (HA P136S) has been seen in only a single dairy cow sequence.

In addition to the HA analysis, no markers of reduced susceptibility to neuraminidase inhibitors and no markers of mammalian adaptation were found. These findings suggest that currently available neuraminidase inhibitors for influenza are expected to maintain their effectiveness and that the virus from this person does not show signs of increased potential to spread from person to person. Sequence data for A/Missouri/121/2024 was submitted to GISAID (EPI_ISL_19413343) and GenBank (not yet available). Additionally, multiple attempts to propagate virus from the clinical specimen were not successful.

Also, since the last update, CDC performed antiviral susceptibility testing of avian influenza A(H5N1) viruses isolated from the previously reported human cases associated with poultry exposure in Colorado. All of the virus specimens were determined to be susceptible to FDA-approved and recommended antivirals (oseltamivir, zanamivir, peramivir, and baloxavir).

On a related note, CDC is awarding contracts that will significantly bolster the United States´ ability to respond to public health emergencies through securing additional nationwide testing capacity and data reporting for new and emerging health threats, including H5N1 bird flu. Under the contracts, public- and private-sector laboratories would be able to begin testing immediately in collaboration with CDC, performing 10,000 tests per week at the beginning of a response. If needed, these same laboratories could scale to over 100,000 tests per week over a short period of time, enhancing access to critical clinical laboratory data across more than 200 reportable conditions to support ongoing situational awareness for the country. These contracts both strengthen CDC´s emergency response and allow private laboratories to develop new tests alongside CDC.

Outreach Update
CDC has posted a new collection of videos aimed at informing the U.S. public in general, and U.S. farm workers in particular, about H5N1 bird flu – what it is, how to prevent it, and what to do if they develop symptoms. The videos, each under a minute long and available on YouTube, answer commonly asked questions, such as "Who is at risk for getting H5N1 bird flu?", "How can I protect myself from getting H5N1 bird flu?", and "What do I do if I get H5N1 bird flu?" Spanish-language versions of these videos are being posted as they are finalized.

CDC continues to support other forms of 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
A CDC team of epidemiologists, veterinarians, and health communication specialists is working with the Ohio Department of Health this week on a serosurvey that will assess past exposure to H5 viruses among a national network of veterinarians and other veterinary professionals. To do this, CDC and Ohio staff are at a bovine practitioner conference enrolling people, asking them questions about exposures, and taking blood samples to look for antibodies to H5 viruses. The goal of this activity is to better understand the extent of prior influenza A(H5) infections among people who work closely with cows and the current risk of infection associated with occupational exposures among people who work frequently with cows in various settings and with different degrees and types of exposures.

CDC has updated its H5 Monitoring page to display results of testing for avian influenza A(H5) virus in wastewater reported to CDC. There are now approximately 168 sites in 41 states testing and reporting to CDC, and these numbers will continue to increase. In each site, wastewater samples are collected and tested for avian influenza A(H5) virus one or more times weekly. If any of the wastewater samples from a site test positive, that site will be classified as "H5 Detection"; if no samples test positive for avian influenza A(H5), the site will be classified as "No Detection." Data will be updated on the CDC site every Friday and displayed in an interactive map. All data are preliminary and may change as more reports are received. Wastewater data cannot determine the source of avian influenza A(H5) viruses, which could come from a human or from an animal (like a bird) or an animal product (like milk from an infected cow). However, while detections of avian influenza A(H5) virus in wastewater do not necessarily indicate human cases, they complement data from traditional influenza surveillance systems and help inform public health planning and response. Efforts to monitor avian influenza A(H5) virus activity using wastewater data are likely to evolve as the methodologies and interpretation are evaluated and refined.

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 4,800 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 47,000 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 CDC. 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.