September 20, 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. 12The 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 213 dairy 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. USDA reports that since April 2024, there have been A(H5) detections in 36 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 with technical assistance from CDC in Atlanta. CDC´s original report about the case in Missouri is available: CDC Confirms Human H5 Bird Flu Case in Missouri | CDC Newsroom. Earlier this week, the Missouri Department of Health and Senior Services collected blood samples from the person who tested positive for H5N1 in Missouri and a household contact who became ill on the same day and was not tested. Both persons have since recovered. The samples are being sent to CDC for serologic testing to look for antibodies to avian influenza A(H5) virus, which would indicate a previous infection. The simultaneous development of symptoms in two people does not support person-to-person spread but suggests a common exposure. As part of the ongoing contact investigation, Missouri identified one additional health care worker contact who had developed mild respiratory symptoms and was not tested for influenza as the illness had resolved before the investigation began. Serologic testing will be offered to this second health care worker. As CDC previously reported, one health care worker with exposure to the infected individual was tested for influenza and was negative. CDC continues to closely monitor available data from influenza surveillance systems, particularly in affected states, and there has been no sign of unusual influenza activity in people, including in Missouri.

Last week, CDC reported on sequencing of the clinical specimen from the Missouri case.

Outreach Update
CDC continues to support outreach to farm workers around the topic of H5N1 bird flu. This includes target 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). CDC is developing an outreach strategy to raise awareness about the free flu vaccinations being offered to livestock workers in affected states this season.

Surveillance Update
CDC posted a spotlight with more information about a serosurvey begun last week that is aimed at assessing past exposure to H5 viruses among members of a national network of veterinarians and other veterinary professionals. The study, conducted by CDC in collaboration with the Ohio Department of Health and the American Association of Bovine Practitioners (AABP), enrolled about 150 attendees of the AABP´s annual conference held last week in Columbus, Ohio.

CDC is also posting a new webpage describing influenza surveillance activities for the 2024-2025 flu season that are aimed at identifying possible spread of HPAI A(H5) to and among people while monitoring seasonal influenza activity. Activities range from individual-level symptom monitoring among those exposed to infected/potentially infected animals (specifically swine, cattle, poultry and other avian species) to monitoring of surveillance data for the general population. These activities will be modified as new information becomes available or the situation changes in a way that warrants a revised approach.

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 4,900 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 49,000 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.

Publication Highlight
A new MMWR report published on September 19, 2024 summarizes data from the first 9 weeks of monitoring influenza A virus and avian influenza A(H5) in wastewater across the United States, including findings from collaborations with state and local health departments to investigate potential sources. The authors report that from May 12 to July 13, 2024, high levels of influenza A virus were detected in wastewater in 4 states, including 3 states with ongoing seasonal human influenza virus activity. During the same period, 9 states (California, Colorado, Idaho, Iowa, Michigan, Minnesota, North Carolina, South Dakota, and Texas) reported one or more H5 detections in wastewater, with follow-up investigations in many of these states pointing to animal-related inputs as the likely source. The findings underscore the importance of a One Health response to this outbreak in dairy cows and the value of coordination across different sectors, including health, agriculture, wildlife, food safety, and others.

Relatedly, a new CDC-authored editorial published on September 19, 2024 in the American Journal of Public Health outlines the challenges and opportunities for wastewater monitoring of influenza viruses in the context of the ongoing threat from HPAI A(H5N1) viruses. The authors recommend actions to ensure the effectiveness of wastewater monitoring and its use for this evolving threat to public health.