A new Morbidity and Mortality Weekly Report revealed that 7% of farm workers (eight individuals out of a sample population of 115) exposed to H5N1 bird flu during outbreaks among dairy cows in Michigan and Colorado showed evidence of recent infection—including several workers who did not recall experiencing symptoms associated with H5 infections.
There is no evidence of any person-to-person spread in either of the two states or anywhere in the United States, the Centers for Disease Control and Prevention reported.
Given these latest findings, the CDC reviewed its existing H5N1 guidance and, jointly with partners, has tailored recommendations to protect workers at the highest risk and help contain the spread of the virus. On Nov. 7, Nirav D. Shah, M.D., J.D., CDC’s principal deputy director, and Demetre Daskalakis, M.D, M.P.H., CDC’s director of the National Center for Immunization and Respiratory Diseases, held a conference call during which they reported the new recommendations.
- People should avoid exposure to sick or dead animals, including wild birds, poultry, other domesticated birds and other wild or domesticated animals (including cows), if possible.
- People should also avoid exposures to animal feces, bedding (litter), unpasteurized (“raw”) milk or materials that have been touched by, or close to, birds or other animals with suspected or confirmed avian influenza A(H5N1) virus, if possible.
- People should not drink raw milk. Pasteurization kills avian influenza A(H5N1) viruses, and pasteurized milk is safe to drink.
- People who have job-related contact with infected or potentially infected birds or other animals should be aware of the risk of exposure to avian influenza viruses and should take proper precautions. People should wear appropriate and recommended personal protective equipment when exposed to an infected or potentially infected animal(s). CDC has recommendations for worker protection and use of personal protective equipment.
CDC continues to respond to the public health challenge posed by the multistate outbreak of avian influenza A(H5N1) virus in dairy cows, poultry and other animals in the U.S. CDC is working in collaboration with the U.S. Department of Agriculture, the Food and Drug Administration, Administration for Strategic Preparedness and Response, state public health and animal health officials and other partners using a One Health approach.
Confirmed cases in 44 people
Since April, CDC, working with state public health departments, has confirmed H5 bird flu infections in 44 people in the U.S. Nineteen of these cases were associated with exposure to H5N1 bird flu-infected poultry, and 24 were associated with exposure to sick or infected dairy cows [A][B]. The source of the exposure in one case, which was reported by Missouri on Sept. 6, could not be determined. Serological testing of the contacts of the Missouri case have been reported, and that investigation has concluded.
The 44 cases include 20 cases in dairy farm workers in California, three of which were confirmed by CDC in the last week of October and three on Monday, Nov. 4; nine cases in poultry farm workers in Washington state, three of which were confirmed by CDC last week; and one case associated with the Washington poultry outbreak that was confirmed by CDC last week and is pending jurisdiction assignment. Not included in that count are four probable cases — one in a California dairy farm worker and three in Washington state poultry farm workers.
While these probable cases were negative on confirmatory testing at CDC, all four met the Council of State and Territorial Epidemiologists probable case definition and have been reported by the states. Cases in California and Washington have occurred in workers on affected farms.
Animal-to-human spread ‘sporadic’
All available data so far suggest sporadic instances of animal-to-human spread. The farm workers in California and Washington state all described mild symptoms, many with eye redness or discharge (conjunctivitis).
Some workers who tested positive in Washington reported some mild upper respiratory symptoms. None of the workers were hospitalized. Updated case counts, including by state and source of exposure, are reflected in a table on CDC’s website. To date, person-to-person spread of H5 bird flu has not been identified in the U.S. CDC believes the immediate risk to the general public from H5 bird flu remains low, but people with exposure to infected animals are at higher risk of infection.
Most infected dairy herds in California
On the animal health side, since March 2024, USDA has confirmed infected cattle in 440 dairy herds in 15 U.S. states. The number of affected herds continues to grow nationally, with almost all new infections identified in herds in California. USDA reports that, since April, there have been H5 detections in 45 commercial poultry flocks and 30 backyard flocks, for a total of 22.37 million birds affected.
Epidemiology Update
On Oct. 30, USDA reported an avian influenza infection in a pig on a backyard farm in Oregon. This is the first time an H5 bird flu infection has been reported in a pig in the U.S. Sequence data from birds in the avian influenza A(H5) virus outbreak on this backyard farm showed no mutations that caused concerns related to disease severity or adaptability to humans.
The discovery that an avian influenza A virus has infected a new mammal species is always concerning, especially when the virus is detected in pigs, which are susceptible to influenza viruses circulating in pigs, humans, birds and other species. These viruses can swap genes through a process called genetic reassortment, which can occur when two (or more) influenza viruses infect a single host. Reassortment can result in the emergence of new influenza A viruses with new or different properties, such as the ability to spread more easily among animals or people. Reassortment events have happened in pigs in the past. A series of reassortment events in pigs is believed to have caused the 2009 influenza A(H1N1) pandemic. Based on available information, the risk to the general public remains low; however, CDC is continuing to gather information.
A multilingual CDC field team continues to assist the California Department of Public Health in its efforts to learn more about how the outbreak in California began and how to lower the risk to farm workers with exposure to infected cows. Two staff members are on the ground in California, and additional staff are ready to deploy if needed. CDC staff are assisting with active surveillance efforts, including field assessments of suspected cases and household contacts; testing and treatment; and dissemination of information to farm workers and the community. A separate CDC field team has returned from deployment to Washington state but continues to work remotely with the local health department on data management and epidemiological summaries. There is no evidence of any person-to-person spread in either of the two states or anywhere in the U.S.
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 H5N1 bird flu. To date, more than 6,700 people have been monitored as a result of their exposure to infected or potentially infected animals, and at least 340 people who have developed flu-like symptoms have been tested as part of this targeted, situation-specific testing. More information on monitoring can be found at Symptom Monitoring Among Persons Exposed to HPAI.
For regular updates, visit the CDC website.
David Murray can be reached at [email protected].
