The report is the latest evidence that the outbreak in dairy herds is spreading undetected in cows, and the spillover into people at highest risk of exposure is going unnoticed.

Out of 150 participating veterinary practitioners across the country, three had antibodies indicating recent infection with Type A H5N1 bird flu, based on blood samples collected in September 2024, noted Samantha Olson, MPH, of the CDC's National Center for Immunization and Respiratory Diseases, and colleagues.

Bird flu infections were detected among cattle veterinarians tested last fall, including two people with no known exposure to animals with the virus, according to a report released by the U.S. Centers for Disease Control and Prevention on Thursday.