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.

A new study shows the bird flu has silently spread from animals to some veterinarians. It echoes two smaller studies that detected evidence of infection in previously undiagnosed farmworkers.