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.

Blood testing of large-animal veterinarians suggests that H5N1 bird flu has spread more widely than US surveillance of the virus is capturing, according to a new study by federal and state disease detectives.

A new CDC report found evidence of previous bird flu infection in veterinary practitioners who work with cattle and who did not know they had been exposed.

The vets had no symptoms, and one worked only in states where no dairy infections had been reported. Three dairy veterinarians, including one who worked only in states with no known bird flu outbreaks in cows,