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.

A CDC study has found H5N1 bird flu antibodies in veterinarians who had no symptoms and no knowledge they had been working with infected livestock.

Bird flu infections were detected among 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.

Three dairy veterinarians, including one who worked only in states with no known bird flu outbreaks in cows, had recent, undetected bird flu infections, according to a new study from the Centers for Disease Control and Prevention. The results are based on antibody testing of 150 veterinarians working in 46 U.S. states.

A new study shows that bird flu has silently spread from animals to some veterinarians. The study published Thursday by the Centers for Disease Control and Prevention echoes two smaller ones that detected evidence of infection in previously undiagnosed farmworkers.

After going quiet on bird flu, CDC scientists have published a report on its spread among veterinarians. The findings suggest a need for better surveillance.

For the first time since 1952, publication of the CDC's Morbidity and Mortality Weekly Report was halted. It returned on Feb. 6, but without planned bird flu studies.

Federal health officials say a dairy worker in Nevada has been infected with a new type of bird flu that’s different from the version that has been spreading in U.S. herds since last year.

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.