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.

Three U.S. dairy veterinarians unknowingly had H5N1 infections, otherwise known as bird flu. This included one person who worked only in states without known outbreaks in cows, according to a new study from the U.

A new Centers for Disease Control and Prevention study suggests that bird flu may be spreading under the radar in the U.S. In September, the CDC tested 150 veterinarians who primarily worked with cattle in the past three months.