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.

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.

Three veterinarians specializing in cattle tested positive, according to the study published in the Morbidity and Mortality Weekly Report Thursday. The vets said they never experienced any flu-like symptoms.