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.

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.

The findings suggest that cases in people and animals in other states have yet to be identified, the authors say.