Tuberculosis in a cattle herd can often go undetected since the early stages of infection do not produce clinical signs. In later stages, signs are lethargy, weakness, anorexia, pneumonia, fever, and chronic moist cough. Most cases of bovine TB in the US are detected by veterinary meat inspectors from apparently healthy cattle, and the animal is then traced back to the source herd. Live cattle are tested for exposure to the Mycobacterium bovis organism by an intradermal injection of the tuberculin purified protein derivative into the tail fold. The site is checked in 72 hours for an immune response (swelling), indicating previous exposure to the organism. A positive result may require more specific testing to rule out exposure to closely-related organisms (false positives). For a previously infected herd to be considered free of the disease, multiple negative herd tests will be required.

Source: Dr. Rick Rasby, Professor of Animal Science, Animal Science, University of Nebraska - Lincoln