Veterinarian performing a tuberculin skin test on a cow. The tuberculin skin test or single intradermal comparative cervical tuberculin test (SICCT) is the primary test for detection of bovine tuberculosis (TB) in herd populations. Bovine TB is caused by the bacterium Mycobacterium bovis. The thickness of the skin on the cow's neck is measured at two different sites. Dead fragments of M. bovis and Mycobacterium avium are then injected into the two separate sites. The skin thickness is measured again after three days. If the cattle has bovine TB, M. bovis will trigger an immune response, producing a swelling or lump at the injection site. If the skin thickness at the M. bovis site is significantly larger than the skin thickness at the M. avium site, it can be concluded that the cattle is infected with bovine TB.