Bovine tuberculosis screening. Vet measuring a fold of skin on the neck of a cow prior to injecting it with tuberculin. This is the first step in the intradermal comparitive tuberculin test, the standard test used to screen cattle for bovine tuberculosis (TB). During the test tuberculin from Mycobacterium bovis is injected into the skin at one site in the neck. In most cattle infected with M. bovis the tuberculin causes localised sweeling. To ensure that any reaction is not caused by other mycobacteria, tuberculin from M. avium is injected at another site. The size and nature of the allergic reactions at each site are compared to determine a negative, positive, or inconclusive result.