Advances in ante-mortem diagnosis of tuberculosis in cattle

The tuberculin skin test is effective in the early detection of pre-clinical cases of Mycobacterium bovis infection in cattle. This allows the rapid removal of infected animals, thus limiting transmission of the disease, and has resulted in the eradication of bovine tuberculosis (Tb) from many countries. This test is very likely to remain the primary screening test for M. bovis infection in cattle as it is a simple, robust and inexpensive test. However, a number of ancillary tests are being used, or are currently being validated. These ancillary tests are likely to provide a more accurate diagnosis following skin-testing. The blood-based BOVIGAM interferon-gamma (IFN-gamma) test is a cellular immune assay which can detect early infection, and has become the main ancillary test in New Zealand. It can be used for re-testing skin test-positive animals, to improve specificity and minimise wastage from slaughtering animals with false-positive tests. Alternatively, it can be used in locations of increased risk of infection in parallel with skin-testing, for examining skin test-negative animals for pre-movement testing or in problem herds to identify M. bovis-infected animals that do not respond to the skin test. Several modifications of the test are now being used to improve specificity by altering the cut-off or using specific antigens present in virulent mycobacteria such as the 6 kDa early secreted antigenic target (ESAT-6) and 10 kDa culture filtrate protein (CFP-10). While antibody based tests generally lack sensitivity, as high levels of antibodies tend to occur late in the disease process, they may have unique desirable properties such as the ability to be used as a cow-side test. The use of these new ancillary tests in association with skin-testing will improve the detection of M. bovis-infected cattle and reduce the unnecessary slaughter of false-positive reactors.