Farmers whose cattle herds are suffering a TB breakdown often say: 'But my reactor animal had no visible lesions of TB at slaughter, so surely that means it wasn’t infected?'.
The TB Hub has provided information for farmers which dispels a common belief that cattle with no visible lesions (NVL) are not infected with bovine TB. It includes a study that was completed at APHA Weybridge which demonstrated the relatively poor sensitivity of commercial post-mortem inspection for confirming M. bovis infection in cattle and the high sensitivity of the gamma test.
- The skin test is highly specific, which means that we can be confident that the vast majority of skin test reactors are truly infected with TB, irrespective of whether or not lesions typical of TB are found at slaughter.
- The gamma test is less specific than the skin test. However although the false positive rate is higher we can still be confident that the majority of test-positive animals are truly infected with TB, regardless of whether visible lesions of TB are found at slaughter.
- Skin and gamma test reactors do not necessarily show lesions typical of TB at post mortem inspection. This is because post mortem inspection in a commercial slaughterhouse setting is relatively insensitive and skin/gamma reactors are often in the early stages of infection when TB lesions are too small to be seen by the naked eye.