If an animal produces a positive response to a test for TB (a.k.a. a test “reactor”) and then shows no visible lesions (NVL) on Post Mortem, your first response may be to think that the test was wrong, assume that it was a false positive result and that your animal was unnecessarily culled.
However, we need to realize that there are other explanations.
If the test is very good at detecting early infection there might not yet have been time for lesions to develop. In that case, it’s a brilliant test, because it could then be a tool for removing animals before they are infectious, allowing us to prevent the spread of infection.
If the test detects an immune response to infection, rather than infection itself, this is still very useful: Once exposed to the tb bacterium an animal could pose a serious risk if it remains undetected. Although a proportion of animals which are exposed to infection may become immune, there’s no way of knowing which ones remain carriers with latent Tb infection. At least knowing which animals have been exposed, gives us a way to halt infection by not giving them the chance to start spreading it. In a herd where TB infection has been identified, by culture of M. bovis, the presumption has to be that all animals may have been exposed to the bacterium. The control of many infectious animal diseases, including bovine TB, is predicated on the early detection and removal of infected animals before they become infectious to other animals, allowing us to break the cycle of infection. Remember that our herds and neighbouring livestock and wildlife are at risk, and as this is a zoonosis, so are we.
The only way to find out whether the Gamma Interferon or Rapid Stat Pak test throw up real false positives, is to test a large number of animals which are not believed to have been exposed to TB, and see how many react to the test. If the BAS/BCL funded trial of the gamma interferon and Rapid Stat Pak tests didn’t go ahead, we will never know how good the tests are; we absolutely can not infer anything about their specificity (i.e. whether they really throw up false positives) from using them only in infected herds.