Mycobacteria such as M. tuberculosis, which is the main cause of TB in humans, and M. bovis (bTB), the main cause in cattle, have a very complex interaction with their host and its immune system and are very successful pathogens. In fact about one third of the world’s human population is infected with human TB and the incidence of disease is increasing. There are many reasons for this and the shortcomings of the available diagnostic tests are important amongst these. The principal method for the diagnosis of bovine TB (bTB) in cattle is the single intra-dermal comparative tuberculin test (SICTT) which has been available for decades and detects the cells in the immune system which respond to TB. Recently another test for these cell-mediated immune (CMI)responses has become available, the gamma interferon test. It is very difficult to develop a ‘gold standard test for bovine TB which will sensitively and specifically detect all stages of infection, from just after the organism invades the body right up until the late stages of advanced disease. Tests for immune responses to TB, rather than directly for the organism itself, have the greatest flexibility and utility for detecting TB. However, immune responses to TB are very complex making good tests difficult to develop. The main reasons for this are:
1. The immune responses to M. bovis, are complicated and variable throughout the course of infection. CMI responses are considered the principal response however humoral (antibody) responses are increasingly being recognised as important.
2. There are at least 120 Mycobacterial species in the environment and they are very homogenous with very similar structural components. The genome has about 4000 genes and these show a high degree of similarity with those of M bovis, and the proteins recognised by the immune system can be very similar too. Differentiation between types is very difficult for the immune system to achieve, and this creates difficulties for developing good diagnostic tests. Recent studies have shown that to make an effective diagnostic test for TB you need to be able to test for responses to several TB antigens at the same time. Conventional testing methods do not permit this due to reasons of cost and complexity.
3. The biology / pathology of mycobacteria – M bovis is very slow growing, clinical signs can take months/years to develop and the bacteria are thought to be able to become latent (where infection remains in a dormant state). During this phase immune responses are at a low level and to test for the infection means that you have to stimulate the immune system to amplify the response. This has been a cornerstone of CMI testing which routinely involves stimulation with antigen, either directly injected into the skin or incubated with cells in vitro. This ‘recall’ method of testing has not been routinely used with tests for antibody.