The Tuberculin Test and its Role in the Strategic Management and Eradication of Tuberculosis in Cattle

Abstract

This thesis explores the necessity for bovine tuberculosis (bTB) eradication; considers the use the tuberculin test as a tool in bTB eradication and explores the need to select the optimal test methodology, to evaluate outcomes of existing procedures and to progressively modify aspects of a bTB eradication programme. Tuberculosis remains a significant disease of animals and humans worldwide. The mycobacteria which cause bTB have an extremely wide host range and serious, although currently probably under-diagnosed, zoonotic potential. Where bTB controls are effective, clinical cases are infrequent in cattle and human zoonotic TB is uncommon. The design of a bTB control or eradication programme should, take into consideration fundamental scientific and veterinary knowledge, the inherent strengths and limitations of the test, the epidemiological profile of disease, non-bovine maintenance hosts and the experience of other eradication programmes. Programme managers need to be mindful of the necessity for robust quality control: including tuberculin assay, monitor of test performance and other surveillance protocols, and assessment of herd health and other bio-security practices. Continuous and rational evaluation of policy efficacy, constraints to progress, optimal desirable goals and appropriate policy adaptations requires data and epidemiological analysis capability. The Irish national bTB eradication programme is utilised in this thesis as an example of science informed policy in a national context. The studies reported were representative of an ongoing monitor and evaluation process of the quality of aspects of tuberculin, the tuberculin test and the continued re-evaluation of existing policy protocols. Tuberculin tests are safe to use and continue to be the primary tool in bTB eradication programmes with the choice of tuberculin test determined by the ecosystem in which it will be used. This thesis assesses the impact of different potencies of bovine tuberculin on the comparative tuberculin test (SICTT) and the single intradermal test (SIT). It also compares the performance of tuberculin from different manufacturers. There was a significant difference in the number of reactors detected using high and low potency tuberculin but no apparent impact if the supplier/manufacturer was changed provided the potency remained constant. However, potency estimates from guinea pig bio-assay conducted during these studies were imprecise. Two examples of policy review exercises forming part of the quality control assessment on the outputs of the SICTT are provided. These affirm the reliability of the SICTT, in Ireland, at current levels of bTB infection and demonstrate that the Irish depopulation policy for bTB (depopulation, disinfection, contiguous testing and local badger removal where implicated) is successful in enabling herds attain and retain bTB freedom following restocking. In conclusion control and eradication of tuberculosis in cattle is a desirable objective for animal welfare, socio-economic and zoonotic reasons. Tuberculin potency is critical to test performance affecting both test sensitivity and specificity and therefore, the accurate determination of potency is particularly important. In the future, as now, eradication of bTB will undoubtedly continue to require a multifaceted approach if it is to be successful.