Despite ongoing eradication efforts, bovine tuberculosis (bTB) remains a challenge in Michigan livestock and wildlife. The overall objective of the studies described in this dissertation was to identify ways of improving eradication of bTB in Michigan and beyond. To first understand bTB in Michigan, a descriptive epidemiological study of bTB in Michigan livestock and wildlife was conducted using retrospective data. An important conclusion was that bTB has been contained to a focal area of Michigan; however eradication may require exploration of new surveillance strategies or diagnostic tests. To further understand current bTB surveillance strategies being used in Michigan and elsewhere, a review of the current literature was undertaken. A focus of the review was to identify gaps that should be addressed. An important conclusion from the review was that adaptation of bTB screening at points-of-concentration (POC) of cattle could aid identification of herds that require targeted surveillance, thereby furthering bTB eradication efforts. Based on the review work, a study was conducted to explore the adaptation of the whole blood interferon-gamma (IFN-γ) assay for use at slaughter facilities, a POC of cattle. The goal of the study was to determine if blood obtained at exsanguination of cattle, experimentally sensitized with inactivated Mycobacterium bovis (M. bovis), can produce measureable IFN-γ to bTB. Results indicated that most sensitized cattle would retain their positive bTB IFN-γ assay result at exsanguination, despite a decline in IFN-γ production at exsanguination. To further understand the decline in IFN-γ production, two trials were performed to evaluate the time of blood collection and total lymphocyte counts as potential factors affecting measurable IFN-γ production at slaughter. Results in both trials demonstrated that sensitized cattle were more likely to retain their positive bTB IFN-γ assay result at commencement of exsanguination than at successive time points. Therefore, it was concluded that time of blood collection is crucial to obtaining valid bTB IFN-γ assay result at exsanguination. A decline in IFN-γ production at exsanguination was associated with a decline in total lymphocyte counts, suggesting that reduced lymphocyte counts could be a responsible factor in the declined IFN-γ production. Therefore, to avoid false negative results, lymphocyte numbers should be standardized before antigen stimulation in blood collected at exsanguination or different IFN-γ assay classification criteria developed for such samples. Finally, based on the results of the sensitization studies, cattle under field conditions were evaluated to determine if valid IFN-γ assay results can be obtained from blood collected at exsanguination. Cattle in this field study originated from herds belonging to one of three risk categories: bTB infected herds, a bTB exposed herd, and herds with no known history of bTB exposure. Cattle with and without bTB-like gross lesions were identified as bTB IFN-gamma; assay positive using blood collected at exsanguination and later one of the cattle was confirmed bTB positive by bacterial culture. All the IFN-γ assay positive cattle originated from either the infected or the exposed herds but none from the non-exposed herds. Therefore, blood collected at exsanguination could be used to identify bTB exposed herds or to monitor bTB status in bTB-free zones, providing an additional tool for the control and eradication of bTB. In conclusion, the entire project identified areas of prime focus in the Michigan bTB challenge and has recommended a novel surveillance strategy that, if integrated at POC of cattle, could advance bTB control and eradication.
Read
