The Incidence of Bovine Tuberculosis in a Dairy Herd Practicing Irregular Skin Test and Slaughter Control Program

The combination of intensified animal husbandry and development of peri-urban systems have corresponded with increased bovine tuberculosis incidence. Its economic impact is primarily driven by direct effects, particularly due to test and culling of affected animals. A retrospective study was conducted to assess the incidence of bTB in a dairy herd practicing repeated irregular skin testing and slaughter control program. The incidence at the subsequent test rounds ranged from 5.4% to 24.8%. These incidences exhibited an oscillating pattern: it initially decreased from 21.3% to 8.4%, then resurged to 24.8% in the third round, and ultimately declined back to 5.4%. Penultimate test result, breed of the animal, and herd composition were significantly associated with the odds of them becoming a reactor to the SICCTT at a subsequent test (P<0.05). The study findings indicated that animals undergoing two consecutive repeated skin tests had an approximately 11 times higher risk of bovine tuberculosis (bTB) infection compared to newly introduced animals. Similarly, animals with an inconclusive penultimate test result were 2.64 times more likely to be infected with bTB than those with a negative penultimate test result. Likewise, reactors that had been embedded by inconclusive penultimate SICCTT result were more likely to have visible lesions at slaughter than those with a negative penultimate SICCTT result. The study herd consisted of a mix of purebred Boran and different crossbred animals. The prevalence of bTB was high in purebred Boran than crossbred animals. In conclusion, the study confirmed the necessity of considering inconclusive SICCTT test results and the retested herd (with inconsistent and extremely prolonged retesting schemes) contribution, which all were likely responsible for the chance to increase the number of new bTB cases.