Tuberculosis Caused by Mycobacterium bovis in a Sheep Flock Colocated with a Tuberculous Dairy Cattle Herd in Central Ethiopia

Mycobacterium bovis (M. bovis) has an exceptionally wide host range including sheep. Information on tuberculosis (TB) in sheep is scarce, and there appears to be conflicting opinions about the relative susceptibility of sheep to infection. In Ethiopia, there was no single previous report on tuberculosis in sheep, though mixed farming of cattle and sheep is a common practice. In this study, following the observation of TB-like lesions on sheep died from sheep flock kept in contact with cattle herd, further investigation was conducted on the flock to assess the magnitude of the infection and identify and characterize the causative M. bovis strain. An outbreak investigation was carried out on 26 eligible sheep out of 33 sheep found on the farm. Comparative intradermal tuberculin (CIDT) test, postmortem examination, Mycobacterium culturing, and spoligotyping were the techniques used in the study. The prevalence of TB in the tested sheep was 15% (4/26). All the sheep that were positive to CIDT had gross lesions suggestive of TB. Three of the positive sheep had extensive and multiple lesions. M. bovis was isolated from all four sheep and the strain was identified as spoligotype SBO134. The in-contact dairy cows were screened for TB and 98% (45/46) of the cows tested positive to CIDT. Furthermore, the same strain, SB0134, was also isolated from the two in-contact cows. The isolation of a matching genotype (SB0134) of M. bovis from both species sharing a known epidemiologic link strongly suggests that the sheep flock might have acquired the pathogen from the dairy cows. This warrants strict physical separation of the sheep flock from the cattle herd to prevent such interspecies transmission of M. bovis.

Appraisal of Interpretation Criteria for the Comparative Intradermal Tuberculin Test for Diagnosis of Tuberculosis in Cattle in Central Ethiopia

ABSTRACT Accurate detection and removal of infected cattle, using immunodiagnostic tests such as the comparative intradermal tuberculin (CIDT) test, are the basis of control strategies for bovine tuberculosis (TB). According to the Office des Internationale Epizooties recommendation, the cutoff point for positivity of the CIDT test, calculated as the difference between skin thicknesses after bovine tuberculin (B) and avian tuberculin (A) injections (B − A), is >4 mm. This cutoff point is used worldwide, although it is likely that local conditions influence test performance. Thus, this study was formulated to determine CIDT test cutoff points applicable to cattle in central Ethiopia. Receiver operating characteristic analysis was performed for the CIDT test, using data from 186 Bos indicus (zebu) and Bos taurus (Holstein) cattle. Detailed postmortem examination for the presence of TB lesions was used to define disease status. At a cutoff of >2 mm, CIDT test sensitivity was 69% (95% confidence interval [95% CI], 58.5 to 79%), while it was 59% (95% CI, 49 to 69%) at a cutoff of >4 mm. In contrast, specificities of the CIDT test at these two cutoff values were identical, at 97% (95% CI, 89 to 100%). Thus, the maximum sensitivity of the CIDT test can be realized using a >2-mm cutoff without affecting specificity. The apparent prevalence was significantly (χ2 = 13.56; P < 0.001) higher at a cutoff of >2 mm (16.0%; n = 5,424) than at a >4-mm cutoff (13.5%; n = 5,424). Nonetheless, no significant difference (χ2 = 2.15; P = 0.14) in true prevalence was observed at a cutoff of >2 mm (19.6%) and at a cutoff of >4 mm (18.5%). Thus, our study demonstrates the importance of defining local, relevant cutoff values to maximize test sensitivity, and we suggest the application of the >2-mm cutoff for testing of cattle in central Ethiopia.