Comparative analysis of tuberculin and defined antigen skin tests for the detection of bovine tuberculosis in buffaloes (Bubalus bubalis)

Background: Bovine tuberculosis (bTB) is a chronic disease that results from infection with any member of the Mycobacterium tuberculosis complex and infected animals are typically diagnosed withtuberculin-based intradermal skin tests per World Organization of Animal Health or similar guidelines. Peptide-based defined skin test (DST) antigens, comprising of ESAT-6, CFP-10 and Rv3615c, are able to differentiate infected from BCG-vaccinated animals and sensitively and specifically identify tuberculin reactor cattle, but their performance in buffaloes remained unknown. To assess the comparative performance of the DST with the tuberculin-based single intradermal test (SIT) and the single intradermal comparative cervical test (SICCT), we screened 543 female buffaloes from 49 organized dairy farms in two districts of Haryana state in India. Results: The results show that 37 (7%), 4 (1%) and 18 (3%) buffaloes were reactors with the SIT, SICCT and DST, respectively. Of the 37 SIT reactors, four were positive with SICCT and 12 were positive with the DST. The results further show that none of the animals tested positive with all three tests, and 6 DST positive animals were SIT negative. Together, a total of 43 animals were reactors with SIT, DST, or both, and the two assays showed moderate agreement (Cohen'sKappa 0.41; 95% CI: 0.23, 0.59). In contrast, only slight agreement (Cohen’s Kappa 0.18; 95% CI: 0.02, 0.34) was observed between SIT and SICCT. Latent class analyses reveal test specificities of 95% for SIT and 99% each for DST and SICCT, but considerably lower sensitivities of 67%, 39%, and 19% for SIT, DST, and SICCT, respectively, albeit with broad and overlapping credible intervals. Conclusion: Taken together, our investigation suggests that DST has a test specificity comparable with SICCT, and sensitivity intermediate between SIT and SICCT for the identification of buffaloes suspected of tuberculosis. Our studies also highlight an urgent need for future well-powered trials with detailed necropsy with immunological and microbiological profiling of reactor and non-reactor animals to better define the underlying drivers for the large observed discrepancies in assay performance, particularly between SIT and SICCT.