Abstract
Bovine tuberculosis (bTB) remains endemic in domestic water buffaloes (Bubalus bubalis) in India and elsewhere, with limited options for control other than testing and slaughter. The prescribed tuberculin skin tests with purified protein derivative (PPD) for diagnosis of bTB preclude the use of Bacille Calmette-Guérin (BCG)-based vaccination because of the antigenic cross-reactivity of vaccine strains with Mycobacterium bovis and related pathogenic members of the M. tuberculosis complex (MTBC). For the diagnosis of bTB in domestic water buffaloes, we here assessed a recently described defined-antigen skin test (DST) that comprises overlapping peptides representing the ESAT-6, CFP-10 and Rv3615c antigens, present in disease-causing members of the MTBC but missing in BCG strains. The performance characteristics of three doses (5, 10 or 20 μg/peptide) of the DST were assessed in natural tuberculin skin test reactor (n = 11) and non-reactor (n = 35) water buffaloes at an organized dairy farm in Hisar, India, and results were compared with the single intradermal skin test (SIT) using standard bovine tuberculin (PPD-B). The results showed a dose-dependent response of DST in natural reactor water buffaloes, although the SIT induced a significantly greater (P < 0.001) skin test response than the highest dose of DST used. However, using a cut-off of 2 mm or greater, the 5, 10, and 20 μg DST cocktail correctly classified eight, 10 and all 11 of the SIT-positive reactors, respectively, suggesting that the 20 μg DST cocktail has a diagnostic sensitivity (Se) of 1.0 (95% CI: 0.72–1.0) identical to that of the SIT. Importantly, none of the tested DST doses induced any measurable skin induration responses in the 35 SIT-negative animals, suggesting a specificity point estimate of 1.0 (95% CI: 0.9–1.0), also identical to that of the SIT and compares favorably with that of the comparative cervical test (Se = 0.85; 95% CI: 0.55–0.98). Overall, the results suggest that similar to tuberculin, the DST enables sensitive and specific diagnosis of bTB in water buffaloes. Future field trials to explore the utility of DST as a defined antigen replacement for tuberculin in routine surveillance programs and to enable BCG vaccination of water buffaloes are warranted.
Highlights
Bovine tuberculosis is a chronic inflammatory disease of cattle caused by members of the Mycobacterium tuberculosis complex, and in addition to being an important animal health problem, bTB poses a significant threat to public health [1]
IGRAs were conducted to compare the performance of the defined antigen skin test (DST) peptide cocktail, composed of ESAT-6, CFP-10, and Rv3615c, with that of the PPDs (Figure 1)
The standard bovine tuberculin antigen (PPD-B), used in the single intradermal test (SIT), induced a significantly stronger skin test response than that induced by the highest dose of DST used in this study (P < 0.001) and identified all 11 reactors as positive, whereas the cervical test (CCT) identified nine of the 11 buffaloes as reactors
Summary
Bovine tuberculosis (bTB) is a chronic inflammatory disease of cattle caused by members of the Mycobacterium tuberculosis complex, and in addition to being an important animal health problem, bTB poses a significant threat to public health [1]. The disease is well-controlled in high-income countries, bTB remains endemic in most low- and middle-income countries (LMICs), including India where bTB has significant impacts in terms of decreased productivity, increased mortality and zoonotic threat. India has the largest livestock population in the world, including nearly 191 million cows and 109 million buffaloes [3]. BTB has been well-studied in cattle generally, studies on this disease in buffalo are scarce, especially for high-producing breeds like Murrah
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