Responses to diagnostic tests for bovine tuberculosis in dairy and non-dairy cattle naturally exposed to Mycobacterium bovis in Great Britain

Abstract

Field surveillance of British cattle using the single intradermal comparative cervical tuberculin (SICCT) test shows a higher incidence rate of bovine tuberculosis (bTB) in dairy compared to beef herds, but a lower probability of post-mortem examination confirmed (PMC) Mycobacterium bovis infection in dairy herds. A cross-sectional study was conducted to compare animal level differences in bTB detection between dairy and non-dairy cattle in Great Britain. During the period from 2002 to 2005, 200 (41% dairy) reactors in the SICCT test (standard interpretation) were randomly selected, and 200 in-contact cattle (43% dairy) were purposively selected from bTB-infected herds. Interferon (IFN)-γ responses in blood to bovine and avian purified protein derivative (PPD), and early secretory antigen target 6 kDa and culture filtrate protein 10 (ESAT-6/CFP10), were measured. The post-mortem examination included gross pathological examination, mycobacterial culture and histopathology. The proportions of cattle positive to ESAT6/CFP10 were 26% (95% confidence interval, CI, 15–39%) in dairy reactors and 62% (95% CI 51–72%) in non-dairy reactors (P < 0.001). PMC risk was 34% (95% CI 24–45%) in dairy reactors and 69% (95% CI 60–78%) in non-dairy reactors (P < 0.001). The odds ratio for PMC risk in dairy reactors compared to non-dairy reactors, after controlling for bTB prevalence, herd size and SICCT test response, was 0.27 (95% CI 0.14–0.53; P < 0.001). In surveillance data, adjusted animal level PMC risks were lower for dairy reactors than for beef reactors aged >2 years (P < 0.001).