Unexpected high responses to tuberculin skin-test in farmed red deer: implications for tuberculosis control

Tuberculosis (TB) in deer is a serious zoonotic disease of worldwide distribution. Detection
of infected animals is usually performed using single or comparative skin-testing (SST/CST),
although false responses due to sensitization to other mycobacteria may occur, hampering
diagnostic specificity. We describe the evolution of the responses to the SST, CST and to
an in-house serological assay in a red deer farm subjected to regular TB testing in southern
Spain in an attempt to understand the dynamics of possible non-specific reactions
occurring under field conditions. We performed 2288 skin-tests and ELISAs in nine sampling
periods between May 2009 and January 2011. In May 2010, a strong increase in skin
fold thickness in response to avian purified protein derivative (PPD) (mean = 4.0 mm, 95%
CI = 3.5–4.5) and bovine PPD (mean = 1.8 mm, 95% CI = 1.6–2.0) was observed in yearling
deer hinds (n = 150), compared to values recorded for the same individuals in November
2009 (avian PPD: mean = 0.7 mm, 95% CI = 0.6–0.8 and bovine PPD: mean = 0.7 mm, 95%
CI = 0.6–0.7) and in January 2011 (avian PPD: mean = 2.2 mm, 95% CI = 1.9–2.4 and bovine
PPD: mean = 1.1 mm, 95% CI = 1.0–1.2). Using SST, 54 animals (36%) of the yearlings tested
in May 2010 would have been classified as positive reactors, while none of them was
positive in the CST. The five animals with highest skin fold increases to mycobacterial antigens
were culled and subjected to post-mortem analysis, which confirmed the absence of
Mycobacterium tuberculosis complex (MTBC) infection but demonstrated the presence of
environmental mycobacteria and closely related bacteria in four out of the five analyzed
animals. Our results demonstrated how non-specific responses to mycobacterial antigens
can adversely affect the specificity of TB diagnosis based on the SST. Thus, once TB infection
has been ruled out using confirmatory techniques, application of comparative diagnostic
tests is highly advisable to maximize test specificity and avoid the slaughter of false positive
reactors