Accurate estimation of the probability of observing a reactor to the skin test in officially tuberculosis free herds in a high prevalence region.

SIT animal-level specificity exceeds herd-level performance in OTF herds; risk-based strategies recommended.

This study evaluated the performance of the single intradermal tuberculin test (SIT) in officially tuberculosis-free (OTF) herds within a high-prevalence region using Bayesian hierarchical modeling. Analysis of 3,039 cattle across 42 OTF beef herds demonstrated a median probability of 1.24% for detecting reactors using strict interpretation criteria, with increased risk in older animals. However, herd-level simulation revealed a 70.8% probability of observing at least one reactor per herd, corresponding to only 29.2% herd-level specificity—substantially lower than field observations showing >98% of Spanish herds test negative annually. Post-mortem examination failed to confirm infection in any reactors. The findings underscore that while SIT maintains reliable animal-level diagnostic performance in OTF herds, herd-level interpretation proves problematic when complete exposure exclusion remains uncertain. The authors recommend implementing risk-based surveillance strategies to mitigate unnecessary OTF status suspensions caused by false-positive results.

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