Comparative efficacity of the skin test and the γ-IFN assay to diagnose bovine tuberculosis in a field assay

A field assay for comparison of the skin test (ST) and the γ-IFN assay has been performed over a total number of 1036 catlle from 18 farms. Cattle reacting positive to either assay were slaughtered and inspected for presence of tuberculous lesions and culture of mycobacteria. The relative sensitivity of the γ-IFN assay varied from 87 to 94%, and that of the ST from 75 to 93%, depending on the criteria used for interpretation of the assays and for establishment of the state of infection. Differences in the sensibility between both techniques were between 1 and 15%. Considering the results of the animals not investigated at the slaughterhouse, the relative sensitivity of the γ-IFN assay would be of 97% and that of the ST of 83% (difference of 14%). These values are higher than the real values of sensitivity, because the infected animals among those reacting negative to both assays have not been considered in the calculation. Our observations in farms with acute outbreaks of tuberculosis indicate that the γ-IFN assay is able to detect infected animals earlier than the ST.