Comparison between a gamma-IFN assay and intradermal tuberculin test for the diagnosis of bovine tuberculosis in field trials in Brazil

Bovine tuberculosis is a major problem in Brazil. The intradermal tuberculi n test is the standard test for detection of bovine tuberculosis in Brazil but can lack both sensitivity and specificity. The purpose of this study wa s to compare a bovine gamma-IFN assay with the tuberculin test under field conditions in Brazil. A total of 1632 animals from 13 dairy farms were test ed using the single cervical tuberculin test (SCTT). Among those animals, a bout 15% of each herd, 220 in total, represented a high-risk group and were selected to be tested using the gamma-IFN test. Of the 1632 animals tested , 207 presented significant reactions representing 12.7% of the cattle stud ied. In the selected group the number of animals positive by the gamma-IFN assay was 126/220 (57.3%) and the total number of reactive cows on SCTT was 106/220 (48.2%). The real number of infected cattle, following standards, was 120/220 (54.5%). From these results the relative sensitivity rate of ga mma-IFN test was 100% including the false-positive results and 88.3% for th e SCTT - a significant (P < 0.01) difference in favour of the gamma-IFN tes t of 11.7%. The gamma-IFN assay also identified some positive animals 60-12 0 days earlier than the SCTT. In conclusion, we believe that the gamma-IFN assay can be used alone or in combination with the SCTT, as a valuable tool for the control of bovine tuberculosis in the Brazilian national herd.