Effect of tuberculin tests and gamma interferon test repeatedly used in tuberculosis-infected and non-infected cattle of Uruguay

Abstract

The first study evaluated the effect of repeated use of single cervical tuberculin test (SCTT) and gamma-interferon (IFN-γ) test at different time intervals in infected cattle. The hypothesis was that the IFN-γ test could be repeatedly used in desensitized cattle for bovine tuberculosis diagnosis without affecting its sensitivity. 32 dairy cattle positive to cervical comparative tuberculin test (CCTT) were selected from an infected herd and randomly divided into 4 groups (n=8). All animals were tested with the in-vitro test on day 0, then a second tuberculin test and IFN-γ test was performed at different time intervals according to the group, at 15, 30, 45 and 60 days post-CCTT. They all were submitted to post-mortem inspection, histopathological analysis and Mycobacterium bovis culture. In comparison with the first tuberculin test, the mean skin thickness decreased significantly when the SCTT was performed at 15 and 30 days, with presentation of false negative results even after 45 days. The sensitivity of the IFN-γ test applied repeatedly in these desensitized animals was not affected. The second investigation evaluated the specificity of the caudal fold test (CFT) and SCTT used repeatedly in non-infected cattle. The hypothesis was that this repeated used generate a sensitization to tuberculin, with the presentation of false positive results. 100 beef cattle were classified into two groups (n=50). One group received 6 rounds of CFT and the other 6 of SCTT, with 60-day intervals between tests. The IFN-γ test was performed in all the animals on day 0 obtaining a specificity of 97%. Regarding the SCTT, a significant increase (0.48mm, p=0.0034) in the mean skin thickness was observed after six rounds of the test compared to the first test, but no positive animal was observed. All the animals remained negative to CFT during the course of this experiment, without affecting its specificity.