Lack of interference with diagnostic testing for tuberculosis in goats experimentally exposed to Corynebacterium pseudotuberculosis.

It has been suggested that infection with Corynebacterium pseudotuberculosis, the pathogen responsible for caseous lymphadenitis (CLA), might interfere with diagnostic testing for tuberculosis (TB), due to antigenic similarities between this particular type of bacterium and those expressed by mycobacteria. The aim of the present study was to investigate whether experimental infection with C. pseudotuberculosis in goats impacted on TB testing, using single and comparative intradermal tuberculin (SIT and SCIT respectively) tests and interferon (IFN)-γ assay. No positive reactors were detected among the CLA-affected goats using the SIT/SCIT tests or the interferon IFN-γ assay. A proportion of goats showed inconclusive results to the SIT test and reactions to Mycobacterium avium. There was no evidence that infection with C. pseudotuberculosis interferes with diagnostic testing for TB using standard interpretation of the SIT, SCIT and IFN-γ tests.

Evaluation of specificity of tuberculosis diagnostic assays in caprine flocks under different epidemiological situations.

The aim of this study was to evaluate the specificity of the most widely used tuberculosis (TB) diagnostic tests, single intradermal tuberculin (SIT) and single comparative intradermal tuberculin (SCIT) tests and interferon-gamma (IFN-γ) assay in 937 animals from eight TB-free caprine flocks under different epidemiological situations. Maximum specificity was found using SCIT test (99.4-100% depending on the interpretation criteria) while SIT test and IFN-γ assay showed a slightly lower overall specificity (97.6-99.2% and 96.4-98.4% respectively). Specificity of the SIT test in a Corynebacterium pseudotuberculosis infected flock was significantly (P<0.05) lower if a severe interpretation criterion was applied. Similarly, specificity values of SIT test and particularly IFN-γ assay in a paratuberculosis (PTB)-vaccinated flock were lower than those observed in non-vaccinated flocks. Higher proportion of false positive reactors to TB tests (SIT and IFN-γ assay) were observed among animals positive in the PTB-ELISA in PTB vaccinated flock. These results demonstrate that TB diagnostic tests show an adequate specificity when performed in goats from TB-free flocks in most situations. However, certain factors such as C. pseudotuberculosis infection and paratuberculosis vaccination can have a negative impact in the most sensitive tests.