Performance of a whole blood interferon-gamma assay for detection and eradication of caseous lymphadenitis in sheep

Caseous lymphadenitis (CLA), a chronic bacterial disease of sheep and goats caused by Corynebacterium pseudotuberculosis, could be controlled by eradication of infected carriers. This study aimed at validation of a whole blood interferon-gamma (IFN-gamma) enzyme immunoassay (EIA) (Bovigam, Pfizer) in naturally infected sheep for use in eradication of infection from a flock. This assay used formalin-inactivated whole bacterial cells as antigen. The sensitivity of the whole cell assay was improved by increasing both the volume of blood and the number of bacterial cells. The assay was validated in experimentally infected sheep and in a flock of known-negative sheep, as well as in a naturally infected flock, a proportion of which was vaccinated with a commercial CLA vaccine. An optical density (540nm) (OD) cut-off of 0.09 was effective in classifying animals as test positive or negative in the naturally infected flock, although there was variation in OD between visits, notably with weakly reacting animals. The test had a sensitivity of 91% and a specificity of 98%. Postmortem data supported the results in test-negative animals. Visit-to-visit variation in IFN-gamma EIA OD in the naturally infected flock as well as CLA disease status was used to develop an algorithm for the eradication of CLA from a known infected flock. The whole blood IFN-gamma assay shows promise for eradication of caseous lymphadenitis from sheep flocks.