Use of the johnin PPD interferon-gamma assay in control of bovine paratuberculosis

Although the interferon-gamma (IFN-γ) assay for measurements of cell-mediated immune (CMI) responses to paratuberculosis PPD (johnin) has been available for close to 20 years, the assay has not yet emerged as the long desired test to identify infected animals at an early time point. Among other issues, this relates to problematic interpretation of the test results and maybe an over-expectation of what can be deducted from this kind of test given the chronic nature and slow development of infection of paratuberculosis. Over a number of years a modified IFN-γ assay with addition of recombinant bovine IL-12 to the PPDj stimulation of blood samples from the heifer group in more than 20 Danish dairy herds which also perform surveillance of MAP antibodies in milk have been performed. The results indicate that IFN-γ assay results are specific for paratuberculosis, but the IFN-γ assay result of an individual animal cannot establish whether the animal is infected or predict the future progression of disease in this animal. The IFN-γ assay should thus be used on a group of animals to test the level of exposure to paratuberculosis bacteria the animals have experienced, and thereby assist in maintaining rational in-herd management procedures and in the establishment of paratuberculosis status of a given herd. Indeed, for any diagnostic test applied in paratuberculosis, both the diagnostic target condition and the purpose of the diagnostic testing must be considered before any meaningful estimates of sensitivity or specificity can be given.