Longitudinal study of a Scottish beef herd affected with Mycobacterium avium subsp. paratuberculosis; monitored by faecal culture, serum ELISA and the interferon-gamma release assay

Paratuberculosis is a fatal, enteritis of animals caused by Mycobacterium avium subsp. paratuberculosis (MAP). Uptake of MAP by macrophages initiates adaptive immune responses, detected as antigen-specific Interferon- gamma (IFN-γ) release or specific antibodies in peripheral blood. MAP can subvert macrophage killing and there is often a long sub-clinical phase. Diagnosis of paratuberculosis by faecal culture or serum ELISA is problematic, particularly during the subclinical phase. Regimens which rely on these tests, may fail as animals may transmit infection for prolonged periods before their detection. Infected animals may develop early Th1 responses to MAP. Measuring IFN-γ produced in response to mycobacterial antigens may give an early indication of animals infected with MAP. Adaptation of the commercially available Bovigam™ test kit utilizing MAP-specific proteins as stimulants of the cell mediated immune (CMI) response discriminates naturally and experimentally infected animals from uninfected ones. Young animals from farms affected by paratuberculosis, exhibit high responses to purified protein derivative from Mycobacterium avium species (PPDa) and MAP–specific antigens. To determine if specific high CMI responses were diagnostic for paratuberculosis they were compared with the inferred true status of each animal in a latent class analysis. Results from this study indicate that the novel interferon gamma release assay (IGRA) test is a sensitive test of MAP infection but does not indicate which animals are likely to subsequently test positive by the established MAP tests. The IGRA test may have an application as a herd test, determining whether infection exists and to what extent it is present within the herd.