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.

IGRA test detects MAP infection early but doesn't predict conventional diagnostic test positivity.

This longitudinal study evaluated three diagnostic approaches for paratuberculosis (Johne's disease) in a Scottish beef herd infected with Mycobacterium avium subsp. paratuberculosis (MAP). The research compared faecal culture, serum ELISA, and an interferon-gamma release assay (IGRA) adapted from the Bovigam™ test kit using MAP-specific antigens. The study found that the novel IGRA test demonstrated high sensitivity for detecting MAP infection by measuring cell-mediated immune responses through interferon-gamma production. However, IGRA-positive animals did not consistently correlate with subsequent positive results on faecal culture or serum ELISA, suggesting these tests detect different stages of infection. Young animals on affected farms showed particularly high responses to both PPDa and MAP-specific antigens. Latent class analysis was employed to determine true infection status. The findings suggest the IGRA test may be more valuable as a herd-level diagnostic tool to determine presence and extent of MAP infection rather than as a predictor of individual animal status by conventional tests. This has significant implications for paratuberculosis control programs, as early detection of infected animals could improve herd management strategies.

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