Neonatal Calf Serum MAP Antibody Titre as a Potential Marker of Early-Life MAP Exposure

Johne's disease (JD), caused by Mycobacterium avium subsp. paratuberculosis (MAP), is typically acquired early in life, yet detection of exposure during this period remains challenging. This study evaluated whether MAP-specific antibodies are passively transferred from dam to calf and detectable in neonatal calf serum, and whether these titres reflect early-life exposure to MAP. Neonatal calves (n = 38) from a commercial dairy herd were blood sampled within the first 10 days of life as part of routine assessment of passive transfer. Serum total protein (STP) was measured to assess colostrum intake, and residual serum was analysed for MAP antibody titres using an indirect ELISA. Multivariable linear regression was used to evaluate associations between calf MAP antibody titres, STP, and dam JD serological status. Calf serum MAP antibody titres were positively associated with STP (p < 0.001) and differed according to dam serological status, with higher titres observed in calves born to MAP-seropositive dams (p = 0.025). A significant interaction between STP and dam status indicated that the relationship between passive transfer efficiency and MAP antibody levels varied by dam infection status. These findings support the biological plausibility of passive transfer of MAP-specific antibodies via colostrum and suggest that neonatal calf serology may reflect exposure to an early-life epidemiological risk period associated with maternal MAP exposure and the calving environment rather than infection status within the calf itself. Given the modest sample size and single-herd design, these findings should be interpreted as exploratory and hypothesis-generating.