Mycobacterium avium subspecies paratuberculosis (MAP) shedding and seropositivity in youngstock in MAP-infected dairy herds in Alberta, Canada

Johne's disease (JD) control and eradication programs aim to reduce the prevalence of Mycobacterium avium subspecies paratuberculosis (MAP) infections and associated economic losses. Youngstock are particularly vulnerable to MAP infection-they can shed MAP in feces, develop antibody titers, and transmit the pathogen to pen mates. However, most JD control programs do not include youngstock in MAP testing strategies, potentially limiting their effectiveness by overlooking this transmission route. This study aimed to investigate the age at which youngstock ≤12 mo old in MAP-positive dairy herds start shedding MAP in feces and become ELISA-positive under field conditions. Eight dairy herds in Alberta, Canada, were sampled 4 times over 14 mo. Median MAP prevalence in animals >12 mo of age using quantitative PCR (qPCR) at the first and second sampling was 1% and 0%, respectively. Using the ELISA, median herd prevalence was 1% at both samplings. Blood and fecal samples were collected from all youngstock aged 2 to 12 mo. The MAP antibodies and fecal shedding were determined using ELISA and qPCR targeting the ISMAP02 insertion sequence. Thirteen percent of animals ≤12 mo of age shed MAP, with earliest shedding observed at 4 mo of age. ELISA seropositivity was first detected at 3 mo, with 4% of animals testing positive. These findings demonstrate that in MAP-positive commercial dairy herds, early MAP shedding and seroconversion can occur earlier than previously assumed under field conditions. Incorporating youngstock into JD testing and control strategies is essential. Monitoring MAP shedding in youngstock may reveal flaws in calf hygiene measures, enhance JD management, and reduce within-herd prevalence over time.