Antibody response early after experimental infection with Mycobacterium avium subspecies paratuberculosis in dairy calves

Serological testing in the early stages of Johne's disease has been successful using specific antigens and in-house ELISA. However, the use of a commercial ELISA has not been evaluated shortly after Mycobacterium avium subspecies paratuberculosis (MAP) infection, nor has it been determined whether this serological response is age or dose dependent. Fifty-six calves were randomly allocated to challenge groups (5 per group) and a negative control group. Calves were inoculated orally on 2 consecutive days at 2 wk or at 3, 6, 9, or 12 mo. Within each age group, 5 calves received either a high or low dose of MAP. Using a commercial ELISA, antibody responses were detected in 42% of the inoculated calves and were present in all age and dose groups (except for the 6-mo low-dose group). Antibody response profiles differed among individual calves; persistent as well as peak and bimodal peak responses existed. Calves inoculated at 12 mo were ELISA positive within 4.5 mo after inoculation, whereas those inoculated at younger ages took longer to become ELISA positive. Furthermore, calves inoculated with a high dose of MAP more often became ELISA positive than low-dose calves when inoculated at a younger age. In conclusion, a dose-dependent antibody response was detected by ELISA in a larger proportion of calves than expected soon after inoculation.