Early infection dynamics after experimental challenge with Mycobacterium avium subspecies paratuberculosis in calves reveal limited calf-to-calf transmission and no impact of Hsp70 vaccination.

Efficient control of bovine paratuberculosis is hampered by lack of a vaccine. The purpose of this study was to evaluate efficacy of a candidate vaccine, consisting of recombinant Mycobacterium avium subspecies paratuberculosis (MAP) Hsp70 with DDA adjuvant, in calves experimentally infected with MAP. Four groups of 14 animals each were used. Animals in group 1 and 2 were all vaccinated with Hsp70/DDA at day 0, 84, 168 and 357, and those in group 3 and 4 were non-vaccinated controls. In each group half (n=7) of the animals were challenged and the remaining half served as contacts. Blood and fecal samples were collected at three week intervals until day 588, and subsequently all animals were subjected to necropsy. The primary outcomes assessed were fecal culture (FC) of MAP, tissue colonization of MAP, and transmission of infection to contact animals. The kinetics of MAP shedding in feces of challenged animals showed a peak around 130 days post-challenge, irrespective of vaccination status. At necropsy no differences in the level of tissue colonization between vaccinated animals and controls were observed in the challenged groups. Only one contact animal (non-vaccinated) was positive at necropsy, indicating limited to no transmission within groups. These findings indicate that Hsp70/DDA vaccination does not influence early infection dynamics after experimental infection. However, early shedding of MAP in calves did not result in efficient transmission of infection to contact animals. The latter implies that introduction of an infected calf in a cohort of susceptibles has limited consequences for spread of infection.