The effect of vaccination on the prevalence of paratuberculosis in large dairy herds.

Eight hundred and sixty-six one-month-old female calves were vaccinated using heat-killed Mycobacterium paratuberculosis in oil during a five-year period. Seven hundred and twenty-one bull calves and 379 female calves served as nonvaccinated controls. Two large herds were used in the study. The calves were tested by faecal culture and by serologic tests on the day of vaccination, at 3, 6, 9 and 12 months of age, at breeding and on the day of parturition. The results were evaluated by trend analyses. Following vaccination, the faecal shedding of bacteria was greatly reduced as determined by annual faecal microscopic tests. During the last 6 months of the experiment as few as 9 samples from 612 cattle proved positive by microscopy and/or culture. The number of seropositive animals and the antibody titres (CFT and AGID) increased during the first three years, then declined.