Effect of unvaccinated Merino wether lambs on shedding of Mycobacterium avium subspecies paratuberculosis in flocks vaccinating for ovine Johne's disease.

Abstract

OBJECTIVE: To determine the prevalence of Mycobacterium avium subspecies paratuberculosis (Mptb) shedding by unvaccinated wethers compared with vaccinated ewes. METHODS: Six self-replacing, ovine Johne's disease (OJD)-infected Merino flocks that had been vaccinating lambs with Gudair(TM) for at least 5 years, but had not vaccinated their last two drops of wethers were studied. Faecal samples were collected in pools of 50 from 350 vaccinated ewes (1-2 years old) and from 350 unvaccinated wethers of the same age and were cultured to determine the presence of viable Mptb. RESULTS: The estimated prevalence of shedding sheep among the unvaccinated wethers was 6-fold greater than among the vaccinated female siblings (0.21% vs 1.27%), with faecal pools collected from unvaccinated wethers being 19-fold more likely to be positive than those from the vaccinated ewes. CONCLUSION: Leaving wethers unvaccinated is likely to considerably increase pasture contamination with Mptb and potentially jeopardise disease control.