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

Abstract

To determine the prevalence of Mycobacterium avium subspecies paratuberculosis (Mptb) shedding by unvaccinated wethers compared with vaccinated ewes. 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. 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. Leaving wethers unvaccinated is likely to considerably increase pasture contamination with Mptb and potentially jeopardise disease control.