Efficacy of a killed Mycobacterium paratuberculosis vaccine for the control of OJD in Australian sheep flocks

A field trial was undertaken from 1999 until 2004 to determine the efficacy of a killed Mycobacterium paratuberculosis (MAP) vaccine, Gudair TM , for the control of ovine Johne's disease (OJD) in Australian merinos run under Australian pastoral conditions. On each of three farms experiencing significant OJD mortalities (5 to 15% deaths per annum), 200 Merino lambs (age 1 - 4 months) were vaccin ated with Gudair™, and 200 lambs were left unvaccinated. Animal assessments and sample collections were conducted twice yearly until 4 or 5 years of age. The impact of vaccination on mortality rate, faecal shedding of MAP (by pooled and individual faecal c ulture), lamb growth, condition score and wool productivity, vaccine injection site lesions and cellular (BOVIGAM™) and humoral (PARACHEK™) immunity were examined. Gudair™ reduced mortalities due to OJD by about 90% and delayed faecal shedding of MAP for t he first year post - vaccination (pv). Thereafter, the prevalence of shedders among vaccinates was reduced by about 90%. The total numbers of MAP excreted by the vaccinated groups were also reduced by at least 90% at most sampling times. However, high levels of excretion by vaccinates were found on some occasions, and although overall only 7 vaccinates died of OJD, all 7 had multibacillary disease. Thus there remains a risk that some vaccinated sheep could transfer the disease. Small (less than 200g per anima l) reductions in liveweight gain were found in vaccinated lambs in the first year pv, but not for condition score or wool productivity. Vaccine injection site lesions were detected in almost 50% of sheep 2 months pv, and these persisted for at least 4 year s in 20 - 25% of vaccinates. The vaccine stimulated both cell - mediated and humoral immune responses in a high proportion of vaccinated lambs which declined over time, accompanied by a significant increase in the proportion of unvaccinated animals with positi ve immune reactions, presumably reflecting an increasing prevalence of OJD in this group.