Evaluation of the effectiveness of Gudair™ vaccination for the control of OJD in flocks vaccinating for at least 5 years

Summary

Ovine Johne's disease (OJD) is an enteric infection of sheep by Mycobacterium avium subspecies paratuberculosis S strain (Mptb.) that has proven difficult to both diagnose and control in Australia. Collaborative research in project OJD.009 demonstrated the efficacy of vaccinating lambs between 1 and 4 months of age with GudairTM, a killed whole cell vaccine imported from Spain, for controlling OJD in high prevalence Australian sheep flocks. Vaccination reduced mortality by 90%, delayed the onset of faecal shedding of Mptb. by 12 months, and reduced the prevalence of shedders by 90% compared to unvaccinated lambs.
This study led to the registration of Gudair. This vaccine and the Assurance Based Credit (ABC) Scheme are now established as the key strategic intervention to control the disease in Australia. Computer modelling suggested that the occurrence of mortalities and shedding would fall rapidly after the commencement of a vaccination control program, depending on disease prevalence at the time of commencing vaccination. However it was acknowledged at the time that validation of this modelling by field research was required, particularly as points for vaccination became incorporated into the ABC risk based trading scheme. Following registration of Gudair many flocks with an apparent low prevalence also commenced vaccination as a precaution against increased mortalities and as a means to improve their ability to sell re-stocker sheep through the ABC scheme. In project OJD.033, we reported on the changes in the prevalence of shedding of Mptb. in the 3-4year and 5-6year old cohorts in 2003-4, 2005-6 and 2007-8 following initiation of vaccination of 1-4 month old lambs in 2002. The study found a significant decrease (1.66% to 0.63%; p0 and =1% and 34.2% (13/38) had a cohort prevalence of >1% Seven of the 14 flocks with initial low prevalence had increased prevalence (medium or high) after 5 years vaccination.
To examine the sensitivity of the initial PFC350 and better understand infection in low-prevalence flocks, sampling was conducted on 600 sheep (PFC600; 12 pools of 50) from 4 of the 7 negative flocks from the initial survey and one was found to be shedding. This flock had ceased to vaccinate wethers. When the PFC 600 was conducted on 16 known infected flocks on Kangaroo Island in South Australia, including 6 flocks that had been recently found not to be shedding by PFC350, no subsequent shedding was found in 14 of the flocks. This indicates a decline in proportion of these 16 flocks as positive for shedding from 100% to a proportion of 12.5% of flocks shedding currently.  It was later identified that both positive flocks on Kangaroo Island had introduced unvaccinated sheep in recent years. Further, it was noted that in addition to whole flock vaccination, a number of other farm management factors designed to minimize the spread of OJD had been introduced on KI. These data indicate that despite a rapid decrease in OJD mortality in flocks following the commencement of a vaccination program, shedding is likely to have persisted for at least 5 years in a majority of infected flocks in NSW and Victoria and is of concern if sheep are to be traded from these flocks or vaccination ceases. However the data from Kangaroo Island are encouraging and suggest that the second generation vaccinates have greater protection from shedding than the first generation vaccinates and will likely present a substantially lower risk of transmission of the disease.