With Gudair® vaccine the major tool in the new national program for Ovine Johne's Disease (OJD) control which began in 2004 (Windsor, 2006), work is in progress to assess its long term efficacy. As the initial study in Australia was performed on high prevalence flocks with OJD losses of 5-15% per annum (Reddacliff et al, 2006), the efficacy of the vaccine long term and on lower prevalence flocks has been questioned. Preliminary results from a recent study indicate there is approximately a 50% reduction in shedding of MPtb in flocks after 3 years of a vaccination control program (Eppleston et al, 2005).
This suggests that the initial findings of 90% decreases may not be an accurate estimate for long term usage. A current trial (P.PSH0309), of which this study is part of, is aiming to evaluate the effects of Gudair® on OJD prevalence levels in infected flocks that have been vaccinating for at least five years. The trial will also evaluate the efficacy of Gudair® in flocks with lower OJD prevalence.
A cross-sectional study was performed to identify potential on farm management factors affecting the efficacy of Gudair® vaccine in controlling OJD. Questionnaire data from 36 flocks from New South Wales and Victoria that had been vaccinating for at least 5 years was analysed. There were two outcomes of interest, namely, current cohort OJD prevalence level (CPREV) and pool OJD status (PSTATUS). These were determined from the results of pooled faecal culture tests performed on sheep from the 36 farms. CPREV was calculated using the pooled prevalence calculator and categorised as <1% or ≥1%. PSTATUS was a binary category, either positive or negative.
Thirty explanatory variables about farm management, farm enterprises, OJD infection history, Gudair® vaccination history and other OJD control strategies were investigated for associations with the outcome variables. Univariable and multivariable logistic regression were performed for both outcomes and a generalised linear mixed model was then used for the outcome PSTATUS.
The majority of the 36 farms included in the analysis were from New South Wales (28) of which 5 were in the Central Tablelands, 3 near Goulburn, 6 near Hume, 6 near Wagga Wagga, 6 near Young and 1 near Braidwood. The remaining 8 farms where located in Victoria. The median size of the farms was 1007 hectares (range 202-2600) and the median altitude and rainfall was 332 metres (range 151-1036) and 620 millimetres (range 500-965) respectively. All 36 farms were self replacing merino flocks and the median adult fleece micron was 19 (range 17.5-21).
The median farm sheep stocking rate was 3.8dse/h (range 0.4-16.6) and most farms lambed in spring (20) while 8 lambed in autumn and 8 in winter. All farms ran multiple enterprises, namely cattle (21), cropping (22), alpacas (1), pigs (1) or other sheep (28). At the time of sample collection, the farms had been vaccinating the merino lamb drops at marking for 5 years (2), 6 years (16), 7 years (9), 8 years (7), 9 years (1) and 10 years (1). Wethers were left unvaccinated in some or all of the drops on 10 farms and of these, 7 farms sold the wethers > 10 months of age. Terminal and cross-bred lambs were not vaccinated on 20 farms and were sold at > 10 months of age on 8 of the farms.
Professional vaccinators were routinely employed on 10 farms to administer Gudair®. None of the study farms shared facilities or rams however 7 shared roads used to walk sheep and sheep strayed between neighbours on 22 of the farms. In the last 5 years, 25 farms used ≥ 2 ram sources and 17 farms introduced ≥ 30 individual rams. The introduction of replacement stock in the last 5 years occurred on 13 farms. The introduced rams and stock had varying vaccination histories however only 4 farms suspected them to be infected with OJD. High loss mobs were sold on 14 farms, however, not all farms had high loss mobs. The sheep were run in age groups on nearly all the farms (32) and sheep lambed and lambs were weaned onto clean paddocks on 22 and 30 farms respectively. In this analysis, pastures were deemed 'clean' if they were spelled or alternated with cattle for at least 1 month due to the drought making 3 month spelling very difficult.
The results of the PFC revealed the majority of the farms had a current cohort OJD prevalence of <1% with 6 farms having undetectable levels. Similarly, 72% of the pools tested negative with only 28% positive. However, 13 flocks still had a cohort OJD prevalence of ≥1% with an outlying flock's prevalence 38.9%. Having sheep stray and using professional contractors for the administration of Gudair® vaccine were both associated with higher cohort OJD prevalence levels. Introducing sheep onto farms was associated with an increased risk of positive pool status. Having a concurrent cattle enterprise was found to be protective as it was associated with not having higher OJD prevalence levels. The findings of this study suggest that while Gudair® vaccine does decrease the OJD prevalence levels in vaccinating flocks, it does not negate the continuing need for farm biosecurity in the control of OJD.
We acknowledge the owners and managers of the 36 flocks for their time and cooperation, without which this study would not have been possible, plus the efforts the local district veterinarians, animal health officers and rangers who assisted in the collection of the faecal samples and the laboratory staff for culture of the samples. Financial support from various sheep industry groups through Meat and Livestock Australian (MLA) and the support of the Australian Wool Education Trust (AWET) for a scholarship is also gratefully acknowledged.