A longitudinal study of OJD and the effects of whole flock vaccination with Gudair

Summary

Ovine Johne’s disease (OJD) is a fatal enteric infection of sheep by Mycobacterium avium subspecies paratuberculosis ‘S’ strain (Mptb.). OJD in the Australian environment has proven difficult to both diagnose and control. The disease has become increasingly important as despite efforts to control spread by regulation of sheep movements, the disease has continued to spread in Australia. Funds allocated to research the many aspects of this disease have included projects that have investigated the level of losses on affected farms and the efficacy of vaccination in the control of the disease.

Project OJD.015 is a case study commenced in a single large self-replacing flock and former fine wool Merino stud farm at Gunning in NSW. The owner was concerned that rapid increases in losses of about 25% per annum were due to OJD. A study was commenced to measure the level of losses and determine the contribution of OJD to the loss. Further, as the owner was seeking to vaccinate the whole flock with Gudair™ OJD vaccine in an attempt to control the losses, an opportunity was presented to examine the efficacy of this strategy. Whole flock vaccination commenced in May 2000 and was completed in September 2000. To assist the research, a cohort of unvaccinated control sheep from each age group was left, to enable comparisons of mortality and Mptb. shedding data between vaccinates and controls over the course of the study.

Over the 4-year study period, adult sheep mortality declined significantly in the Merrill flock. By completion of the study in 2004, mortalities were uncommon, due to a decline in the flock mortality risk from 24.2% to 2.8%, mostly attributed to a decline in the OJD attributable mortality risk from 19.0% to 1.4%. However following commencement of vaccination, significant OJD mortalities did continue, particularly in adult vaccinates. The prevalence of faecal shedders declined from a high to a moderate prevalence following vaccinating of 2-year-old sheep, although shedding at rates considered sufficient for transmission of infection did continue, particularly in wethers. However, a low prevalence of faecal shedding in sheep vaccinated at 3 or 8 months of age, suggests that vaccination is similarly efficacious in these age groups. This finding suggests that vaccination may be beneficial in sheep when delayed to weaner age or older, even when exposed to a heavily contaminated environment since lambing. A decline in the prevalence of Mptb. shedders in the unvaccinated control sheep during the project was also observed. However it is considered more likely that a series of management changes, including the culling of clinical cases, sale of ‘at risk’ stock and introducing of replacements and reduction in stocking rates leading to a decrease in ‘sheep-years at risk’, would have had more significant impact on pasture contamination with Mptb. than the whole flock vaccination strategy.

In conclusion, whilst an increase in flock immunity from whole flock vaccination may have contributed to the control of OJD, it was probably a minor component of the successful control program achieved over such a short time frame. The study concludes that by accompanying vaccination with intensive management changes, a significant decline in the risk of OJD mortality can be obtained and rapidly lead to control of OJD in a heavily infected flock. As the study was conducted in a single environment with high mycobacterial challenge, which may have provided sustained stimulation of flock immunity, it is recommended that field evaluation of vaccine performance be conducted in low challenge environments.