Longitudinal study of OJD and the effects of whole flock vaccination with Gudair : whole flock OJD vaccination at Merrill.

Abstract

Ovine paratuberculosis or Johne’s disease is an insidious mycobacterial infection of sheep capable of causing wasting and death. It has emerged in recent years in Australia as a cause of significant losses in some infected sheep flocks. MLA Project OJD.015 was commenced to enable a detailed longitudinal case study of an attempt to control the disease in a large self-replacing Merino flock at Gunning in NSW with an owner-estimated annual mortality rate up to 25%. Vaccination of the whole flock with exception of a small group in each age cohort was conducted, with documentation of the effects of vaccination and other strategies on mortalities and mycobacterial faecal excretion rates and mortalities in all age groups of animals. The study concluded that the significant decline in mortality risk from 24.2% to 2.85 during the study may only be partially attributed to the increased flock immunity following vaccination. By accompanying vaccination with extensive changes in flock management, including culling of clinical cases, sale of ‘at risk’ stock and introducing of replacements and reduction in stocking rates, control of the losses can be rapidly achieved in an infected flock with a high prevalence of OJD. This project has enhanced the knowledge and understanding of the performance of this vaccine under Australian conditions in a high OJD prevalence environment and recommends that field evaluation of the performance of the vaccine continue, particularly in low challenge environments.