Where Ovine Johne's disease (OJD) infection has been confirmed in one or more animals on a property, the owners/managers of the flock may consider implementing a Property Disease Management Program (PDMP). This is a whole-of-farm approach to dealing with OJD.
A PDMP should comprise appropriate measures with the purpose of:
- reducing pasture contamination on the infected property
- controlling or reducing the level of infection on the infected property
- minimising or removing risk of transmission to neighbouring properties
- reducing exposure of non-susceptible age groups to higher risk areas
- establishing lines of low-risk sheep for production and trading purposes.
Guidelines for Property Disease Management Programs
In addition to the disease minimisation and management strategies listed below, it is important that, whenever possible, measures are instituted to minimise the risk of spread of infection to unaffected flocks. An appropriate 'barrier', incorporating both physical and management factors, should separate the infected flock from neighbouring flocks and land.
Examples of physical barriers include land not stocked with susceptible species (eg crops and cattle), double fencing, tree plantations, laneways or roadways, contour banks, stone walls or other physical separation which minimises the risk of sheep straying and/or environmental spread of infection to neighbours. Management procedures may include running low-risk sheep such as adult vaccinates adjacent to boundaries.
Infected Flock Profile (IFP)
Before implementing a PDMP, knowledge of the apparent prevalence, and age/mob distribution, of infection in the flock/property will be helpful.
An infected flock profile (IFP) involves a progressive assessment of apparent distribution and prevalence of infection, based on information such as flock history, property management, on-farm testing results, abattoir monitoring, tracing investigations and other measures.
Options for PDMPs
Consideration needs to be given to enterprise type, property design and facilities, resources, alternative enterprises, environmental conditions, and the situation on neighbouring properties, before selecting appropriate options for disease management/reduction.
Options for PDMPs include:
- selective culling
- land segregation
- breeding strategies
- segregating test-negative sub-flocks
- grazing/watering strategies
Clinical cases: monitoring of all mobs and culling of clinical cases (i.e. sheep showing signs of OJD) should be mandatory in all PDMPs, on both disease control and welfare grounds.
Test-positive mobs: where on-farm testing indicates the presence of infected mobs, it will be advantageous to selectively cull any mobs with evidence of shedding.
Age-related resistance: current indications are that adult sheep are relatively resistant to low/medium challenge. Strategic culling of sheep which may have been exposed to any level of infection as lambs or weaners is recommended.
By combining selective culling with other disease reduction measures, it may be possible to significantly reduce on-property challenge and provide options for successful implementation of further disease reduction measures.
The provision to segregate portions of a property or enterprise immediately offers flexibility, and the option of undergoing progressive land decontamination.
Progressive area destocking for 15 months (including 2 summers) and decontamination within properties, or between separate land parcels comprising a property, may be combined to advantage with other disease-reduction measures.
Short term destocking, for example for 6 months including one summer, is likely to remove a significant number of infective bacteria from pastures.
Various specialist breeding techniques can be used to reduce exposure of young animals to contamination, e.g. multiple ovulation embryo transfer (MOET), juvenile in-vitro embryo transfer (JIVET) and lamb snatching, but most of these are resource intensive and my not be available in many situations.
Disease elimination and genetic recovery may be effected in one generation by using artificial breeding techniques, provided that recipient stock are isolated from potential sources of infection and recipient stock and the land they use are free of infection.
Segregating Test-negative Sub Flocks
Animals from infected flocks that are at least four years of age and are serial faecal test negative and/or biopsy negative are unlikely to be infected. Segregating such animals from subsequent exposure, e.g. moving on to decontaminated land or into sheds, will provide a nucleus of low-risk animals which may be used as a breeding sub-flock. Preliminary observations in New South Wales indicate that by establishing a test-negative breeding sub-flock and applying further minimal disease breeding strategies, it may be possible to re-establish an uninfected age group within two generations.
Susceptibility to infection appears to be highest in very young animals, and appears to be related to the degree of challenge and the amount of stress to which the lambs/weaners are exposed. These factors are interrelated, e.g. early weaning may reduce exposure but increase stress and susceptibility.
Challenge to young animals may be reduced by lambing on to decontaminated or low-contamination pastures, and/or by removing lambs as quickly as possible from infected ewes. Stress may be reduced by providing adequate nutrition and shelter, worm control and prevention of other diseases.
All PDMPs should provide strategies for pasture management, especially the provision of lower risk pastures for more susceptible young stock. In addition, prevention of nutritional stress, fencing of swampy ground, parasite control and reduction of other sources of stress should all be standard components of any PDMP. Where possible, provision of reticulated low-risk above-ground water supplies should be implemented.
Particular care should be taken when utilising pasture management for OJD control. Many of the recommendations may jeopardise parasite control programs. Producers need to consider implementing strategies to optimise concurrent control of both OJD and internal parasites.
Vaccination is a vital tool that can be used with the aim of both containing and minimising infection in a flock.
Vaccination before 16 weeks of age has been shown to reduce mortalities, shedding of the OJD bacteria and the overall number of sheep that are shedding by at least 90%.
Vaccination of re-stocker lambs before 16 weeks of age (preferably at marking) is recommended. Successive vaccination of each year's drop will result in a fully vaccinated flock within 4 -5 years. In some cases, particularly where heavy shedding and contamination associated with high mortalities is a feature at the time of implementation of a PDMP, vaccination of hoggets, adult stock and introduced rams could be considered.
Certain conditions apply to the supply and use of the vaccination, as well as the identification of vaccinates.
Implementation of a Property Disease Management Program
When developing and implementing a PDMP, it is recommended that producers seek advice from a veterinarian.Acknowledgment
The NSW Department of Primary Industries is acknowledged for permitting the use of their Ovine Johne's Disease policy.