A Property Disease Eradication Program (PDEP) is a management strategy with the purpose of eradicating Ovine Johne's disease (OJD) from land by destocking and decontamination. A PDEP is implemented under the supervision and guidance of an approved veterinary practitioner. It provides an auditable program which ensures that actions required to eradicate the disease occur. The satisfactory completion of a PDEP provides a high level of confidence that the disease will no longer be present on the land.
Guidelines for the Eradication of Ovine Johne's Disease
A PDEP is a program to eradicate OJD from a property and consists of two parts:
- removal of all infected and potentially infected susceptible species from the land (destocking) and,
- management of the land to ensure the level of Mycobacterium avium subspecies paratuberculosis (M. paratuberculosis) contamination on the pasture is reduced to a level at which susceptible species are unlikely to ingest a dose sufficient to cause infection (decontamination).
Principles of Destocking
All sheep, goats and deer other than fallow deer (Dama dama)
are regarded as susceptible species and should be considered in any destocking program. Generally, a PDEP will start at the beginning of summer and all animals regarded as "shedders" should be removed from the land as early as possible in the summer, and by 15 January if the PDEP for that summer destock is to be approved.
Sheep do not develop a strong age-acquired resistance to infection, as in cattle. However, observations support the belief that lambs are more susceptible to infection than adult sheep and that the course of the disease in animals infected as lambs will be shorter. It has also been shown that clinically affected sheep deposit significantly more bacteria on to the pasture than sub-clinically affected animals.
The onset of clinical disease, and therefore shedding, is hastened by the size of the infective dose. Experimentally, a massive challenge can produce clinical signs in sheep within six months of infection. In heavily infected flocks, clinical disease has been observed in animals as young as 10 months of age. In such flocks, significant losses will start to be seen in hoggets as a result of exposure to high challenge as lambs. In flocks that have a low or moderate level of infection, clinical disease tends to occur in older animals (between two and four years of age).
Shedding of significant numbers of bacteria in faeces does not occur early in the course of the disease, and under most circumstances is not detectable until sheep are at least nine months of age.
It is generally accepted that it takes 12 months from initial exposure until adult sheep pass significant levels of contamination. Hence some classes of susceptible animals in the early stages of infection may not be regarded as "shedders". Destocking of such animals may be delayed, albeit for a strictly limited period of time.
In heavily contaminated environments, it is desirable to await the passage of one summer before introducing clean adult sheep for short-term grazing.
Principles of Land Decontamination
The decline in the number of M. paratuberculosis
bacteria in the environment normally follows an exponential decay curve, i.e. rapid initial decline followed by a reduced rate of reduction of viable organisms over time. Assuming climatic and environmental factors remain constant, the initial amount of contamination may determine the period of time before the contamination drops below the critical point, i.e. an infective dose for one animal. Harsher environmental effects will hasten the decline.
Under cool, moist conditions, the sheep strain of the organism has been shown to survive for several months. Dryness, heat and exposure to UV light are detrimental to the survival of the organism. It follows that summer will have the greatest effect in reducing environmental contamination. Shade is the most important factor prolonging survival.
If owners/managers ensure that there is no further contamination of the land with M. paratuberculosis
, the passage of one summer will significantly reduce the number of organisms in the environment. By including a second summer and the intervening autumn, winter and spring, the likelihood of sufficient organisms being present to infect clean, introduced animals is exceedingly low. The exception would be in heavily contaminated areas subject to significant degrees of shading.
Strategies for the Use of Land During the Decontamination Phase
While it may be desirable to leave the land unstocked to ensure no further contamination of the land with M. paratuberculosis
occurs, the resulting pasture growth may well provide a more favourable microclimate for the organism to survive. In addition to economic reasons, it is therefore undesirable to leave land unproductive over this time.
Alternative land uses include:
- grazing with non-susceptible species, e.g. adult cattle
- cropping (including hay making)
Assessment of the Risk from Neighbours
The status of neighbours is an important consideration in determining the long-term success of a PDEP, particularly in high prevalence areas where there is a reasonable chance that some of the neighbouring flocks are infected.
The following options are currently available to address the risk posed by neighbours.
- Ongoing assessment of the OJD status of all neighbours as part of a group approach to allow the early detection of any infection in adjoining flocks.
- All infected neighbours undertake eradication at the same time.
- The PDEP ensures there are adequate fences to minimise the chance of straying and there is a suitable barrier to the environmental spread of infection.
- The risk posed by neighbours with untested flocks is assessed as provided for in the sheep Market Assurance Program (SheepMAP). The PDEP must address the need for appropriate barriers against those flocks assessed as being a risk.
Aspect to be covered by PDEP
Map of the property showing:
- the location of sheep handling facilities,
- all paddocks; including the name/number of all paddocks referred to in the plan,
- watercourses with arrows indicating direction of drainage,
- the names of neighbouring properties,
- an indication of property size.
including the source of infection if known and an assessment of the level of OJD contamination on the property. In flocks where infection has been detected only in introduced sheep and partial destocking only is proposed, reasons need to be provided as to why the disease is not suspected in other sheep.Livestock Inventory:
list of all livestock on the property at the commencement of the program by species, sex, age, breed and origin (home bred or provide details of introduced sheep).Destocking Strategy:
a plan for destocking the property, including all proposed dates of movement, and the proposed date by which destocking should be completed. Decontamination Strategy:
the strategy for management of the land during the decontamination period. The means of identifying "low risk" sheep for short-term grazing must be specified. Risk Assessment:
an assessment of the risk posed by neighbours (from both strays and environmental spread) and a plan describing how that risk is to be addressed. This includes a requirement to maintain fences to exclude straying sheep, and to establish barriers against environmental spread from INFECTED neighbours.Monitoring Program:
the owner must
- maintain records of stock movements on to and off the property during the program (both introductions and strays), including dates, numbers, origins and/or destinations,
- identify sheep introduced for short-term grazing in a manner such that their date of introduction and their origin can be readily determined,
- notify the supervising veterinarian: when destocking is complete, prior to re-introduction of any sheep on to the property, prior to making any changes to the program.
times for property inspections by the supervising veterinarian, particularly to ensure that destocking has occurred, fences are adequate, and short-term sheep are accounted for.Acknowledgment
NSW Agriculture is acknowledged for permitting the use of their Ovine Johne's Disease policy, which this document is based on.