Key features of ovine Johne's disease (OJD)
- Signs of OJD can include ill-thrift in adult sheep (that does not respond to feeding or treatment) and a 'tail' in adult mobs.
- Animals are infected by ingesting pasture or drinking water contaminated with infected faeces.
- The most common means of introducing OJD onto a property is the movement of infected sheep either as bought, stray or agisted animals.
- Disease spread is also possible through the movement of infected faeces in water run-off or slow moving creeks, or via movement of contaminated footwear, equipment or vehicles.
- Most OJD bacteria are killed within six weeks of exposure to sunlight on the ground. However, in moist shaded areas, the bacteria can survive for more than12 months.
- Animals under 12 months of age are the most likely to become infected. Older animals are more resistant to infection but can still be infected if their resistance is reduced by concurrent disease, or if the level of bacterial challenge is very high.
- Sheep with higher levels of immunity, whether natural or induced by vaccination, are more likely to withstand the bacteria and remain disease-free.
- OJD has a very long incubation period. The time between infection and an animal starting to shed the bacteria in their faeces is generally one to three years. Shedding of bacteria can start before the animal shows visible signs of the disease.
- It will generally be several years before signs of the disease (wasting, scouring) are seen in an infected animal.
The effects of the disease varies from property to property. Some properties, particularly in the lower rainfall areas or the sheep/cropping zone, will experience little or no effects. Significant annual death rates may occur in other areas, especially those in areas with higher rainfall, high stocking densities, or with permanent pastures. There may be no signs of the disease for the first few years following infection, but then death rates will increase over a number of years (in high spread areas death rates of up to 10% per year can occur).
Consider all management options carefully before choosing those best suited to your business. Factors that need to be taken into account include:
- sheep losses
- trading implications for the business
- the cost of the disease to the farm business compared to the cost of control measures.
OJD can be eradicated from some farms by complete destocking over a period of 15 months (including two summers). To date this measure has only been successful on 50-60% of properties due to restocking with infected animals or spread from neighbouring flocks.
Eradication through long-term vaccination programs is a theoretical possibility but needs to be tested. Research in this area is ongoing. Vaccination will result in much lower levels of disease and of faecal contamination of pastures.
If you are considering eradication you need to discuss the options and costs with your veterinarian and farm management consultant.
A property control program needs to suit the property's individual business circumstances. The main principles in controlling OJD are to reduce exposure of sheep to the bacteria, and to reduce the effect of the disease in exposed animals.
No single measure can be relied on for OJD control; rather a combination of measures will give a higher chance of success. You wil need to assess the best mix for your operation.
Vaccination is very effective at reducing death rates and the number of bacteria shed by infected sheep. It can also reduce the chances of introducing OJD to an unaffected flock, but should not be the only measure relied on for prevention.
Maintaining a younger flock structure will reduce pasture contamination, as younger animals are less likely to shed bacteria in their faeces. It also reduces the opportunity for infected animals to progress to the clinical stage of the disease when they may become unsaleable or die. Infected sheep shed large numbers of bacteria in the terminal stages of the disease, increasing the level of paddock contamination and the level of bacterial challenge to susceptible sheep.
Key steps in flock management include:
- culling any sheep showing early signs of OJD such as wasting (as these animals will be shedding the greatest numbers of bacteria)
- culling entire mobs if there are significant signs of the disease
- avoiding feeding on the ground
- avoiding leaving young sheep in holding paddocks (as they are likely to be heavily contaminated with the bacteria)
- using portable yards for young sheep
- crutching ewes before lambing to reduce faecal contamination of the udder when lambs are suckling
- ensuring good worm control and nutrition to minimise other stresses
- ensuring good nutrition to lambing flocks to maximise milk supply and reduce early grazing by lambs
- developing a policy with neighbours regarding stray sheep. Ask them to notify you of strays rather than just returning them to the mob. If you find stray sheep, isolate them and inform the owner.
Prepare low-contamination paddocks for lambing ewes and weaners, as young sheep are the most susceptible to infection. This will lower the infection challenge. Lower risk pastures include:
- previously ungrazed stubbles
- paddocks grazed by cattle
- paddocks grazed only by sheep vaccinated against OJD
- padocks grazed by known low-risk mobs (sheep under 12 months old, tested free mobs)
- paddocks cut for hay
- paddocks destocked for at least six weeks, particularly in summer.
- Fence off wet, shaded areas in paddocks or reserve such areas and paddocks for sheep about to go for slaughter.
- Fence off common waterways.
- Prevent contamination of water supplies with sheep faeces.
Prevent spread to and from neighbouring properties
Good fencing is essential to prevent straying stock. It may be cost-effective for neighbours to vaccinate their flocks so all parties are better protected. Graze cattle or low-risk sheep on any boundaries where water drains between properties. Investigate possible barriers to sheep and water movement such as contour banks, laneways and tree plantings.
Regardless of which management options are available for your property, it will take time to see any significant results.
Mortality rates are a guide to the level of OJD on your farm. Monitoring the progress of control measures can include monitoring mortality rates of different age groups of sheep, on-farm testing and abattoir testing.
It is important to seek advice on the most cost-effective management options for each property and you should discuss options with your veterinarian or farm consultant to develop an individual plan for your property.
Maintaining a biosecurity plan can reduce the risk of introducing infected animals onto your property. Control of stray sheep or goats is a critical part of any biosecurity plan and can also reduce the risk of diseases such as OJD, footrot, lice and drench-resistant worms.
A closed flock with good fencing, including fenced waterways, is the best protection against introducing disease.
The safest way of introducing genetic material is via semen or embryos.
Any introductions of sheep should be from a low-risk source, identified using the national sheep health statement.
Other general biosecurity measures which reduce the risk of introducing disease include:
- visitors' vehicles to stay in designated area and not travel across paddocks
- visitors to have clean footwear
- isolate introduced stock until you are confident they are disease free
- clean footwear after visiting other properties
- clean machinery coming on and off the farm to remove faecal material and soil.
- The impact of OJD will vary from one property to another.
- OJD can be managed through grazing and flock management strategies, along with vaccination in some cases.
- A biosecurity plan that helps prevent OJD will also help to prevent other flock diseases such as footrot and drench-resistant worms.