General information on Ovine Johne's Disease (OJD)
Since it was first found in Australia in 1980, Ovine Johne's Disease (OJD) has proven to be a costly disease. It spreads slowly, is difficult to detect early on, causes lowered weight gain and wool production and can kill about 10% of adult sheep each year if left unmanaged.
What is the disease?
- OJD is a wasting disease caused by the sheep strain of the bacterium, Mycobacterium paratuberculosis, which grows mainly in the small intestine.
- The intestine wall slowly thickens and the animal has increasing trouble absorbing nutrition from its food. Infected animals slowly lose condition and usually die within 3 to 6 months of showing signs.
- Scouring may occur, but is not seen routinely.
- Merinos are more susceptible than crossbreds and British Breed sheep.
- Goats and cattle can be infected but infection is rare. Alpacas may be infected by OJD.
- There is no cure for OJD.
How does it spread on farm?
- The OJD bacteria are shed in huge numbers in faeces.
- Animals pick up the infection from contaminated teats, pastures and water.
- The bacteria can survive many months in shaded environments.
- Sheep are most commonly infected as weaners, with deaths often showing up in the two and three year olds.
- Lambs can be infected before birth.
How does it spread between farms?
- Bought sheep and strays are the main risk.
- Sheep droppings and contaminated run-off can also spread the infection between farms.
Why bother about OJD?
- In Australia, annual death rates around 10% are common if OJD is not managed.
- Recent New Zealand research shows losses of 17% of weaner weight and 10% of wool cut in sub-clinical cases in crossbred sheep.
- Once it gets into an area all flocks are at risk. The earlier you act, the less the impact of OJD on your business and other flocks in your district.
- Assuring sheep buyers about the OJD status of your sheep is becoming the norm.
How do I know if my sheep have it?
- There is often no sign for the first few years.
- Some animals can be infected and spreading the disease but the flock can still look fine.
- The first thing you might notice is a persistent tail in one or two mobs that won't respond to drenching.
- You might notice a few poor doers that aren't around next time you check in the paddock.
- By the time you see obvious OJD deaths the disease is likely to be well established and it will take some years to get the situation under control.
What can I do to keep the disease out?
- This depends to some extent on whether your flock is already infected and how much OJD is in your district, but there are some basic rules:
- Only buy in vaccinated sheep or sheep that are low risk according to the accompanying Sheep Health Declaration.
- Vaccinate your flock if there is a risk of OJD coming in with purchases or from neighbours.
- Improve your flock's resistance to disease through good nutrition and worm control.
- Work with neighbours to reduce spread between farms.
What can I do if my flock is already infected?
- Vaccinate all animals as lambs (before 16 weeks of age).
- Cull all sheep showing signs.
- Use the dung test (pooled faecal culture-PFC) to identify and cull heavily shedding mobs.
- Manage your grazing patterns to reduce exposure of young sheep to OJD.
- Clean up contaminated pasture by:
- grazing with adult cattle,
- rotating with crops,
- re-sowing pasture,
- grazing with 'clean' sheep, then selling them straight to slaughter
- Avoid feeding on ground - go for troughs or automatic feeders.
- Consider changing your enterprise mix so that you are running fewer susceptible animals.
What happens if I do nothing?
- If your flock is already infected and you do nothing:
- The disease will slowly spread throughout your flock.
- Each new drop of lambs will be exposed, and eventually a significant proportion of your animals will be infected.
- You may see mortalities early on, or they may not start until your animals are older or under stress, eg when feed gets tight or worms get going.
- There is a strong chance that your sheep will spread the infection to neighbours.
- If your flock is not infected and you do nothing:
- You run the risk of eventually ending up with infection.