Ovine Johne’s disease (OJD) is an incurable, infectious wasting disease of sheep that can result in significant economic losses on infected farms due to sheep deaths and lost production in meat, lambs and wool.
OJD is caused by the sheep strain of the bacterium Mycobacterium paratuberculosis, which leads to the intestinal wall slowly thickening, causing reduced absorption of nutrients from the intestine. This eventually leads to severe loss of condition – infected sheep can waste away and die.
The bacteria are passed in the manure of infected animals, contaminating pasture and water supplies and spreading infection to other susceptible sheep. Once a flock is endemically infected with OJD, it is difficult to eradicate.
For more information, visit the other pages in this section, download the OJD Factsheet or view an online slide presentation.
Spot the OJD sheep
Causes/Symptoms
OJD Sheep
OJD is caused by the sheep strain of the bacterium Mycobacterium paratuberculosis, which leads to the intestinal wall slowly thickening, causing reduced absorption of nutrients from the intestine. This eventually leads to severe loss of condition – infected sheep can waste away and die.
OJD is often not diagnosed in a flock until a significant proportion of the flock is already infected and deaths are occurring. There is often no sign for the first few years – that is why it is often referred to as a ‘silent but costly disease’. Infected sheep can be shedding the bacteria in their manure for a considerable period (sometimes years) even though the flock still looks healthy, but they are contaminating the pasture and infecting other sheep.
The classic sign of the disease in a mob is a distinct ‘tail’, with sheep ranging in condition from good to very poor, then the sheep in the ‘tail’ start dying. The number of sheep in this ‘tail’ may be constant but it’s made up of different animals over time, insidiously eating away at flock numbers and profits.
The infected sheep continue to eat and drink normally until they are too weak to graze and eventually die. Scouring may occur, but is not a common feature of Johne’s disease in sheep. In some large flocks, the number of deaths may only be appreciated when big discrepancies occur in counts of adult sheep.
Divided mob – OJD sheep on left
When obvious OJD deaths are noticed, the disease is likely to be well established and it will take some years to get the situation under control, during which time deaths will continue. The best place to look for the disease is in two and three year olds, but sheep from weaners through to older adults can also die from the disease.
Diagnosis
Accurate and rapid detection of ovine Johne’s disease (OJD) infection, in both individual sheep and flocks, continues to be a major challenge for the management and control of the disease in the sheep industry. Current diagnostic tests for OJD have a limited ability to detect individual infected sheep, particularly in the early stages of infection. However, they can be used with good accuracy to assess the infection status of flocks.
The main test currently used for on-farm flock screening is the Pooled faecal culture (PFC) to detect OJD bacteria in dung. A single dung pellet is collected from at least 350 adult sheep in the flock and cultured in ‘pools’ of 50 for the OJD bacteria. On average, the PFC is able to detect OJD in 98% of infected flocks if 2% or more of sheep in the flock are infected.
A blood test to detect antibodies to OJD bacteria in the blood (serology) is also available, but has its drawbacks. It is less sensitive than PFC and so more sheep must be tested to provide a similar level of confidence in the results. Antibodies are also produced in the blood following vaccination. Approval by the state Chief Veterinary Officer may be needed before using the blood test.
Abattoir surveillance is also a practical and cost effective means of checking for OJD. Trained inspection personnel at abattoirs in all states now inspect lines of adult sheep for visible signs of OJD in the intestines and lymph nodes. Find out more about the abattoir monitoring program in Australia.
Ongoing research is focusing on the development of new tests that can quickly and accurately detect infected sheep, particularly in the early stages of infection.
More information is available from Meat & Livestock Australia’s fact sheet on Diagnosing Ovine Johnes Disease.
Spread
OJD usually enters a flock through the introduction of infected sheep (bought in or agisted) or stray animals (from infected neighbours). Sheep droppings and contaminated run-off can also spread the infection between farms once it has ‘seeded’ into an area. The risk of spreading infection via natural waterways and floodplains is also present.
OJD bacteria are shed in huge numbers in the manure of infected sheep. Animals pick up the infection from contaminated pastures, water and teats. The bacteria can survive for many months in shaded environments.
Sheep can become infected with OJD at any age, but are most commonly exposed and become infected as lambs or weaners, with deaths most often occurring in two and three year olds.
Economic impact
OJD infection can result in significant economic losses on infected farms due to sheep deaths, lost production and trading restrictions. Owners of flocks that are infected with OJD who undertake no appropriate management measures will, over time, experience heavy production losses and deaths as the level of infection within the flock increases. Annual death rates in infected Australian sheep flocks of up to 25% annually have been seen and, in a self-replacing flock, the lambing percentages can fall to an unsustainable level.
More information is available from Meat & Livestock Australia’s brochure on the Economic Impact of OJD.