Understanding ovine Johne's disease

Page last updated: Thursday, 12 May 2016 - 4:26pm

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Ovine Johne's disease (OJD) is caused by the bacterium Mycobacterium avium subspecies paratuberculosis (commonly referred to as Mycobacterium paratuberculosis). The bacteria infects sheep, goats and deer but does not readily transfer to cattle and other species. OJD is a quite different disease to bovine Johne's disease (BJD).

Ovine Johne's disease (OJD) occurs in most countries of the world. The origin of OJD in Australia is not known but it is thought that infected sheep or goats were imported from New Zealand during the 1960s or 1970s.

The first diagnosis in Australia was made in 1980 in the Central Tablelands area of New South Wales. Since then, the number of infected flocks has slowly increased.

The disease was first detected in Western Australia in 2000. By 2003 it was found to be on a number of properties and was shown to have been on those properties for some time.

The slow and insidious nature of OJD means it can spread via sheep movements well before the most observant owner has any indication that the disease is present on their property.

Identifying signs of OJD

  • Look for ill-thrift in adult sheep that do not respond to feeding or treatment.
  • Look for a 'tail' in adult mobs.
  • Do not assume that ill-thrift is due to worms, mineral deficiencies or under-nutrition, especially if other sheep in the mob are doing well.

Contact your veterinarian if you see any of the above.

OJD is a chronic infection that causes thickening of the bowel, reducing the absorption of nutrients. This results in weight loss and ill-thrift that does not respond to feeding or treatment. The main sign of an OJD-infected flock is a 'tail' in adult mobs. It can be mistaken for internal parasites, under-nutrition or nutritional diseases. Unresponsive or unexplained wasting and death in adult sheep, particularly in spring, should arouse suspicions of OJD. There is no treatment for OJD — once weight loss has started, death generally occurs within three to four months.

OJD has a long incubation period. The time between infection and the bacteria being passed in faeces is generally between nine months and two years, and it usually takes three to four years before an infected sheep shows any signs of the disease.

How OJD spreads

OJD bacteria pass out in the faeces of some infected animals. The disease is spread when sheep consume pasture or water that has been contaminated by these infected faeces. Most OJD bacteria die within six weeks of being exposed to sunlight, but they may survive in moist, shaded areas for 12 months or more.

Susceptibility to OJD infection decreases with age; animals over two years old are much less likely to become infected. Lambs and weaners are most at risk of picking up infection. Stress and the presence of other diseases increase susceptibility.

The most common means of spread between properties is movement of infected sheep, either as purchased or agisted sheep, or as strays. Sheep can carry the bacteria and spread the disease without showing any obvious signs. This means that OJD can spread to unaffected properties by the movement of apparently normal-looking sheep.

Sheep droppings and contaminated run-off can also spread infection, as may contaminated vehicles, machinery and footwear.

Testing for OJD

OJD is diagnosed in live sheep by finding the bacteria in the faeces. The standard test to detect OJD in a flock involves testing a sample of faeces from 350 adult sheep by means of the pooled faecal culture (PFC) test. This culture test can take three months or more to complete. A new test, a faecal polymerase chain reaction test (PCR) is now available. This gives results in two to four weeks, but still requires testing large numbers of sheep (as per the PFC test).

Not all infected animals shed bacteria in their faeces, particularly in the early stages of the disease. For this reason current OJD tests that rely on finding the bacteria in the faeces (the PFC and faecal PCR tests) have a limited ability to detect individual infected sheep. Young animals and those in the early stages of infection are particularly difficult to diagnose. Testing individual sheep is impractical and costly. It is not recommended that animals less than two years of age are tested as they will not be shedding the bacteria.

Blood testing for OJD is not very reliable and very large numbers of animals need to be tested, making it costly.

Post-mortem testing involves finding the bacteria in the intestines and related lymph nodes.

Market implications of OJD

The greatest cost of OJD in Australia has been the loss of domestic markets as a result of regulatory movement restrictions. The economic impact is far greater for studs and flocks that sell store or breeder sheep than for those who sell mainly to slaughter. Potential purchasers of sheep may not want to risk introducing OJD to their flock.

The effect of OJD-related deaths and lost production varies greatly between farms. Some infected flocks show few or no signs of the disease, whereas others may experience significant losses as the level of infection within the flock rises. Death rates range from 1-15% per year.

Severely affected flocks can show an associated decrease in wool cut and weaner weights. Vaccination and management procedures help minimise the impact of the disease on a farm, however these can be costly. Assess the economic losses associated with OJD against the costs of managing the disease before starting a management program.

Abattoir surveillance

Examining the intestines of adult sheep at abattoir provides a practical and cost-effective method of detecting OJD in flocks. Trained inspectors inspect lines of adult sheep for visible signs of OJD in the intestines and lymph nodes. In areas with a high prevalence of long-standing OJD, this method is thought to detect up to 90% of infected flocks. However, detection of OJD in sheep from areas with a lower prevalence of the disease, and therefore fewer infected animals, is considerably lower.

If you would like to arrange an inspection for adult sheep going to abattoir, contact Dr Anna Erickson on the details below.

Reducing the risk of OJD entering your flock

A few simple precautions will significantly reduce the risk of OJD entering your flock:

  • Maintain a closed flock. Introduction of infected sheep, which may be shedding bacteria despite showing no obvious signs of the disease, is the biggest risk of introducing OJD to your flock.
  • If you do buy in sheep, follow good biosecurity practices, including:
    • Only purchase from flocks with a health status as good as, or better than, your own. Follow the same rules if bringing in agistment stock.
    • Ask for a national sheep health statement (SHS). This requires the vendor to disclose the OJD status of their flock. Be aware that the SHS can only describe what the vendor knows so you may wish to ask for additional testing.
    • Only transport purchased sheep in clean vehicles. Discuss vehicle cleaning protocols with your transporter.
    • Maintain good records of movements on and off your property to ensure whole-of-life traceability.
  • Maintain fences to prevent sheep straying on and off your property. Work together with neighbours to reduce the risk of straying.
  • Check regularly for strays in your flock. Promptly remove and isolate strays.
  • Assess your risk and consider vaccination if you decide this is economically practical.
  • Restrict visitor and vehicle movement onto your property to designated driveways and access roads.
  • Use semen and embryos for introducing new genetics; this greatly reduces the risk of introducing disease.
  • Check the health protocols for shows and breeding centres and isolate sheep as much as possible.
  • Inspect your sheep regularly, especially those more recently introduced and report any unusual signs to your veterinarian.

Health status declarations

Before purchasing sheep you need to assess the OJD risk of a flock, including looking at:

  • Current OJD status — is the flock known or suspected to be infected? Has the flock been tested, and if so, when and what was the result?
  • Trading history — is it a closed flock? If not, does it buy sheep from multiple sources? What is the status of the flock of origin for introduced sheep?
  • Area prevalence — OJD is more common in areas with higher rainfall and stocking densities.
  • Vaccination history — vaccination alone does not eliminate the risk of OJD but it significantly reduces bacteria being shed in the faeces.
  • Is the flock in the sheep market assurance program (SheepMAP)? The higher the SheepMAP status, the higher the level of assurance OJD is not present.

Much of this information is included in the national sheep health statement (SHS). Always request a completed SHS when buying sheep.

Ask the owner or stock agent for any additional information that is not included on the SHS.

For more information, contact Dr Anna Erickson on the details below.

Contact information

Anna Erickson
+61 (0)8 9881 0211