Ovine Johnes Disease

Ovine Johnes Disease

Ovine Johnes Disease (OJD) is a disease of major significance to the Australian sheep industry which has been growing in prevalence in our previously low risk area. Since the discovery of of OJD in the early 1980s, there had been no confirmed cases within the sheep flocks of our clients. However, in the past two years we have diagnosed several cases of OJD and we suspect that number will continue to grow in the near future so we urge producers to be on the alert.

 

What is Ovine Johnes Disease?

Ovine Johnes Disease is caused by the sheep strain of the bacteria Mycobacterium paratuberculosis. The bacteria is shed in the faeces of infected animals and can survive in the environment where it can infect further animals. Infection of individual animals typically occurs at a young age and can be from suckling contaminated udders or grazing contaminated pastures. Animals become less susceptible to infection as they get older however can still contract the disease at any age.

Once infected the disease has a long incubation period before clinical signs become apparent. The bacteria slowly causes thickening of the walls of the small intestine, reducing the amount of nutrients absorbed into the blood stream. Clinical signs often take up to two or more years to show as the infected animals will slowly start to waste away due to lack of nutrition. Affected animals will continue to lose condition despite any treatments and will die typically within 6-12 weeks of signs beginning. The most common age for clinical signs to become apparent in sheep is 2-4 years of age (usually infected as lambs) but can occur in younger or older sheep on occasion. It is important to note that unlike in Johnes disease of cattle, scouring only occurs occasionally in affected sheep.

Due to the insidious nature of the disease, it is often not detected for several years after the initial introduction by which stage many animals may already be infected. These infected animals shed large numbers of bacteria (especially those showing clinical signs) which contributes to environmental contamination and the source of future infections within the flock. Some infected animals may not progress to showing clinical signs but can still act as carriers and shedders of the bacteria for their entire lives. The bacteria can live for long periods in the environment, over 12 months in moist, shaded areas.

Once established on a property, if not managed the disease can cause massive losses with up to 10-20% adult deaths annually recorded on some properties.

 

How is OJD introduced onto a property?

The vast majority of new Ovine Johnes infections result from sheep introduced from infected flocks or stray sheep from infected neighbouring properties. These introduced sheep shed bacteria into the environment and subsequent generations born on the property are then infected.

A less common cause of introduction is via waterways/etc carrying the bacteria from contaminated land onto clean land and subsequently exposing sheep to infection.

 

How is OJD diagnosed?

Testing for Ovine Johnes Disease is complicated by a number of factors including:

  1. Prolonged incubation period of the disease
  2. Intermittent shedding of the bacteria in faeces of infected animals
  3. The low number of infected animals within a flock early in the course of the disease
  4. Difficulty and time taken in growing the bacteria in a lab environment
  5. Presence of similar bacteria species which can cross react with some methods of testing

All of these factors influence the sensitivity, specificity and predictive values of the available tests which is a whole discussion on its own. However the bottom line is that there are no definitive laboratory tests which can both rule in or rule out OJD infection with absolute certainty. As such we are required to utilise the available tests and interpret results in light of risk factors, clinical signs, etc.

The diagnostic and monitoring tests currently available are:

  1. Pooled Faecal Culture (PFC) – faecal samples are collected form a large number of adult animals and pooled into lots of 50 to be cultured for the Johnes bacteria. This test is typically used as a flock screening test. Results take approximately 2-3 months to return. A positive result is definitive for OJD however a negative result does not definitively preclude OJD in a flock. The PFC350 is often used as a screening test and a marketing assurance and involves the pooled faecal culture of 350 adult animals within the flock.
  2. High Throughput Faecal PCR – this method of faecal testing has been widely used in cattle (the method we used in the JBAS testing programs recently) however is not widely used in sheep at this point. Its major advantage is that results can be obtained on pooled faecal samples in a matter of days rather than months however it is limited by the same constraints as the faecal culture – negative results are not 100% definitive due to reasons 1-3 noted above.
  3. Abattoir Surveillance – the intestinal contents of lines of adult animals can be examined post slaughter in abattoirs for signs consistent with OJD infection. This monitoring is a cost effective way of increasing assurance that OJD is not within a flock for trading purposes however it is not sensitive in detecting animals with early stages of the disease. The AB500 test status requires 500 adult animals to have been inspected in abattoirs over a period of 2 years and is used by some flocks as a marketing tool to increase buyer assurance of low risk disease status.
  4. Post mortem examination & histopathology – for individual, clinically suspicious animals, a post mortem will be performed to examine areas of the gastrointestinal tract with samples sent to the lab for histology examination (under microscope) to look for changes classically associated with OJD infection. This method is highly accurate in ruling in or out OJD infection in individual animals showing signs of the disease but for obvious reasons is not suitable as a flock screening test.

 

What industry wide management practices are in place?

Since the early 1990’s, the management of Ovine Johnes Disease Australia wide has been reviewed and modified several times with varying levels of industry and government involvement throughout that period. Over this time government involvement and regulation has decreased with more emphasis placed on industry bodies such as Sheepmeat Council of Australia and Woolproducers Australia to provide management guidelines and individual producers to control risk through their own management practices.

 

There have been a number of schemes throughout the past 20 years which have been regularly changing, providing confusion to many producers about what is current practice. A few points to try and clear up some of this confusion are:

  • Prevalence Area Determination was discontinued in 2013
    • Designed to class areas as High Prevalence (Central & Eastern Victoria, Eastern Riverina & NSW Central Tablelands), Medium Prevalence (North Western Victoria, Tasmania, Monaro, Western Australia) and Low Prevalence (South Australia, Northern Territory, Queensland, Northern & Western NSW)
    • Our region was in the low prevalence zone
    • Movement restrictions based on prevalence zones were found to be unsuccessful in preventing the spread of OJD and as such were discontinued
  • ABC Points System was discontinued in 2013
  • Government mandated quarantine - control/eradication programs and movement restrictions on properties suspected or diagnosed with OJD have been discontinued
    • These programs were the source of much angst amongst sheep producers and as a result there was a stigma associated with OJD. As such, many producers preferred to avoid investigating suspect clinical cases in fear of the repercussions if a positive diagnosis was confirmed.
    • With the reduction in Government involvement in the current national management plans, there are no longer any mandatory quarantine or trade restrictions placed on properties with OJD
  • Regional Biosecurity Plans (RBPs) were introduced in 2013
    • These were voluntary plans for groups of producers in low prevalence areas who were willing to adhere to certain management practices regarding sheep movements, biosecurity protocols and disease investigation.
    • They allowed individual producers to market their sheep as low Johnes risk.
    • RBPs were typically formed along RLPB/LLS boundaries and in NSW included Coonabarabran, Coonamble, Walgett, Nyngan, Darling, Northern Tablelands, Riverina & Western.
    • With the cessation of the previous National OJD Management Plan which ran from 2013-2018, most of these RBPs have ceased to exist with Coonamble and Nyngan being the only ones continued in NSW.

Currently, OJD management is run by Animal Health Australia in conjunction with industry bodies as part of the Sheep Health Project which focuses on a number of important endemic diseases in sheep across the nation. As mentioned, government intervention and regulations have been lifted and the focus is on individual and groups of producers to control the disease as best suits their operations. Tools available to producers to ensure that they can control the risk of disease and to provide a source of low risk stock for purchase include:

  • Vaccination – GudairÓ is a registered vaccination for OJD which is ideally administered as a single dose at less than 16 weeks of age (at lamb marking). It is important to note however that it is not a silver bullet and vaccinated animals may still develop infection in a small number of cases. The vaccine may be used in older animals however it will not prevent deaths in many animals already infected but it will reduce the degree of bacterial shedding and subsequent environmental contamination. The downsides of the vaccination are its cost, vaccination site reactions and the OH&S implications – inadvertent injection into humans will likely cause severe and on-going injury.
  • Sheep Health Declarations  –  provided by vendors, these are vital documents which contain information on a number of diseases which help purchasers consider the risks involved in the purchase. Ensure to always ask for the Sheep Health Dec when making any purchases.
  • SheepMAP – is a market assurance program that aims to provide a source of sheep that provide a low risk of OJD infection. Through routine testing (Pooled Faecal Cultures), accredited flocks are able to demonstrate that they have a decreased likelihood of being infected with OJD.
  • Abattoir Surveillance – is carried out as part of the wider National Sheep Health Monitoring Project. Producers can request that adult animals over 2 years of age and that have been on the property for a minimum of 2 years be examined post mortem for changes consistent with OJD. These examinations can be used to demonstrate a flock’s low risk level as detailed previously with either an AB150 (150 sheep in past 12 months) or AB500 (50 sheep in past 2 years) status.

 

How do I minimise my risk of OJD entering my flock?

As mentioned previously, by far the most common source of OJD infection is through introducing new stock or through stray animals. The risk of introducing OJD into your flock is obviously dependent on the type of operation that you run.

Maintaining a closed flock will substantially decrease your risk of introducing OJD compared with an open flock however there are still possible avenues where the disease may get into your flock including:

  • Stray animals (both from or to an infected neighbouring property)
  • Sharing of common facilities with neighbours including yards, woolshed, etc
  • Purchased rams
  • Via water running from contaminated land
  • Contamination of common grazing areas such as roadsides/TSRs/etc

In order to minimise the risk of OJD entering from the above routes a few practices can be implemented:

  • Maintain standard of boundary fencing, especially with neighbours with sheep flocks
  • Minimise contact with other sheep flocks. If thinking about sharing facilities consider the risk status of any other flocks using the same shed/yards/etc and prevent sheep from grazing whilst in these areas
  • Introduce rams from low risk flocks – ideally from producers who are part of the SheepMAP or have recently performed Pooled Faecal Culture or Abattoir Surveillance testing
  • Minimise contact of at risk sheep (i.e. lambs & weaners) with areas of concern such as waterways from neighbouring properties if there are any concerns over their OJD status

For those producers that run an open flock, much greater care needs to be taken if trying to avoid introduction of OJD onto your property. Whilst all of the above points still apply, the major consideration is sourcing low risk sheep when buying. Different classes of sheep will provide varying degrees of risk:

  • Weaners/lambs to grow out and finish for slaughter provide a low risk of introducing OJD as even if they have been infected, they will likely be turned off prior to shedding of the bacteria into the environment
  • Replacement ewes provide the greatest risk of introductions if sourced from an unknowingly infected flock as they will likely be shedding bacteria by the time they arrive, will be on the property for some time and will likely pass the bacteria onto the their offspring.

When introducing new sheep some considerations to take to minimise your risk may include:

  • Always ask for a Sheep Health Statement from the vendor – this provides vital information on disease status for OJD and a number of other important diseases
  • If possible, source sheep from producers who are part of the SheepMAP program or have a current PFC350 or AB500 test status.
  • Consider the location of where sheep are coming from – certain areas will be much lower risk than others (e.g. western NSW vs the central tablelands)
  • Consider the vaccination status of the sheep. Although not 100% reliable in preventing infection, sheep that were vaccinated as lambs will be much lower risk than non-vaccinated sheep or sheep vaccinated as adults
  • For non-vaccinated sheep consider implementing your own vaccination program. If infected sheep are inadvertently introduced, vaccination at the time of introduction will at least help to minimise the degree of bacterial shedding and slow the spread to the rest of the flock (if they are not already vaccinated as well)

 

What do I do if OJD is diagnosed in my flock?

For many years a stigma was associated with the diagnosis of Ovine Johnes Disease within a flock and producers were fearful that mandatory quarantine and trade restriction measures would severely impact their businesses and their profitability. As outlined above, all of these mandatory measures have been discontinued and the impacts of an OJD diagnosis have diminished greatly. The onus is now on individual producers to determine their own course of action to determine how best to manage the disease in their operations.

As such, if there is any suspicion of OJD infection on your property you are much better off investigating it and the being able to manage it rather than sweeping it under the carpet.

Although still a notifiable disease, there is no longer any automatic notification of neighbours or trace forwards of sold stock. It is up to the producer to let neighbours know of the diagnosis as well as recent purchasers of any sheep.

Following confirmation of a positive diagnosis, two major options are available for sheep producers – attempt eradication OR opt to manage as an endemic disease.

Eradication:

Eradication programs were regularly implemented in the early days of the OJD control plans however the focus recently has tended to shift away from eradication due to a number of reasons.

As the bacteria can survive for such long periods of time in the soil, eradication programs involve destocking of all sheep for at least 15 months which must include two Summers. Whether this is achievable will depend on your operation; a mixed sheep and cattle trading operation may be more flexible when it comes to destocking all sheep compared with a self-replacing flock with many years of genetic improvement invested.

A critical consideration when deciding whether to attempt an eradication program is the likelihood of reintroducing the disease when restocking. A low risk source of replacement stock must be available and biosecurity measures to prevent spread from neighbouring properties must be addressed.

Overall the reported success rate of eradication programs is only 50-60% and given the expenses involved, a plan to manage the disease may be more appropriate.

Management:

The aim of any management program is to minimise environmental contamination, reduce exposure of susceptible sheep to the bacteria and minimising the effect of disease on animals that are exposed. No single measure will provide complete control of the disease and as such a range of measures will need to be implemented which can be tailored to individual properties depending on their circumstances.

  1. Vaccination:

Vaccination is likely to the be the mainstay of most control programs as it is very effective at reducing death rates and the amount of bacterial shedding into the environment. As previously mentioned however, it is not 100% effective and some losses will still occur.

Vaccination is carried out as a single dose at lamb marking (prior to when most animals are infected) and the process continued annually as part of routine management. In most cases, the vaccine can be limited to female animals only if all male animals are destined for slaughter as lambs.

Adult animals may also be vaccinated when either introduced to a property or in the immediate term following detection of the disease. This will help to reduce but not eliminate deaths and shedding in animals already infected.

 

  1. Flock Management:

In the short term following the initial diagnosis of OJD in a flock, the deaths of affected animals will continue to occur despite management practices being implemented. Most often, these animals will be in the 2-4 year old range and as such consideration may be given to sell these mobs prior to further deaths occurring (on these properties, these animals are likely to be the first drop of lambs exposed post disease introduction and as such the older ewes should not experience the same issues as they were adult sheep before they were potentially exposed).

Longer term, some key steps in relation to flock management to reduce death rates and shedding of bacteria include:

  • Maintaining younger flock structure to reduce pasture contamination as shedding is more likely to increase as sheep get older
  • Immediately culling any sheep showing any clinical signs of OJD as these are likely to be shedding large amounts of bacteria
  • Cull entire mobs of sheep if a large number of clinical cases are becoming apparent
  • Ensuring good parasite control and nutrition to minimise other stresses which may impair immune function
  • Avoid leaving young sheep in holding paddocks which will likely have heavy environmental contamination
  • Use portable yards when handling young sheep
  • Crutch/drench ewes prior to lambing to reduce faecal contamination of udder
  • Early wean lambs to minimise period of time where they are co-grazing with ewes on potentially contaminated pasture. Tight joining/lambing periods will facilitate early marking and weaning with lambs ideally weaned 11 weeks from the start of lambing. This will also help to reduce worm burdens in lambs and increase grazing efficiency as lambs start to compete with their mothers for feed.

 

  1. Paddock Management:

Paddock management aims to provide low-contamination grazing for higher risk stock classes which in the case of OJD are lambs and weaners. Low contamination paddocks should ideally be prepared for lambing ewes and for lambs post weaning. If such paddocks are limited, priority should be given to weaners following early weaning. Lower risk pastures may include:

  • Previously ungrazed stubbles
  • Paddocks grazed by cattle
  • Paddocks grazed only by sheep vaccinated against OJD
  • Paddocks destocked for a least 6 weeks in Summer or 6 months in cooler months
  • Paddocks grazed by low risk sheep – i.e. sheep under 18 months of age which are unlikely to have started shedding or mobs which have been tested negative
  • Paddocks that have been cropped or had pasture resown

Other considerations to make regarding paddock management include:

  • Fencing off wet, shaded areas in paddocks as these will likely have a higher bacterial contamination. Alternatively, graze these paddocks with adult or vaccinated sheep or sheep about to go for slaughter
  • Fence off waterways
  • Prevent contamination of water sources with sheep faeces

 

In summary, the key points to take home regarding Ovine Johnes Disease are:

 

  1. OJD is becoming more prevalent in our area and that trend is likely to continue in the near future – be aware and monitor your flocks for any suspicious cases
  2. Do not ignore the issue – if you have any suspect cases of wasting sheep you are much better off investigating them and then being able to manage the disease rather than burying your head in the sand
  3. Beware of introduced stock – consider the risk status of the flock and request a Sheep Health Declaration
  4. Vaccination will likely be the mainstay of most control programs however is not a silver bullet and other management practices should be implemented

If you have any queries relating to Ovine Johnes Disease or are concerned that you may be affected please do not hesitate to call the clinic for further information.