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D14

Johne's Disease

  • Jock Simmonds, "Rowan Park", NSW.

Web site editor: This needs to be updated, vaccine is now available in Australia.

Introduction

At the time of writing, there was no generalised "Agfact" on Johne's disease specifically relating to goats. Therefore, this Goat Note has been compiled from a number of sources relating to cattle and sheep, and to research results involving goats.

Johne's disease is caused by the bacterium, "mycobacterium paratuberculosis". This bacterium causes thickening of the gut wall which, increasingly as the disease progresses, disrupts the animal's ability to take in nutrients from food. Infected animals simply waste away and die.

Johne's disease affects sheep, cattle, goats, alpaca and deer, and has been reported in other species. While the disease in all species is caused by mycobacterium paratuberculosis, there are a number of strains, some of which seem predisposed to infecting cattle, called Bovine Johne's Disease (BJD); and others to sheep, called Ovine Johne's Disease (OJD). In the field it appears that spread of infection between cattle and sheep is rare, but not unheard of. It is believed that, given the right conditions, all susceptible species could become infected by any strain.

Both the sheep and cattle strains have been found in goats. However, the existing evidence indicates that goats are more resistant to the disease than sheep or cattle. Moreover, the evidence indicates that goats are unlikely to spread the infection to sheep, cattle or any other stock.

Symptoms and Diagnosis

The progress of the disease in infected herds is usually very slow. Symptoms are unlikely to be seen in young animals, and only a proportion of stock will develop the symptoms. For example, in self-replacing sheep flocks carrying the disease, loss rates of up to five percent per year might be expected, with all or most of these sheep aged five years and over.

Growers often attribute losses of this nature to forms of ill-thrift such as internal parasites, fluke or "just a poor doer", without seeking veterinary confirmation.

The first indications of the disease are chronic weight loss, progressing to emaciation and death. Once symptoms become apparent, the infected animals usually deteriorate and die within six months. Scouring may occur, but it is not normally a feature. If these symptoms are not cured by anthelmintics or other treatments, the possible presence of Johne's disease should be investigated.

Diagnosis is hampered by the composition of infected herds, which comprise:

  • uninfected animals,
  • subclinical cases where animals are infected, but not showing any symptoms, and cannot be distinguished from uninfected animals (these animals are important to the spread of the disease because they can be shedding bacteria onto the pasture), as well as,
  • clinically affected animals, which shed large amounts of bacteria onto pastures.

Postmortem examination, with laboratory testing, is necessary to confirm a diagnosis in an individual animal. Blood testing is useful to determine whether a herd is infected or not, however, the reliabilities of the current tests are fairly low, which dictates that a large number of animals must be tested to provide adequate confidence in the result. This is an expensive exercise. Further research into tests promises less expensive test techniques, but postmortem examination is likely to remain in order to confirm positive test findings.

Spread of Johne's Disease

Animals become infected by ingesting food or water that has been contaminated by the faeces of infected animals. It takes about twelve months for these newly infected animals to begin shedding bacteria in their own faeces, thus continuing the cycle.

Johne's disease is not highly infectious and it requires exposure to a significant number of bacteria over a period of time for older animals to become infected. It would seem that most animals, that become infected, do so at, or soon after, birth, whilst their immune systems are not well developed.

Whilst there is no conclusive evidence, it is believed that, under ideal conditions, the bacteria may survive on pasture for at least twelve months. During hot, dry summers, survival may be much shorter. Currently, pastures are deemed to be decontaminated if stock, capable of recontaminating the land, are removed for a period that includes two consecutive summers.

Transmission of the disease on tyres or boots is not significant, providing normal hygiene standards are met. The introduction of infected stock is by far the most important source of the infection. Although, infected stock moved or straying onto clean land is another source of infection. Similarly, clean, susceptible stock straying onto infected land might infect other stock on returning, but the chances of this are not very great.

Bovine Johne's Disease is largely carried and passed on to other cattle, and other species, by infected cattle. Similarly, Ovine Johne's Disease is carried and passed on to other sheep, and other species, by infected sheep.

Control Strategies

Remove clinical cases. Clinically affected animals shed large quantities of bacteria onto pasture. Early recognition and removal of these animals will reduce contamination and, thereby, reduce subsequent infection in the herd.

Reduce the herd age structure. The mortality rate and the amount of pasture contamination can be reduced by culling at an early age, but note that these culls can only be sent to slaughter. Buy adult replacement stock, as these are believed to be more resistant to the disease than younger animals. This strategy should delay the onset of clinical symptoms and, thereby, reduce the burden of pasture contamination.

Eradication. Johne's disease can be eradicated from an infected property by removing all infected and potentially infected animals, for a period that includes two consecutive summers. During decontamination the land can be used for cropping or grazing of species deemed not to be susceptible to the disease strain. The genetic base of stud stock can be retained, at a price, by transferring disease free embryos into clean animals maintained on a disease free property.

A plan should be developed, in consultation with an approved veterinarian, before attempting eradication. This plan should consider the status of neighbouring properties and sources of disease free replacement stock. Official recognition that the disease has been eradicated may depend upon official approval of the original plan.

Vaccination. Vaccination has been used overseas to control the disease, but there is no vaccine in Australia at present. The vaccine has been known to mask the disease, such that an infected animal can shed bacteria, without showing any  symptoms of the disease. Authorities believe that the introduction of the vaccine could seriously impede the national eradication programme.

Treatment. There is no effective treatment for infected animals.

Sale from infected properties

Stock sold off an infected property must be sold directly for slaughter, or by auction at special slaughter-only sales.

It is illegal, and subject to heavy fines, to:

  • Move stock infected with Johne's disease, or cause them to be moved to, on, or across any land other than that owned or occupied by the owner of the stock, or
  • Sell, or attempt to sell, diseased stock, except for slaughter.

There have been a number of cases where vendors have been sued for damages, following the introduction of Johne's disease into another herd. Damages are likely to be many times greater than the returns received from the sale of the infected stock.

Finally, there is a moral obligation to protect people from unwittingly introducing diseased stock onto their property.

Purchasing new stock

Frequent introductions of stock into a herd places it at considerable risk - particularly when they are from many different sources. It increases the likelihood of introducing Johne's disease as well as other unwanted problems such as footrot, lice and anthelmintic resistant internal parasites.

Check with your District Veterinarian to avoid buying from high risk areas.

Only purchase stock that come with some guarantee that they are free of Johne's disease. Testing of individual animals will not give protection against the introduction of this disease. There are different levels of assurance that Johne's disease is not present in a herd. Ranked from highest to lowest assurance they are:

  • Market Assurance Program
  • Enhanced Vendor Declaration of OJD status
  • Vendor Declaration

The Market Assurance Program (MAP). The Market Assurance Program identifies herds with the lowest risk of Johne's disease. Because there is testing and an assessment of management and property factors, buyers can be confident that stock from herds with Monitored Negative (MN) status have a very low risk of being infected.

The cost of participating in the MAP is likely to restrict it to studs, thereby providing a source of low risk males, but only a limited number of females and wethers.

Enhanced Vendor Declaration (EVD) of OJD status. The Enhanced Vendor Declaration is a signed declaration that the vendor has no suspicion of OJD in the herd, supported by limited blood testing within the herd.

Purchasers of stock accompanied by an EVD can have high confidence that the owner's declaration is genuine.

Owners are able to sign an "enhanced" declaration for 12 months from the date of the test. Over time, testing repeated at 12 monthly intervals will increase the confidence that the herd is OJD free.

Verdor Declaration (VD). The Vendor Declaration is a signed declaration from the owner or manager that to their knowledge they have no reason to suspect Johne's disease is present in the herd of origin, or on any land on which the stock have ever grazed. There is no testing involved.

© 2000 ACGA