Disease Description

Sheep can become infected with Johne's disease at any age by eating pasture or drinking water contaminated with faeces carrying the bacteria from infected animals. Susceptibility to disease can be influenced by age, breed, stress and the presence of other diseases.

The Johne's Disease bacteria have the ability to survive in the environment under ideal conditions for up to 12 months, particularly in permanently wet, boggy areas.

Prolonged dry conditions and direct sunlight kill the bacteria.

Many sheep infected with Johne's Disease may carry the disease in their intestines and spread the bacteria in their faeces without ever showing any obvious clinical disease. The disease has a long incubation period and most infected sheep do not begin to show any signs of illness until after 2 years of age.

Clinically affected sheep show severe wasting which eventually ends in death. The average time from onset of clinical illness to death is about 6 to 12 weeks. Chronic scouring may sometimes be seen but this is not a common feature of the disease in sheep.

The classic symptom of the disease in a mob is a distinct "tail" with sheep ranging in condition from good to very poor. The infected sheep continue to eat and drink normally until they are too weak to graze, and eventually die.

Although generally only seen in older sheep, young sheep in some flocks are affected because of a shortened incubation period. This may be caused by high stocking rates or heavy bacterial contamination of pastures.

Johne's Disease is characterised by a chronic infection of the intestinal wall causing the lining to thicken. Post mortem examination of animals with suspect clinical signs is currently the only reliable method of diagnosis.

Any suspicious lesions found in the intestines or lymph nodes are sent to a laboratory for microscopic examination to confirm the diagnosis of Johne's Disease.

There are several blood tests available but these may not detect all infected sheep. The value of blood tests is as a screening procedure during flock surveillance. It is not appropriate to define the OJD status of an individual animal on the result of a single blood test.

Recent advances in growing the bacteria from faecal and tissue samples show promise as a new diagnostic test.

There is no successful treatment available for infected sheep and before making any management decisions, it is wise to discuss the issues with a veterinary adviser.

OJD - The Disease (.PDF) 94KB gives a detailed description of the disease including aetiology, distribution, clinical disease, testing and transmission.