Ovine Johne's Disease
Ovine Johne’s Disease (OJD) is a chronic intestinal infection of sheep and goats and is caused by the bacterium Mycobacterium avium subsp. paratuberculosis. It was first diagnosed on the Central Tablelands of NSW in 1980 and is now widespread across much of South Eastern Australia as a result of the movement of infected animals and the lateral spread between adjoining properties. OJD is capable of causing significant production losses on individually infected properties. Regulations and trade restrictions imposed on producers as a result of OJD have also had a significant impact on the sheep industry in Australia.
Transmission of M.paratuberculosis is primarily by the faecal-oral route. The bacteria enter through lymphoid tissue, called Peyer’s Patches, in the small intestine and infect the terminal ileum, caecum, colon and associated mesenteric lymph nodes and vessels. This results in inflammation and thickening of the bowel wall, which interferes with the normal absorption of energy and nutrients. This can lead to severe weight loss, emaciation and eventually death.
The disease is characterised by a long incubation period. Young lambs and kids appear to be more susceptible to infection, with transmission usually occurring from older infected adults to juveniles and neonates. Most animals, however, do not show clinical signs until two years of age. During this time, infection can be very difficult to detect and infected animals can shed bacteria, often intermittently, throughout the course of the disease and even before showing outward clinical signs of infection.
The characteristic features of clinical OJD include weight loss, emaciation and weakness, which progress ultimately to death due to malnutrition. In sheep, however, only about 10% of clinical cases show diarrhoea in the end stage of the disease, unlike cattle where this is characteristic of the disease. Once clinical signs become apparent, animals may die within 2-6 months. Clinical cases of OJD are considered to be the “tip of the iceberg”. For every clinical case of OJD, it is likely that many other animals are infected, depending on the level of infection within the flock. Due to this long incubation period, when infection is first recognised, it is likely that transmission has occurred a few years earlier.