The Disease

Johne's Disease is a chronic enteritis of ruminants caused by the aerobic bacterium Mycobacterium johnei (syn. M. paratuberculosis) (Beeman et al., 1989). The disease is widely distributed and causes substantial economic losses through death and loss of productivity during the prolonged preclinical stage (Johnston and MacLachlan, 1986). The disease has remained a problem for so long because of the absence of a simple, accurate diagnostic test (Collins et al., 1993; Jones and Kay, 1996; Scott et al., 1995). Recently, Johne's disease has been implicated in Crohne's disease in humans.

Mycobacterium johnei is spread via faeces. It is excreted by infected animals before clinical signs appear, although bacteria numbers increase once clinical signs have developed. A sheep strain of Mycobacterium johnei found in the United Kingdom is highly pigmented due to a bright yellow, non-diffusible pigment (Gilmour and Angus, 1991).

After ingestion with faeces, Mycobacterium johnei invades the lymphatic tissue in the mucosa of the small intestine, where it multiplies over the next 2-3 months and spreads to the draining mesenteric lymph nodes. The outcome of the infection depends on both the ability of the host to mount a cell-mediated immune response and the dose of the initial infection. Sheep that are infected as lambs are more likely to develop clinical signs than sheep infected as adults (Gilmour and Angus, 1991).

The course of the infection depends on the host's response: the initial infection is overcome; the infection persists for many months or years (sheep can be carriers, not developing the clinically apparent disease); or the intestinal lesions slowly progress until they interfere with normal function. There is loss of plasma proteins into the lumen and malabsorption of amino acids (Jones and Kay, 1996; Scott et al., 1995). Faecal pellets are often soft rather than diarrhoea and anaemia occurs in some animals. Appetite usually persists. Enhancement of protein metabolism in the liver has been reported, which may be the cause of the muscle wasting found during the disease (Gilmour and Angus, 1991).