Johne’s disease is a chronic enteritis of ruminants caused by the aerobic bacterium Mycobacterium johnei (syn. M. paratuberculosis). The disease is widely distributed and causes substantial economic losses through death and loss of productivity during the prolonged preclinical stage. The disease has remained a problem for so long because of the absence of a simple, accurate diagnostic test.
M. johnei is spread via faeces and milk. It is excreted by infected animals before clinical signs appear, although bacteria numbers increase once clinical signs have developed. It is believed that most infections occur during the first day of life of a calf and originate from the dam. After ingestion with faeces or milk, M. 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.
The course of the infection depends on the host’s response: the initial infection is overcome; the infection persists for many months or years; 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. Faecal pellets are often soft rather than diarrhoeic, and anaemia occurs in some animals. Appetite usually persists, until the disease reaches advanced stages and animals become emaciated and weak.
Johne’s disease has been associated with reduced milk production and increased culling of infected cows in dairy herds.
Recently, Johne’s disease has been implicated in Crohne’s disease in humans, as it has been shown that there is a high risk for the presence of residual M. johnei bacteria in milk even after pasteurisation.
The prevalence of subclinical Johne’s disease in UK cattle was estimated in an abattoir survey in the south-west of England in 1996. A total of 2.6-3.5% of the animals were infected, depending on the diagnostic test used. In a questionnaire survey of southern English and Welsh farmers, 17.4% of the farms reported having had the disease at some point, but only 0.9% of the farms had had recognised clinical cases in the past 4-year period.