Johne's Disease (Paratuberculosis)

Infection is silent for years. In cattle, the first sign is often falling milk yield and weight loss despite a normal appetite, then diarrhoea that starts mild and becomes severe, with soft swelling under the jaw (bottle jaw), weakness and eventually recumbency and death. Sheep and goats more often show progressive emaciation, poor appetite, depression, poor hair or fleece and severe disability. At necropsy the small intestine is thickened with a corrugated mucosa and the mesenteric lymph nodes are enlarged.

Paratuberculosis (Johne's disease) is caused by Mycobacterium avium subspecies paratuberculosis, a slow-growing acid-fast bacterium related to the agents of tuberculosis and leprosy. The organism settles and multiplies in the wall of the small intestine and its associated lymph nodes, where it provokes granulomatous inflammation built of macrophages and epithelioid cells. Early, subclinical lesions are small and focal with few bacteria; in advanced cases the enteritis becomes diffuse and packed with organisms, and the gut wall thickens and corrugates. The thickened intestine absorbs nutrients poorly and leaks protein, and this protein-losing enteropathy drives the wasting and the fluid swelling under the jaw.

Cattle, sheep, goats and other ruminants are affected, and control guidance also names deer and camelids. Infection spreads mainly through manure, and also milk, colostrum and in utero; newborns typically swallow contaminated faeces from the calving area or the dam's udder. Young animals, especially under six months, are far more susceptible than adults. The organism survives long in the environment, and animals shed it for long periods before any test is positive. Clinical signs usually take two to four years to appear. In Saudi Arabia, infection with both cattle-type and sheep-type strains has been confirmed in sheep and goats, and a survey in Khuzestan, Iran, estimated true seroprevalence near 9% in cattle and 15-16% in sheep and goats. A possible link to Crohn's disease in people is debated.

In adult sheep and goats, chronic wasting has several causes: caseous lymphadenitis, which leaves caseous abscesses with onion-ring layering in lymph nodes and often the lungs; maedi-visna, which brings respiratory disease; chronic parasitism; and poor teeth or jaw disease. Paratuberculosis is separated by thickened intestine with granulomatous enteritis and acid-fast bacteria at necropsy, or by PCR, faecal culture or ELISA in the living animal.

Small intestine, mesenteric lymph nodes, gastrointestinal tract, mammary gland (shedding in milk)

Closed herds or test-negative replacements, clean separate birthing areas, early calf removal, test-negative or pasteurised colostrum and milk, keeping manure out of feed and water, testing and culling positives, separate grazing for young stock.

No cure. Clinically affected cattle do not respond to treatment, which is uneconomic; the disease is usually fatal, with animals losing condition until they cannot stand and die.