Even in herds with widespread infection, at any one time only a few animals may be showing signs of disease. Infection results in invasion of the terminal portion of the small intestine. The result is a reduced ability to absorb and utilize nutrients. The intestinal wall may be thickened, with a reddened surface. Although the early signs are similar, Johne's must be differentiated from coccidiosis, winter dysentery, and parasitism.
Malnutrition increases the severity of the disease signs.
Diagnosis Diagnosis from necropsied or slaughtered animals is made by histopathologic examination of the ileocecal portion of intestine and the regional lymph nodes. If an animal is infected, intracellular, acid-fast organisms and characteristic granulomatous cellular changes are present. The organism can also be cultured from ileum, lymph node, or fecal material. In the individual live animal, the best diagnosis is made by a culture of the manure, although the animal must be shedding the organism at the time. The fecal culture is specific and does not give false positive results.