Diagnosis of the disease in animals with clinical signs is more difficult in sheep than in cattle. Sheep suspected of dying because of Johne’s disease should be necropsied to assist in the diagnosis. Bacteriologic culture of tissues or feces, commonly used in cattle, may or may not be successful in sheep. Microscopic examination of tissues of sheep dying from the disease can establish a diagnosis.

In all ruminant species, detecting subclinically infected animals is a great challenge. Numerous tests have been developed and all have limitations. Currently, those most often employed in diagnosis are fecal and tissue culture, and serologic tests such as enzyme-linked immunoassay (ELISA) and agar gel immunodiffusion (AGID). There is a DNA probe recently approved to detect Mycobacterium paratuberculosis in bovine feces.

Fecal culture has been the standard test used; it can demonstrate and quantify the presence of the bacteria and, therefore, identify the shedding animal. It is, however, time consuming, expensive to perform, and not widely available. In cattle, cultures usually need to be incubated eight to 16 weeks before growth of the organism is evident. Further-more, animals must be shedding bacteria to be detected. Only a very small proportion of the young stock may be shedding bacteria, and some older animals may be shedding numbers too small to routinely detect.

There also may be host-adapted strains of the causative agent for sheep, cattle, and goats. It is not known how often transmission across species occurs in nature, but epidemiologic evidence suggests that it probably does. Many investigators now believe cultural methods to be unsatisfactory for routine identification of infected (clinical and subclinical) sheep.

No USDA-licensed ELISA or AGID test is commercially available for sheep at this writing. The AGID test, a quick and inexpensive test also performed on blood serum, is primarily useful to quickly support a diagnosis of Johne’s disease in an animal showing compatible clinical signs. Work in cattle and sheep suggests that it may identify animals shedding the highest numbers of Mycobacterium paratuberculosis. It will not identify all infected animals in a flock, and it is least useful for animals in the early stages of infection. An AGID test may be useful in screening flocks in which a high prevalence of infection is suspected. ELISA tests may also be useful for flock screening; however methodology has not been standardized for sheep.

Choosing an appropriate test should be done in consultation with your veterinarian and diagnostic laboratory.