Relation between serologic response and pathologic findings in sheep with naturally acquired paratuberculosis.

OBJECTIVE

To investigate the relation between lesions and serologic response in sheep with naturally acquired paratuberculosis and to evaluate the efficacy of agar gel immunodiffusion (AGID) and ELISA as diagnostic tests.

ANIMALS

134 adult sheep, culled by the owner from flocks with history of severe chronic weight loss.

PROCEDURE

Before euthanasia, blood samples were collected and serum was obtained. AGID and ELISA were performed, using a protoplasmic antigen of Mycobacterium paratuberculosis. Sensitivity and specificity of these techniques were evaluated, using histopathologic findings as a reference. Histologic studies were carried out focusing on specimens containing intestinal organized lymphoid tissue.

RESULTS

Of the sheep examined, 46.3% had lesions that were classified in 5 categories, according to their location, extension, cellular types, and presence of acid-fast bacilli. Regarding serologic test results, 37.1% of sheep with lesions had a positive AGID result, and all sheep without lesions had negative results. However, ELISA positivity was slightly greater--48.4% of sheep with lesions; however, by use of ELISA, 11.1% of sheep without lesions had positive results.

CONCLUSION

A close relation between pathologic findings and serologic response was determined, especially between presence of acid-fast bacilli (potentially heavily shedding sheep) in the intestinal lesions and positivity to serologic tests, especially AGID.

CLINICAL RELEVANCE

On the basis of these results, together with its high specificity, reliability, and the difficulty of fecal specimen culture in sheep, AGID is proposed as a useful tool for seeking evidence of paratuberculosis in live sheep, and therefore, is suggested as the basis for control programs in areas in which the disease is a serious threat to sheep production.