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.