Detection of Mycobacterium avium subsp. paratuberculosis in ovine tissues and blood by the polymerase chain reaction

A direct polymerase chain reaction (PCR) test was applied to DNA extracted from blood, liver, ileocecal lymph node and ileum from twelve ewes in poor condition with histologically confirmed paratuberculosis and ten clinically normal sheep which had no evidence of paratuberculosis. The assay was compared with four serological tests: complement fixation test (CFT), gel diffusion test (AGID) and two enzyme-linked immunosorbent assays (ELISA). The PCR detection rate of Mycobacterium avium subsp. paratuberculosis, when results of single tests were interpreted in duplicate, was 72% for ileocecal lymph node, 90% for liver, and 100% for ileum in sheep with confirmed paratuberculosis. A single PCR test detected the target DNA in 66% of 0.5 ml blood samples. Sensitivities of serological tests compared with histological diagnosis were: 33% for CFT, 66% for AGID, 75% for the Central Animal Health Laboratory (CAHL) ELISA, and 83% for the 'modified' Commonwealth Serum Laboratories (CSL) ELISA. The PCR assay gave no positive reaction in samples collected from 10 sheep considered to be free of paratuberculosis. Similarly, all four serological tests were also 100% specific. The results raise some hope for the development of a PCR-based test using liver biopsy specimens, or possibly blood, in the diagnosis of paratuberculosis in sheep.