Use of the polymerase chain reaction assay for the detection of Mycobacterium avium subspecies paratuberculosis in blood and liver biopsies from experimentally infected sheep

Objective To evaluate a polymerase chain reaction-based assay for the detec tion of Mycobacterium avium subsp paratuberculosis in blood and liver biops ies from subclinically infected sheep. Study design A direct PCR assay for the detection of M a paratuberculosis w as applied to liver biopsy specimens and to samples of blood that were sequ entially collected over 53 weeks from 14 sheep infected experimentally with the organism. Results Of 117 blood samples from the 14 experimentally infected sheep, two tested positive in the PCR assay. Both positive results were obtained in t wo subclinically infected sheep that had paratuberculosis later confirmed b y histological examination at necropsy. However, the assay failed to detect the target DNA in samples of blood from five other sheep with histological ly confirmed paratuberculosis. Similarly, the PCR assay on liver biopsy spe cimens collected 32 weeks after administration of M a paratuberculosis gave only two positive results, both of which were obtained in sheep with histo logical evidence of paratuberculosis. Conclusions The PCR assay on blood and liver biopsies does not provide a us eful method for the diagnosis of M a paratuberculosis infection in subclini cally infected sheep.