Introduction.

More than two decades after its introduction to South Africa in 1967 ovine Johne's disease (JD) spread to the sheep farming areas of the Western Cape Province and subsequently to two other provinces. Although JD is a controlled disease in South Africa, no surveillance is currently done to determine the prevalence in sheep flocks on provincial or national level and no measures are implemented to eradicate outbreaks of JD on infected farms. For this reason and because of the low sensitivity of the AGID test used in previous surveys, it must be assumed that ovine JD is more widely spread than currently known.

Materials and methods.

Between April 2000 and March 2002 a field study was carried out in sub-populations of two naturally infected Merino flocks in the Western Cape. Seventy-five adult sheep were identified per farm and tested for JD every four months. The diagnostic tests used included the Parachek^TM Johne's Absorbed EIA (Commonwealth Serum Laboratories), HerdChek^TM Mycobacterium paratuberculosis Antibody ELISA (IDEXX Scandinavia), Bovigam^TM gamma interferon assay (CSL) and the AGID. A certain number of sheep with one or more positive test results were slaughtered and subjected to macroscopic and histopathological examination of the ileum and the ileo-caecal valve area, followed by bacterial culture on Herrold's egg medium. Culture isolates were identified by IS900 PCR. At the end of the study all remaining sheep were slaughtered and examined accordingly.

Preliminary Results.

To date, preliminary data analysis indicates a JD prevalence in the two sub-populations of at least 38.4% and 40%, respectively. Only 7/150 sheep (4.6%) remained test negative for the entire study period. A total of 126 sheep (84%) showed reactions to more than one test. Despite repeated, multiple test reactivity typical microscopic lesions were often absent and bacterial culture was unsuccessful. On completion of data collection the sensitivity and specificity values for the different tests applied and their potential value in a test-and-slaughter approach to control ovine JD in sheep flocks will be evaluated.