Validation of a diagnostic strategy for detection of ovine Johne's disease in New South Wales sheep flocks

The New South Wales sheep industry has developed a strategic plan to progressively eradicate ovine Johne's disease (OJD) from the Australian sheep flock (1). This initiative relies on a suitable diagnostic strategy to identify infected flocks and those flocks unlikely to be infected to provide a source of clean replacement sheep. Flock diagnosis of OJD is limited by the unreliability of fecal culture, the low sensitivity of serological tests, and the impracticality of sample collection for histology. We have determined the prevalence of OJD is selected infected sheep flocks by histological means and seroprevalence using an agar gel precipitin test. All sheep were condition scored prior to slaughter. A comparison of the prevalence of Johne's disease nad the seroprevalence within each condition score group is presented for each flock. A significant but not unexpected finding was that sheep infected with OJD are more likely to be found amongst sheep of low condition score within the flock. These results demonstrate that the sensitivity of serological diagnosis of OJD in sheep flocks can be improved by deliberately biasing the sample of sheep tested to those animals with lowest body condition score.