Individual animal tests for ovine Johne's disease.

Abstract

Routine diagnostic tests for ovine paratuberculosis have poor sensitivity in the early stages of the disease, and transmission often occurs before detection. Currently there are no tests to accurately confirm early infection in individual sheep. Such tests are required to provide trading opportunities for producers who may have valuable stock at low risk of infection. Surgical biopsy is one means of disease detection using relatively sensitive laboratory procedures, but was unproven. 77 sheep grazing on a heavily infected farm were examined at 12, 18 and 24 months of age by histopathology and culture of biopsied ileum and mesenteric lymph nodes. Results from biopsy were compared to those from routine tests (ELISA, AGID, IFN-γ, skin testing, faecal culture and direct PCR) applied at six-monthly intervals, and to necropsy findings at three years of age. A total of 170 biopsies were performed without serious complications, and the samples collected were adequate for culture and histological assessment of paratuberculosis. Overall, 36% of sheep were shown to be uninfected at 3 years of age. Of these, 16 were uninfected at all sampling times and 11 sheep had recovered. (ie. They had been infected at an earlier sampling.) The remaining 64% of sheep were classified at necropsy as infected. Biopsy was consistently the most sensitive nonlethal technique for identification of infected sheep, although even at 36 months it detected only 2/3 of infected sheep. It may be useful as an additional tool in the management for individual valuable sheep from infected stud flocks.