Immunohistochemical distinction between preclinical bovine spongiform encephalopathy and scrapie infection in sheep

Sheep are susceptible experimentally to bovine spongiform encephalopathy (BSE), the clinical signs being indistinguishable from those of scrapie. Because of the possibility of natural ovine BSE infection, laboratory tests are needed to distinguish between scrapie and BSE infection. The objectives of this study were to determine whether (1) PrPSc accumulates in biopsy samples of the tonsil or third eyelid, or both, of BSE-infected sheep before the appearance of clinical disease, and (2) such samples from BSE- and scrapie-infected sheep differ in respect of PrPSc accumulations. Homozygous ARQ sheep (n = 10) were dosed orally at 4-5 months of age with a brain homogenate from BSE-infected cattle. Third eyelid and tonsillar biopsy samples were taken at < or = 6 monthly intervals post-infection and examined immunohistochemically for PrPSc. Third eyelid protuberances were difficult to identify, resulting in many unsuitable samples; however, third eyelid samples shown to contain lymphoid follicles were invariably negative for PrPSc. In contrast, tonsillar biopsy samples became positive for PrPSc from 11 to 20 months post-infection. Consistent differences in the morphology of PrPSc granules in tingible body macrophages (TBMs) between BSE- and scrapie-infected sheep were detected with anti-peptide antibodies directed towards amino acids 93-106 of the ovine prion protein: thus, PrPSc appeared as single granules in TBMs of tonsillar sections from BSE-infected sheep, whereas clusters of PrPSc granules were observed within TBMs in the tonsils of scrapie-infected sheep. In contrast, antibodies against epitopes situated N- and C-terminally from the 93-106 region of the ovine prion protein revealed no differences between BSE- and scrapie-infected sheep in terms of PrPSc granules in TBMs.