Photoreceptor degeneration preceding clinical scrapie encephalopathy in hamsters.

Hamsters experimentally inoculated with scrapie had histologic and ultrastructural changes in their retinas before clinical signs of central nervous system disease became evident at 50 days postinoculation. The retinal changes were limited to the photoreceptors and began with degenerative changes in the inner segments as early as 8 days postinoculation. As the inner segment lesions became more prominent, associated degenerative changes within outer segments were seen (38 days postinoculation). An influx of macrophages was associated with increasing degenerative changes; these cells engulfed cellular debris did not appear to contribute directly to the degenerative process. Retinas having the greatest damage to the inner segment area also had moderate reduction in outer nuclear layer density and outer segment length, and in numbers of outer segments. All of the retinal changes described occurred almost exclusively in the central to mid-region of retina with relative sparing of peripheral retina.