Pathological findings in retina and visual pathways associated to natural Scrapie in sheep

This work represents a comprehensive pathological description of the retina and visual pathways in naturally affected Scrapie sheep. Twenty naturally affected Scrapie sheep and 6 matched controls were used. Eyes, optic nerves and brain from each animal were fixed and histologically processed using hematoxylin-eosin, followed by immunohistochemical staining for prion protein (PrPsc) and glial fibrillar acidic protein (GFAP). Retinal histopathological changes were observed in only 7 clinically affected animals and mainly consisted of loss of outer limitant layer definition, outer plexiform layer atrophy, disorganization and loss of nuclei in both nuclear layers, and Müller glia hypertrophy. PrPsc was detected in the retina of 19 of the 20 sheep and characterized by a disseminated granular deposit across layers and intraneuronally in ganglion cells. The inner plexiform and the ganglion cell layers were the structures most severely affected by PrPsc deposits. PrPsc exhibited a tendency to spread from these two layers to the others. A marked increase in the number and intensity of GFAP-expressing Müller cells was observed in the clinical stage, especially at the terminal stage of the disease. Spongiosis and PrPsc were detected within the visual pathways at the preclinical stage, their values increasing during the course of the disease but varying between the areas examined. PrPsc was detected in only 3 optic nerves. The results suggest that the presence of PrPsc in the retina correlates with disease progression during the preclinical and clinical stages, perhaps using the inner plexiform layer as a first entry site and diffusing from the brain using a centrifugal model.