Following bilateral intraocular (i.o.) infection of s7 mice with ME7 scrapie, sequential tissue pools were taken from retina, optic nerve, superior colliculus (SC), dorsal lateral geniculate nucleus (dLGN), visual cortex and cerebellum. The infectivity levels in these pools were estimated by intracerebral (i.c.) assay in C57BL/FaBtDk mice. Infectivity was first detected in retina at 35 days post-injection (as an increase above residual injected inoculum), SC at 56 days, dLGN at 77 days and in optic nerve, visual cortex and cerebellum at 98 days. Pathological lesions were shown to develop in the same sequence later in the incubation period. Comparison of sequential retina and SC assays in congenic mice, which differ only in the vicinity of the locus, revealed a difference in the initial detection and progression of i.o. infection of between 60 and 100 days, indicating that acts by delaying the initiation of replication. Higher levels of infectivity were found in retina and SC of mice infected with 79A scrapie, which destroys the photoreceptor layer in the retina, than with ME7 scrapie, which does not. Retrograde transport of infection was indicated by the levels of infectivity in the retina after i.c. infection with ME7 or 79A scrapie. These results indicate that scrapie spread within the central nervous system is restricted to neuroanatomical pathways, and that controls the initiation, but not the rate of replication.


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