Early and late pathogenesis of natural scrapie infection in sheep.

The pathogenesis of scrapie infection was studied in sheep carrying the PrP(VRQ)/PrP(VRQ) genotype, which is associated with a high susceptibility for natural scrapie. The sheep were killed at sequential time points during a scrapie infection covering both the early and late stages of scrapie pathogenesis. Various lymphoid and neural tissues were collected and immunohistochemically examined for the presence of the scrapie-associated prion protein PrP(Sc), a marker for scrapie infectivity The first stage of scrapie infection consisted of invasion of the palatine tonsil and Peyer's patches of the caudal jejunum and ileum, the so-called gut-associated lymphoid tissues (GALT). At the same time, PrP(Sc) was detected in the medial retropharyngeal lymph nodes draining the palatine tonsil and the mesenteric lymph nodes draining the jejunal and ileal Peyer's patches. From these initial sites of scrapie replication, the scrapie agent disseminated to other non-GALT-related lymphoid tissues. Neuroinvasion started in the enteric nervous system followed by retrograde spread of the scrapie agent via efferent parasympathetic and sympathetic nerve fibres innervating the gut, to the dorsal motor nucleus of the vagus in the medulla oblongata and the intermediolateral column of the thoracic spinal cord segments T8-T10, respectively.