Cerebrovascular amyloidosis in scrapie-affected sheep.

Cerebrovascular amyloidosis (CA) was found in the cerebral and cerebellar cortices in 11 of 20 sheep, of six different breeds, with naturally-occurring scrapie. The occurrence of the lesion did not appear to be influenced by the age of the sheep. This was not seen in 20 brains from age- and breed-matched sheep with other conditions. Furthermore, it was not found in 20 sheep experimentally infected with scrapie by sub-cutaneous inoculation, or in 22 sheep with natural scrapie in a closed breeding group, but it was present at low frequency in a production flock, closed apart from the introduction of rams. It is concluded that this lesion is strongly associated with sheep scrapie disease, but that its occurrence varies between different groups of scrapie sheep. A major source of this variation is likely to be the strain of the agent, though other factors may be involved as has been shown for cerebral amyloidosis in experimental mouse scrapie.