The unusual properties of CH1641, a sheep-passaged isolate of scrapie.

An isolate of scrapie designated CH1641 was identified from a natural case of scrapie in a Cheviot sheep by passage in sheep and goats. It has not been possible to transmit scrapie to mice from this source. The Sip gene which controls the incubation periods of experimental scrapie in Cheviot sheep has two alleles; sA which shortens and pA which lengthens the incubation periods of most strains of scrapie after the first experimental injection in sheep (the A group of strains). The CH1641 isolate differs from them in that the alleles of Sip act in the opposite way, with incubation being shorter in the pA homozygotes. There is some evidence that one or more genes, in addition to Sip, may be implicated in the control of scrapie incubation in sheep and the possibility of a carrier infection with CH1641 is also discussed.