Design of the different studies.

Age in months for naturally exposed sheep of study 1; months post-infection for all other studies. Sheep pool, pool of 17 scrapie brains (VRQ/VRQ = 6, VRQ/ARQ = 6, ARQ/ARQ = 4, VRQ/ARR = 1).Murine strains, sheep dosed with 22 A or 87 V killed at 35 mpi; sheep dosed with ME7 or 79 A killed at 47 mpi. Dose in grams: n/a, not applicable;*, four sheep were boosted with 2 g of inoculum by the subcutaneous route (months post-inoculation correspond to those of original challenge).