A single strain, from where?:

BSE is a single source strain, as indicated from no variation in mouse-incubation tests, perhaps it came from the brain of a single cow. This is unlike the several pathotypes of Scrapie with varying mouse-incubation periods.

BSE may have been of spontaneous bovine origin or of sheep origin and its ability to infect cattle by mouth enabled it to spread. This single strain became widely distributed as bovine-derived protein by the large scale rendering plants.

Host susceptibility.

Methionine at PrP codon 129 is important in human susceptibility to CJD. Cattle, but not primates, are monomorphic for methionine at 129 and this may explain their universal susceptibility to BSE. Thus a single strain, BSE, is able to spread by ingestion in a species with uniform susceptibility, the cow.