Actually, scrapie itself is conjectured to trigger BSE.

"Cattle are believed to have been infected by being fed meat-and-bone meal that contained either the remains of cattle who spontaneously developed the disease or scrapie-infected sheep products...

"The British Government enquiry took the view that the cause was not scrapie, as had originally been postulated, but was some event in the 1970s that could not be identified."

https://en.wikipedia.org/wiki/Bovine_spongiform_encephalopat...