Scrapie (the name for it in sheep) has been around for over 250yrs cattle beca\me infected through the demand for cheap protein rich feeds and dead animal by products are cheaper than natural vegetable proteins, so cheaper feed!
I think that the guess that artificial feeding of deer is where it originates from is right. what a lot of people dont know is there are variants of this in many animals including North American squirrels. I read an article by some US medical journal where there were more cases of CJD in people who regularly ate squirrel than those who didn't ( dont eat squirrel now).
More information here if your interested :-
http://www.aphis.usda.gov/lpa/pubs/f...ahscrapie.html