John, one of the most fascinating books about this whole scenario is the following link.
Deadly FeastsIt talks about how it all started; and how it goes from one animal to another = sheep to cow to deer to mink to pig to chicken, etc. Whatever kind of animal they feed the "madcow" brain to, dies of the same thing.
At the end of the book, WHO is estimating that between the years 2012 and 2015 millions of people are going to die from this.
It is well worth the time to read it, and getting a used copy for less than US $1.00 is a great price.