A study published in PLoS Pathogens last year, BSE Case Associated with Prion Protein Gene Mutation, elucidates the origin of BSE in cattle. (PLoS stands for 'Public Library of Science' and their Journals are all 'open access' - this reverses the traditional fee structure by charging authors for publication but making the research available to everyone).
The origin of the original case(s) of BSE still remains an enigma. Hypotheses include (i) sheep- or goat-derived scrapie-infected tissues included in meat and bone meal fed to cattle, (ii) a previously undetected sporadic or genetic bovine TSE contaminating cattle feed or (iii) origination from a human TSE through animal feed contaminated with human remains.
The paper describes a novel mutation in a prion protein gene in a cow that is similar to the mutation in the human genome that causes the genetic form of CJD. This suggests that the BSE epidemic could have begun by a random genetic mutation. If that is the case then there are implications for the US beef industry. Concentrating on keeping out infected cows isn't a bad idea but should not be the only strategy since spontaneous cases of BSE will appear. The important point is to prevent the multiplication via cow cannibalism of those spontaneous cases that occur.