Where does mad cow disease come from? THEY say it comes from eating contaminated feed. A cattle farmer named Mark Purdy in England has come up with an alternate theory that seems to bear more fruit than the contaminated feed theory. Here are a series of articles that dispute the feed theory. In this article from Alberta Beef Magazine it states that government scientists have been unable to pass on BSE thru contaminated feed.
Alberta Beef Magazine - BeefNews.com

The Ecologist (interesting how Icelandic farmers eat their scrapie infected sheep, brains and all and have not developed vCJD)
Ad for Mark Purdy's video(a connection between organophosphate insecticide and BSE in Britain?)
Mark Purdey's website