I read somewhere that it was animal brains amoung other organ meats that were being all ground up and fed back to fatten the cows ........which caused the Mad Cow desease!!
i dont know specifics, but seems like what you note the brains of other cows in the feed defintely was a source of spreading it, but there is some discussion on root causes. The other issue was for when that disease jumps from bovine to humans i.e. cruetzfeldt jakob disease.