Getting rid of the waste parts of sheep is far more difficult then getting rid of beef, so on the other side of the pond they decided to grind the carcasses up (called offal) and feed them to the cows. This included cows as well, but the majority of it was from sheep since the slaughter percentage (amount going into food) is lower for sheep then beef.
This resulted in the Mad Cow disease.
I am part of the Scrapie Program here in the US so that this does not happen. I am required to send any animal that dies unusually in for testing, and always have, even when it has been a real pain to do so. If any Scrapie (similar to Mad Cow Disease) was ever discovered or traced back to my farm, I would be under quarantine and is something the USDA takes VERY seriously. Luckily in the USA it is not a big deal at all...very little Scrapie has been discovered because we never relied on cannibalistic feeding of livestock here.