Actually, USA does have mad cow disease, but their farmers simply refer to it as "stumbling disease" and "shoot, shovel and shut up". As regulation of carcass disposal becomes more stringent, more and more cases will show up. The US is way behind as far as regulation of their food industry is concerned. The real reason they claim not to have any mad cows is becasue they don't look to hard for it.
BSA, like Scrapies in sheep, can develop spontaneously but very rarely, however it can be passed on in feed when ground up animals are used to boost protein in feed. Canada has to outlaw any animal parts being used in feed period. We still allow it for chicken feed. and yes, feed does get mixed up on farms, or in trucks.
Some reporter could make a career blowing the lid off this process in the US, providing he lives long enough for the story to air, and his network/paper has the jounalistic balls to run the story.
rickoshadows