I'm not sure of the method they use to purify the materials, but I would imagine if you feel safe eating beef or even milk, then you would really be fine with this..
NOW is a reputable company and if anything like the agent causing Mad Cow Disease may be present in their products I believe that 1) our community would have experienced the ill effects, and 2) they would immediately pull all of that inventory off the shelves and issue a warning.
So I would say you're 99% safe! But then again, what the hell is 100% safe today anyways?
My opinion -- If I had the money or needed a Bone supp, I would have no problem using it! Good luck either way man..