blood meal is a high quality feed better used for pigs than bovines. Cows you can virtually give urine as a nitrogen source, since the bacteria in the rumen turn low quality protein into mid-quality protein (and also high quality protein into mid-quality protein, which is why feeding them blood meal is a waste of a resource.)
That said, there really hasn't been any funding for investigation of alternative causes of mad cow disease. The timing of breakouts coincided with a number of things, including excessive widespread organophosphate pesticide application to cows in order to control botfly. I'm not saying that's the cause, just that there is circumstantial evidence for it and no one has bothered to investigate. Yes I've read the studies by Pruisner, and no he hasn't *proven* that it is caused by a protein, he's just established a strong body of evidence supporting that assertion. One which has yet to be challenged by alternative hypothesis, as is normally done with good science.