Here is a good article that explores the problem some more. Taken from the Montreal Gazette.
Beef disaster follows years of sloppy regulation
The diagnosis of mad cow has nothing to do with detecting the specific prion that may cause the disease. There is no way of assuring that animals less than 30 months are mad cow free. They are just less likely to show symptoms and are less likely to have been fed higher risk food.
I must reiterate I strongly believe no ruminent animals should be fed any kind of meat. No animal should ever be fed canabalistacly or ever fed manure.
Not entirely sure how the diagnosis of BSE has nothing to do with the diagnosis of the prion, since the two are THE SAME THING.
How many times do people have to hear that animals less than 30 months are incredibly unlikely to contract BSE before they believe it?
Let me repeat: It takes anywhere from 3-6 years for BSE to develop in cattle. Cattle who don't show signs of the BSE prion have not been proven to be infective. The majority of cattle slaughtered in the North America are less than 30 months of age, which they haven't had time to develop the disease--even if they've contracted it--which means even if you test them, you're not going to get a positive result for BSE.
Tests have also shown that cattle that don't have the prion CANNOT SPREAD THE DISEASE.
The Harvard Centre for Risk Analysis has shown that the most effective way to stop the spread of BSE is to eliminate the infective material in the feed. This was done in 1997 by both the U.S. and Canada. The two North American BSE cases were born before the feed ban, which prohibited ruminants from being fed to ruminants. This means they most likely got the disease from contaminated feed.
Canada and the U.S. have also recently changed their rules regarding specified risk materials--the infective agents which spread BSE--further reducing the risk of BSE being spread.
also p.s. there are few cattle producers that feed their cattle animal-based proteins, at least in Alberta. Grain--and assorted vegetable-based feeds--are the rule, because Alberta has an abundance of grains/etc. with a high enough level of protein to promote growth.