I mean isn't the problem that we weren't eating the brains but feeding them back to the livestock allowing prion diseases to spread horizontally and vertically.

I mean isn't the problem that we weren't eating the brains but feeding them back to the livestock allowing prion diseases to spread horizontally and vertically. If a prion infected livestock is processed and consumed completely by us, we'd still probably have some infections, but far less than the entire population of a country eating infected meat that has been eating prion infected meat all the time.