If the scientists are correct and all it takes is a couple of grams of pre-feedban contaminated feed, then I would expect that the odd cow would be coming down with bse for years to come as the stuff could lurk in feed bins for years into the future.
The other way is that the offspring are born with it and every herd should be either tested with a bse blood test when avaiable or a bse urine test which works now.