I’d just like to point out that BSE is not an infectious disease spread by a foreign virus or bacterium. It is a spontaneous random refolding of a normal brain protein in cattle (and humans). The thing that makes it special is that the refolded form of the protein, called a ‘prion’, can catalyze the refolding of other copies of the prion.
It’s possible, perhaps even probable, that you have at least one of these prions in your own brain - like every human or higher mammal that ever lived. A single protein in a whole brain can’t catalyze much, or do much damage, and eventually degrades, like any protein. However, through time, bad luck or predisposition, aome unfortunates accumulate hundreds or thousands of prions-- enough to persist long enough for the process to take off exponentially. Over months or years, they build up million or billions of prions, catalyzing and doing damage at an ever accelerating rate.
In short: given the hundreds of millions of cows in the US, there will ALWAYS be a few random “spontaneous” cases no matter how tightly we control contagious spread, just as we’ve always had a few spontaneous cases each year of the human equivalent: spontaneous Kreutzfeld-Jacob disease.
The normal PrPP proteins in cows, sheep, humans, etc. are not identical, and may not be identical between individuals. There may also several altenative foldings of each subtype, with different catalytiic efficiencies and varying abilities to catalyze the refolding of other subtypes within or between species. This contributes to the spread between species, and the appearance of ‘outbreaks’. It also sometimes allows us to do a limited degree of tracking of to tell if an outbreak is independent or linked to others.
I’m not saying that transmitted BSE, scrapie, vCJD etc. aren’t serious concerns-- but a few US cases are inevitable and normal. Barring a re-engineering of the affected species, It’d be impossible for them to ever stop. In fact, if our government reported zero BSE cases for too long (I don’t have the exact spontaneous incidence figures handy), I’d strongly suspect either a systematic coverup or very sloppy monitoring. BSE has been around longer than humans have