I think prions are a bigger problem in crowded single-species environment than in comparatively sparse mixed-species pre-industrial world.

In particular when you feed cows proteins made from dead cows it's making it more likely to create prion problems than when wild animals live in small herds in forests (and the ones that are sick are killed by predators or starvation before they can infect others more than likely).

Feeding the output back into the input definitely makes it a bigger problem, however Chronic Wasting Disease in Elk and Dear in North America is becoming a huge problem and is a prion disease, and they’re living wild.

So it doesn’t require it, it just increases the odds (and they don’t eat each other to spread it - it’s highly infectious and demonstratively infects if they share browsing/grazing land).

It doesn’t appear to spread between them and humans, but it definitely makes me nervous.

The advent of agriculture and animal husbandry is generally thought to be much more recent than your comment makes it sound. More like ten thousand years ago.

The slight problem there is, prions probably did end a bunch of tribes which were sufficiently local - and then left the area alone long enough that the prion contamination in the environment broke down.

Today we have international animal shipping and trade, and global distribution. Prions are able to spread from exposure to shared pastures. A couple years of contamination used to be no big deal - today, if missed, it can spread all through the food supply.

I always wonder how much more truth there is to haunted houses and forbidden lands than we think. Just superstition? Maybe, but maybe not.

It is hypothesised that Kuru (and vCJD) in some individuals (due to genetics) might take 40-50 years to develop if introduced through intestine. Well after the individual had a chance to reproduce.

We're actually having a worse situation re/ animals than our ancestors. We nowadays cram farm animals in stables so tightly that without antibiotics and medical treatment the animals would almost immediately perish. Our farms and our animal "care" practices have been the breeding ground for all recent global pandemics (swine flu, bird flu, SARS/MERS/SARS2). Not to mention the conditions that farm and slaughterhouse workers have to work and live in (as evidenced by massive COVID outbreaks).

The chance of any infectious agent "jumping the species" due to cramped conditions or because of exposure of workers to contaminated animal parts in unclean slaughterhouses is immense - the current SARS-CoV-2 pandemic in my opinion is a warning signal of something much worse to come, and no matter what kind of agent it will be, we're pretty much screwed. Half the population or more of the world doesn't believe in vaccination, a quarter or more don't have access to vaccines or any kind of modern medicine at all, we are already running out on reserve antibiotics...