why the hell do prion diseases mostly hit ruminants?

...these are the questions that occupy me. But it's a good question, because these are really curious diseases.

I went digging a while back trying to figure out why the fuck prion diseases primarily hits ruminants–in fact, all the examples I’m aware of outside human (scrapie, BSE, chronic wasting disease) are ruminant diseases. That’s kind of weird, right? Deer and bovids aren’t normally in the habit of running down their fellow deer and cannibalizing them? It’s not like rabies, which mostly hits the Carnivora and which makes sense in that context: carnivores bite things more in the context of foraging. So what’s up with prions?

Turns out that because prions are so incredibly stable, when an animal that dies of a prion disease decays… well, the prion doesn’t die itself at all. They actually attach to the soil itself! Once a prion-infected carcass has decayed on the soil, depending on soil type the prion may remain present and infectious for years–up to sixteen years, according to one study. The prion can certainly stick around long enough for the soil to be pushed up by a new growing bit of grass, and ruminants tend to eat bits of soil anyway in the course of grazing. So when new animals come by the place where a TSE case died… well, that’s probably pretty lush grass, and there’s no way for them to know it’s infectious.

Prions are weird.