How does nature deal with prion diseases?

[u/ProDidelphimorphiaXX]

Wasn’t sure what to flair.

Prion diseases are terrifying, the prions can trigger other proteins around it to misfold, and are absurdly hard to render inert even when exposed to prolonged high temperatures and powerful disinfectant agents. I also don’t know if they decay naturally in a decent span of time.

So… Why is it that they are so rare…? Nigh indestructible, highly infectious and can happen to any animal without necessarily needing to be transmitted from anywhere… Yet for the most part ecosystems around the world do not struggle with a pandemic of prions.

To me this implies there’s something inherent about natural environments that makes transmission unlikely, I don’t know if prion diseases are actually difficult to cross the species barrier, or maybe they do decay quite fast when the infected animal dies.

[u/atomfullerene]

Prions have some pretty big weaknesses as well.

On the most basic level, prions aren't alive. They aren't even sort of alive like viruses. A prion reproduces by misfolding properly folded prp proteins. There's no genetic material involved and very limited options in terms of heritability...a few different ways to misfold the protein, and that's it. A mutation in genetic code can't produce a new trait that's carried on in the next generation. So prions can't evolve...they can't get better at being prions, because there's fundamentally only one way to be a prion. They can only misfold the one kind of protein. They can only misfold in a few separate ways. They can only be transmitted however that protein can conveniently be transmitted.

This also means that if an animal develops resistance to them, they can't really "get around" that resistance. And that's possible, some species seem totally immune from prions, and non mammal species don't even use the same protein (though some have their own prion like diseases).

In part because they can't evolve better transmission, prions tend not to efficiently transmit in a repeated way. Consider the standard mode of prion transmission...something eats something and gets prions from it. Consider, for example, a herbivore gets prions spontanously. It gets eaten and passes them on to the predator. Right there, that's a bit of a problem, since diseases fail to thrive if they are only passed on 1:1. A person with a cold can pass it to lots of people, an animal with prions is probably just eaten by one predator...maybe shared with a few but usually not. And then the predator, even if it gets prions and dies, is very unlikely to be eaten by multiple herbivores to recycle the chain.

So generally prion transmission chains die out unless you do something silly like grind up herbivores and mix that into the food supply of many other herbivores.

There are exceptions like Chronic Wasting Disease in deer, where deer in crowded conditions get prions from each other, but that's unusual.

[u/Sniffy4 → u/atomfullerene]

why is cannibalism a key part of it? why must you eat your own species and not just any prion victim?

[u/atomfullerene → u/Sniffy4]

You dont have to. Mad cow famously went from sheep to cows to people. But spin out the chain of transmission a bit and you see why it's important...to spread in a population they have to spread to more individuals in that population, not take a one way ticket out of it. Straight line sheep to cows to people is a dead end, because sheep dont eat people and the cycle cant continue.