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/EricTheNerd2]

There is a fundamental flaw with your assumptions: prions are not terribly infectious. Animals with prions die off pretty quickly and to catch the disease you must consume the misshapen protein directly. It isn't like Covid where someone coughs and spreads it.

And once the host dies, unless something consumes it pretty soon thereafter, the prions won't be spreading.

[u/The_Frostweaver → u/EricTheNerd2]

I thought prions could survive for a a long time?

[u/ElysiX → u/The_Frostweaver]

"A long time" is relative. Misfolded proteins are still proteins, and proteins rot.

[u/throwawaystedaccount → u/ElysiX]

But isn't there an entire ecosystem of insects, rodents and scavengers that eat carcasses?

With so many different species feasting on that prion infected body, one would assume it would spread somewhat?

EDIT: Other answers in this thread clarify this to an extent.

EDIT2: Thank you for all the answers!

[u/MsNyara → u/throwawaystedaccount]

Each animal order and clade uses different proteins, and uses them in different ways and extent. This generally means that you need the same specie of animal scavenging the dead animal fresh, and then when such a scavenger dies, it also needs to be eaten by a similar scavenger. Generally this chain does not repeat for long and the prion is dead.

It can also be transmitted by blood and fluids, but just if the protein is used in the tissues it is getting contact at. This will be extremely specific in nature and potentially wipe an animal's family or geographical limited group, or nothing happens (aside the source individual dies).

It would be a fairly common disease if it was not for the fact that such protein misfold already just happens once every a couple hundred million living beings every a dozen years or so, so it really does not have much chances to spread outside farming, among farm animals.

[u/CoffeeFox → u/MsNyara]

So, hypothetically, let's say there was a blanket ban on using slaughterhouse byproducts in cattle feed: would it follow that BSE/mad cow disease be expected to die off on its own?

[u/MsNyara → u/CoffeeFox]

Bingo. It would stop altogether. If we did not use water contaminated by poop bird to pluck off feathers then we would not need to cook poultry neither, among a few other things that we do or that happens exclusively due to industrial farming being the way it is, since our demand is higher compared than the arable land available per human, which is just a fifth part of an hectare, which is just enough for 10kg of meat per person per year, not 60kg, or 120kg like in the US.

So to produce that surplus of meat, practices like this must be done to optimize space use. Or we would reduce our demand for meat, or have 6 times less humans (worldwide), any works.

[u/LotusMoonGalaxy → u/throwawaystedaccount]

There was a paper published, I wanna say in the last 6 months, that found that cougars intestines were capable of destroying deer wasting disease prions. So in a "natural world", nature does have ways of preventing cross contamination in species. And that's just mammal to mammal predation. Insects and birds have different everything which again, would prevent cross species contamination. Part of the chronic wasting deer problem is that there isn't enough predation, enough food and overcrowding so deer are spreading it faster than prions usually spread.

[u/MATlad → u/LotusMoonGalaxy]

Looks like a few papers coming out of that research group!

https://scholar.google.com/scholar?q=cougar+intestine+chronic+wasting+disease

[u/throwtrollbait → u/ElysiX]

Nah bro. Prions are so resistant to this that they might as well be immune. They've found a few weird microbes that can break them down, kind of. They remain infective for at least several years in soil in natural conditions.

But these bastards are so terrifyingly stable that even burning them is tricky. Like, incineration needs to be north of 900C for an extended period of time. Aluminum, brass, and bronze all melt before prions stop being infectious prions.

[u/ElysiX → u/throwtrollbait]

Everything filters down one way or another, and oxygen is the death of all.

Trying to sterilise medical tools is not the same situation as a damp forest floor. Yes the soil will be contaminated, but not forever. Every time that something eats it, it will either be destroyed, made smaller, or kill what ate it and possibly multiply a bit. But the chance that that thing gets eaten by something up the food chain rather than down is small. So eventually it ends up destroyed or so deep in the soil that nothing can reach it and it eventually decomposes.

[u/throwtrollbait → u/ElysiX]

As I mentioned, in the case of CWD, prions are infectious for years in soil under natural conditions.

Sure, they will eventually decay, one way or another, but that might happen years after their host species has gone functionally extinct.

[u/KeyofE → u/The_Frostweaver]

How often do you eat things that have been long dead? And if you do and get a prion disease, how likely is it that you will be eaten? It’s a pretty big barrier to propagate.

[u/xiaorobear → u/KeyofE]

This is not the full picture- chronic wasting disease for example is a prion disease in certain types of animals that can be spread through contaminated soil and water for years. They don't have to eat the animal directly, indirectly works too. Fortunately humans haven't been affected by it.

https://www.cdc.gov/chronic-wasting/animals/index.html

[u/The_Frostweaver → u/KeyofE]

I was thinking of scavengers eating the body, or maybe insects eat the body then jump to a nearby animal and bite that animal spreading it.

There must be some sort of vector spreading it from one deer to the next

[u/Vitztlampaehecatl → u/The_Frostweaver]

I don't think an insect would spread prion diseases very well. There are only like 1 or 2 prions that affect mammals, and insects don't use the affected proteins, so any prions they eat wouldn't be able to replicate inside the insect.

[u/SquirrellyBusiness → u/EricTheNerd2]

This is not true. Cwd prions can persist for years in the environment, where they can then be eaten by other deer, contracting it. They can be shed via saliva and even urine, which deer spread amongst themselves during rutting. 

[u/Lizardcase → u/SquirrellyBusiness]

Velvet is also a transmission source.

[u/Emu1981 → u/EricTheNerd2]

Animals with prions die off pretty quickly

It depends on what proteins the prions are. vCJD (aka human mad cow disease) can lay dormant for years or even decades before it starts to snow ball into being symptomatic and the inevitable death.

to catch the disease you must consume the misshapen protein directly

Not only this but it also has to be a protein that you use for it to cause any issues for you - prions cause issues by causing the left handed version of themselves to misfold into the right handed version and the body doesn't recognise that these right handed proteins even exist. Scrapie is the sheep and goat version of BSE (aka mad cow disease) yet it is thought that the prions that cause it pose no risk to humans despite it being thought to be behind the mad cow epidemic in the UK back in the 1980s.

[u/Uturuncu → u/Emu1981]

Yes, and how long the BSE-derived prions can persist in a human body before causing the cascade failure that kills actually barred anyone who lived in the UK during the epidemic from donating blood(at least in the US) until just a couple years ago. We are, finally, at the point where medicine's sure that everyone who was going to die from that epidemic, has done so, and that there is no longer concern of us transferring those prions via blood transfusion. So many layers of horrifying, both the inevitable nervous system cascade failure that leads to death, the inability to test someone to see if they have said prions, and the fact that exposure can end up hopscotching to someone who didn't even consume the prion the traditional way via blood.

[u/3rdbasemonkey → u/EricTheNerd2]

They’re very infectious. Prions are among the most infectious agents! What you’re saying is that they’re not very transmissible.

[u/Lizardcase → u/EricTheNerd2]

In the case of CWD, there is evidence that the prions bind preferentially to some soils, allowing them to persist longterm. Since it infects grazers, this may be a major factor driving transmission. So, the carcass doesn’t have to be directly consumed to spread it.

However! There is also evidence that some felid species (mountain lions) can consume and degrade prions, limiting their spread on the landscape.

[u/RamonaLittle → u/EricTheNerd2]

It isn't like Covid where someone coughs and spreads it.

Plot twist: "Most recently, there have been some case reports indicating that the systemic immune response in COVID-19 could accelerate the clinical course of sCJD [sporadic type Creutzfeld Jakob Disease], however, a potential causal link remains unclear."

Another article: "Importantly, the SARS-CoV-2 ‘S1’ spike proteins contain both self-associating “prion-like” regions, amyloid peptide-binding and other domains that appear to play roles in pathological “seeding,” amyloidogenesis and/or spreading that supports the formation of pathogenic lesions in the brain and CNS which contribute to pro-inflammatory neurodegeneration, neural cell atrophy and/or neuronal cell death (Tavassoly et al., 2020a,b; Lukiw et al., 2022; Tetz and Tetz, 2022)."