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I think the question is based on a misapprehension. Kuru and Mad Cow Disease are not caused by cannibalism. This seems to be quite a common belief and is a misunderstanding. The cannibalised individual has to be infected in the first place. Which is relatively rare. What can be said, perhaps, is that cannibalism is possibly the only way the infection can be transmitted from individual to individual as consumption of infected tissues is pretty much the only way it can pass from person to person. However, kuru is not a fundamental risk of cannibalism in and of itself. If there isn't kuru in the population already, cannibalism itself isn't going to cause it to emerge.

I think the myth came from a group of people who engaged in ritualistic cannibalism of their dead. They had kuru in the population already from eating infected ungulates and people were dying of kuru and then being eaten due to the traditional practices, which just passed the problem on. But the cannibalism wasn't the source of the condition, just the means of transfer.

Are prions completely absent in healthy individuals? If they're present at low levels, then cannibalism would allow bioaccumulation of them.

Cannibalism is just a form of transmission, prions self propagate by causing misfolding of healthy PrPs which accumulate into amyloids and cause disease.

Kuru is absolutely caused by cannibalism. It started off because sporadic CJD happens in about 1/1m people and because of ritualistic eating of the brain and spinal cord it became transmissible variant CJD.

About 90% of CJD is sporadic.

Prions only affect mammals. Cows also don't partake in cannibalism, we grind up cattle into protein powder and mix it into cow feed as a supplement. All mammals can get sporadic cases so there are laws about feeding animals to their same species because while rare, there's a risk.

I think we're talking past each other because some basic assumptions are left unstated. I think these are:

• Healthy individuals have zero prions
• The body cannot defend against any level of prions. Once they're in, they're bound to replicate until they reach fatal levels.

If those are true, then I agree that transmission is all that matters. If they aren't true, then the prion level would matter, and bioaccumulation (or really biomagnification) could come into the picture. I read something recently about defense mechanisms called "chaperones" that restore prions to the correct shape. It would also be odd for cells to have no defense against a problem that's surely been around for billions of years. So that's the background for my question.

Why would prions only affect mamnals? Something about high body temperatures?

We don't really know, a lot of mammals are also highly resistant to prions so it's thought it's an issue of susceptibility.

Did some googling, and it looks like only mammals have the specific protein sequence needed: https://www.cell.com/trends/biochemical-sciences/fulltext/S0968-0004(07)00060-6

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This is an interesting question! I knew a little bit about Transmissible Spongiform Encephalopathies from being a vet tech (scrapie in sheep & goats, CWD in deer, TME in mink and ferrets, and of course BSE/vCJD in cattle and humans— especially because I lived in Germany and the UK for big chunks of the 80’s and 90’s and so was banned from donating blood until very recently just in case I was a carrier myself!) but because I don’t work in a farm setting it’s not something I really need to keep up with so my knowledge is pretty shallow. This question got me googling and I found this interesting article:

Animals Resistant to Prion Disease: Intrinsic vs. Extrinsic Factors

https://www.frontiersin.org/articles/10.3389/fncel.2020.00254/full#h3

It’s super long and gets very technical at times so I will freely admit that I skimmed big chunks of it, but my “Today I Learned” takeaway is: horses, dogs, and rabbits are very highly resistant to prion disease, as in not developing disease even when injected directly into their brains, let alone when being fed contaminated material. I knew so many different species had their variants (though I also knew not all diseases seem to be able to cross to all species), and that we have evidence of cats and zoo animals becoming ill during the BSE outbreak, so I think in the back of my mind I had an assumption that all mammals would be at risk.

Dogs being resistant makes sense to me— they’re highly generalized scavengers whose whole schtick is being able to thrive on whatever garbage they find lying around, lol. But horses and rabbits are both infamous for being so medically fragile! I guess now I know what superpower they traded away everything else to gain… (/facetious in case that wasn’t clear!)

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>So I'm aware that, most notably, humans and cows suffer quite a debilitating condition if they partake in cannibalism, as Kuru and mad cow disease from prion infection.

I think it's important to understand exactly how Kuru happened.

Specifically, it happened in a group of people who engaged in ritual cannibalism of members of their own group who had died. Someone in that group had a spontaneous case of CVJD. They died, presumably of the disease, and were eaten by group members...specifically by women and children. Men occasionally at meat, but never organ and brain tissue. And nearly all cases of Kuru were in women or children. People who ate infected tissue and became infected themselves died, were eaten, and passed the disease along.

So it wasn't just cannibalism that allowed kuru to spread, it was the specific circumstances of cannibalism. First of all, a rare case of CJVD had occurred in the population to kick the whole tragedy off. The people who were eaten had died of natural causes, so people who had advanced stages of kuru were themselves eaten and able to infect others. The people being consumed were group members, so the disease could form a continuous chain.

In other circumstances, Kuru would not have been as likely to propagate. For example, if this was cannibalism where enemies killed or captured in battle were eaten, there'd be less chance for the disease to spread. A person killed in combat would be less likely to be suffering from the disease in the first place. If the disease did spread from the first individual to those eating them, it would be less likely to get a second chance to spread (Because those warriors would have to themselves be taken by enemies that also practiced cannibalism and eaten). And if the tissue being eaten was flesh and not brain and nerve tissue, spread would also be less likely.

This plays in to the spread of the disease in other mammals as well. It's exactly what happened in cows, for example. Older animals that had the disease were ground up and their organ meat was fed to many other cattle, who were themselves ground up and refed when they died. That made for a continuous chain of transmission. In contrast, using your example of cannibalism in the wild of young animals, they are less likely to have a spontaneous prion disease in the first place. And even if it does, the chain of infection would not persist because the adult that ate the offspring would be unlikely to themselves be cannibalized, especially by more than one individual (which is required for the disease to actually spread).

TLDR

To actually spread prion diseases by cannibalism, it's not enough for cannibalism to occur. The individuals which contract the disease by cannibalism must themselves be cannibalized, and by multiple individuals, and so on in a chain of transmission.

That said, cannibalism does also expose you to other, more ordinary diseases. It's also often risky, since adults of the same species are usually dangerous prey. And you also run the risk of harming your fitness by eating a relative.

Thank you all for the responses. I believe I misspoke when I brought up Kuru and MCD, as I was just trying to find a more... tangible disease than just "the jitters" that I'm familiar with. Most of the videos I've watched on the subject seemed to imply that cannibalism in any capacity will eventually cause some sort of extremely debilitating disease/condition, and was not aware how specific those two examples actually were.

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That said, I am still quite confused on the actual chain of events when cannibalism occurs (without prior infection like Kuru and MCD), for species that are affected negatively.

> Are prions completely absent in healthy individuals?

Yes, otherwise everyone would die from them pretty fast. Despite similarities, prions aren't viruses or bacteria, they can't lie dormant

Very interesting paper! Thanks for the link.

As far as we can tell, you can't fix a misshapen PrP (aka prion). Those "prions" in the paper you are referencing are yeast proteins that have distinct conformations that can propagate like human prions. All humans have tons of PrP but it's rare for one to become a prion. However, once one does it can start a chain reaction.

We have tons of PrP^wt of which it is very rare for one to become PrP^sc . If PrP^sc forms it's estimated to take around a decade to kill, at least in humans.

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>Those "prions" in the paper you are referencing are yeast proteins that have distinct conformations that can propagate like human prions.

Thanks, I missed that they were only roughly analogous to prions. That's an important distinction.

And to address my other point myself, it looks like only a few groups of mammals are vulnerable to them, so evolution hasn't had billions of years to work on this, more like tens of millions, I think.