Just wanted to add two things: 1) the so-called mad cows disease is also caused by prions - that's why it is so scary, 2) cannibalism, or any other loop in the food chain, causes retention and amplification of such diseases in a population.
> In Iceland in 1978, a program was implemented to eradicate scrapie, and affected flocks were culled, premises were disinfected, and sheep houses were burnt; after two to three years, the premises were restocked with lambs from scrapie-free areas. Between 1978 and 2004, scrapie recurred on 33 farms. Nine recurrences occurred 14–21 years after culling as a result of persistent environmental contamination with PrPSc.
A human variant transmissible through cannibalism is Kuru (against which the locals have developed some resistance):
> In 2009, researchers at the Medical Research Council discovered a naturally occurring variant of a prion protein in a population from Papua New Guinea that confers strong resistance to kuru. [...] G127 polymorphism is the result of a missense mutation, and is highly geographically restricted to regions where the kuru epidemic was the most widespread. Researchers believe that the PrnP variant occurred very recently, estimating that the most recent common ancestor lived 10 generations ago.
There's a quite decent documentary on the life and work of Mike Alpers put together with a lot of early family footage if you've an interest in the Kuru story.
Technically you could refold the proteins with, say, a vortex fluid device. But that would require... liquefying the proteins... and then re-folding them mechanically. :-/
Squirrel brains, cow brains, human brains ... mammal brains in general.
More interestingly it's rare - you have to eat infected brains that have a variation that side steps not one but two prion defence mechanisms that most humans have ... which suggests that almost all humans have ancestors that engaged in enough cannabalism to develop layered defences against brain transmitted diseases.
Why would we jump to cannibalism if it is merely sufficient to eat a bunch of other mammal brains (which I vaguely remember reading are a "delicacy", though I feel like I hear that about enough extremely dangerous foods that I consider it an anti-endorsement at this point)?
As a general rule of thumb, diseases varients that evolved within mammal type X proliferate the most within mammal type X populations - the varient can jump to mammal type Y but tends to be a relatively rarer occurrence and tends to go with a bit of further evolutionary variation.
FIV is not the same as HIV (Feline, Human, but they're similiar) and so to with prion diseases.
Mad Cow disease came about from feeding large quantities of ground up cow offal from abattoirs mixed up with grains to make 'tasty' feed pellets .. eventually a cow prion disease 'perfected' itself and went on to occassionally(?) affect humans - more common in cows than humans, etc.
With human cannabilism, we know about Kuru from the Fore people in PNG who, like many other groups in the highlands, practiced cannabilism as a mortuary ritual, a funeral practice to retain the essence of someone respected. Kuru was a specific human to human variation that evolved to become more common and overcome the defences against human -> human prion defence I mentioned earlier.
We have other reasons to suspect cannibalism was common elsewhere .. such as the documented European practice of medicinal cannibalism and the fashion of consumming bitumised mummies.
But, ahh, yeah - these days genetics is the thing, markers for defences against human prion diseases are a thing talked about in a group of papers post Kuru.
They're high in fat even in otherwise lean animals so would have been a totally normal thing to eat in much of the world for most of history. In some places & seasons possibly the only ready source of saturated fats, so probably a delicacy for that reason.
Prion diseases https://www.cdc.gov/prions/index.html lurk within brains and nerve stems, with cows it became a big thing when the industry started making mega tonnes of offal waste mixed with grain feed pellets .. so all cows were being feed ground up other cows including brains.
Like most other diseases species -> same species is an easier vector than species -> different species.