Is it a bacteria? Is it a virus? Is it both? Is it neither? Is it a fungus? An entirely different life form?
I once saw a tv documentary (possibly on PBS) that said it was merely a poisonous protein (which somehow replicates itself in a host). But even this tv documentary pointed out their were many critics of this theory. So what exactly is it?
And these next questions might deserve another thread, but while we’re at it, how do kill it (on inanimate objects and in people and other animals)?
It is a prion, a misfolded protein. Using most conventional definitions of the word “life,” it is not alive. It does not self-replicate, but it does convert proteins not like it to proteins like it, so it is infectious.
Stan Prusiner came up with the prion hypothesis and won the 1997 Nobel for it. It was a relatively crazy idea with many critics, but evidence has borne it out (so far).
We all have normal copies of PrP© (Cellular Prion Protein) that serves a useful function in neurons. Recent literature suggests that it interacts with stress proteins in the cell; other literature shows that it binds copper and may regulate its concentration at the synapse. The protein is evolutionarily conserved – cows and sheep have it as well.
Sometimes, the protein can become misfolded. This can be because of an inherited mutation, it can be sporadic, or it can be because of contact with another misfolded PrP. Think of a crystallization type reaction that spreads throughout the nervous system. A useful analogy may be Kurt Vonnegut’s “ice 9” from Cat’s Cradle – it was a new crystal form of water that had a melting point at around 50 degrees centigrade. It was a Doomsday Weapon – a single crystal dropped in water would instantaneously change all of the water into ice-9 at regular temperatures. So drop it in the ocean and all water turns into ice-9.
When PrP misfolds, the misfolding spreads. The new misfolded protein aggregates, or forms unprocessable chunks of protein, which eventually lead to neuronal dysfunction and death. This causes a fast-progressing dementia. In sheep, it is called scrapie; in cows BSE (bovine spongiform encephalopathy); in humans CJD (Creutzfeld Jacob Disease).
It was shown that cows eating feed enriched with sheep neural tissue were contracting BSE. Some of the sheep had scrapie. This led to worries – would eating the brain (or neural tissue) of an infected cow or sheep lead to CJD in humans? The epidemiology is scarce, because the disease is rare. A few cases of vCJD (variant CJD) thought to be caused by eating contaminated cow neural tissue were reported in England. I don’t know if these were 100% linked to eating infected cows – they could have been the rare sporadic or inherited cases – but the suspicion was high enough to take such big measures.
Nice post, edwino. One final comment: because BSE prions aren’t actually alive, they can’t really be killed. BSE prions have been shown to be remarkably unaffected by high temperatures, ultraviolet light, ionizing radiation, etc.
Art
Great answer edwino. Here’s a link to a disease called Kuru caused by a prion that has been around for quite some time (early 1900s). The prion hypothesis was put forward by the Nobel laureate you mentioned (Prusiner) in 1982. According to the link above:
The clinical picture really isn’t pretty. One of my attendings had made a CJD diagnosis on a patient three years before I was on his service. The patient was already dead. There are very few dementias that can progress that fast.
Fatal Familial Insomnia sounds like the worst. You go from normal to total permanent insomnia in 9 months or so; death follows within another 9 months.