During my career as a neurologist so far, including my med school and residency training, which covers the past 16 years altogether, I’ve seen a total of 5 patients with confirmed CJD.
It is pretty rare, but also pretty horrible. In none of the cases, which ranged in age from 45 to 65, could we identify a likely cause. A couple of the patients had a history of remote blood transfusions and surgery. I think all of them had had dental work at some time. None, so far as I recall had tissue transplants or were exposed to Human Growth Hormone (a known transmitter of prions in the old days when hGH was derived from cadaver pituitaries…) One of the patients was a cattle rancher from Northern California, which didn’t ring any bells with me at the time (before this paranoia about MCD).
How do prions cause disease? That’s a damn good question. There is no DNA or RNA in prions, so they are not viruses or bacteria, or indeed, from the point of view of classical biology, living things. They are proteinaceous particles that have the interesting ability to “infect” organisms, in which they cause disease and “reproduce.” Because of the latter characteristic, they cannot be viewed simply as “toxins.” Toxins may cause damage, but they are not living and do not “reproduce.”
One theory is that these prions incorporate themselves into the host genome, or there is some latent gene in the host that they “trigger” to produce more prions. According to this theory, the “Prion Protein Gene” (PrPG) behaves somewhat like an “oncogene.”
The explanation that I find most compelling is that the prion acts like some sort of “seed crystal.” It gains access to the organism and reproduces by simple thermodynamic molecular interactions with host molecules, in the same basic way that a seed crystal can percipitate a condensation chain reaction in a supersaturated solution. That, at least, is they way I view the process, and although I have not read about this idea elsewhere, it seems to make sense, and I’m sure others must have considered it.
The good news is that there seems to be a good biochemical test for BSE in cattle, that gives a result in 24 hours. See Nature 2001; 409:476-478. The conventional mouse bioassay took 1-2 YEARS to give a result about a questionable cow…