Wiki reminds us that the incurable brain disease, Creutzfeldt-Jakob Disease (CJD) is sometimes incorrectly called mad cow disease. It does belong to the class of spongiform encephalopathy that as the disease progresses, creates vacuoles in the brain, hence the designation spongiform. However, this form apparently self-develops without a transmitted infection.
The CDC advises the occurrence rate is about 3.4 per million, with the United States experiencing about 300 cases a year. The median age of death is 68 years with a 5 month disease progression. The Variant Creutzfeldt-Jakob Disease, vCJD, is a transmissible disease believed to be passed by prions. With vCJD, the median age of death is 28 years, and it has a 14 month period of illness. The incubation period though, is measured in years.
Composed almost completely of a sialoglycoprotein called PrP 27-30, a prion has no cell nucleus. The precursor of PrP 27-30 is a healthy protein, PrP 33-35. In a process not well understood, this folded version of the normal protein is able to convert other healthy proteins into its folded form, perhaps a primitive form of reproduction. These proteins then accumulate into a mass similar to starch that eventually kills the cell.
Because a prion is essentially a protein, it is difficult to disable it other than physically destroying it. Hence heat, radiation, etc., are ineffective at preventing the transmissible disease which normally transmits through ingestion of food containing the prion. There is strong evidence that vCJD is a cross-species of the variation that infects cattle, Bovine spongiform encephalopathy (BSE).
BSE itself was first detected in 1986, possibly originating from cattle being fed meat-and-bone meal containing scrapie-infected sheep products. (Scrapie is prion disease of sheep and goats.) Although it is a brain disease, the animals apparently experience a burning sensation in their skin, and they attempt to scrape the fleeces off against fences and trees. Hence, the name, scrapie.
Prion Protein in Cell Culture explains that Norwegian scientist, Christoffer Lund has studied scrapie and genetically modified its PrP with a green fluorescent marker. This permits the observation of how the 'bad' PrP turns 'good' PrP into 'bad'.
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