Prions

Prions are infectious proteinaceous particles or, more simply, proteins that lack nucleic acid. They were discovered by Stanley Prusiner, who received the Nobel Prize in medicine in 1997 for his work on them. Prions are biologically unique, existing somewhere in the border zone between living things and nonliving matter. Although they show none of the characteristics associated with life, such as the need to metabolize and the capacity to reproduce, they are in some manner capable of replication in the body of a human or certain other mammals.

Prions apparently gain entry to the body mainly by ingestion, or else in contaminated human growth hormone, or, possibly, in contaminated blood or blood products. They selectively attack the central nervous system, causing a relentless and progressive destruction of neural tissue, leaving in its place microscopic vesicular globules. The pathological name for this is spongiform encephalopathy. Conditions in this category, all of them invariably fatal, are all transmissible. They include kuru, Creutzfeldt-Jakob disease, scrapie (a degenerative neural disease of sheep), bovine spongiform encephalopathy (mad cow disease), and variant Creutzfeldt-Jakob disease, which appears to be acquired by ingesting beef contaminated by the prions that cause mad cow disease.

As of September 2000, it remains unknown what other mammalian species are vulnerable to prions; in research laboratories they have been shown to infect rodents and primates. It is possible that all domestic farm animals are at risk, though so far only sheep, beef and dairy cattle, and wild ungulates such as deer and elk have been confirmed as vulnerable. There is no vaccine or serum to protect against infection, and no agent that can arrest or retard the progress of the spongiform degeneration once it begins