A few infectious diseases are caused by prions. In 1982,
American neurobiologist Stanley Prusiner proposed that
infectious proteins caused a neurological disease in sheep
called scrapie. The infectivity of scrapie-infected brain tissue is
reduced by treatment with proteases  but not by treatment with
radiation, suggesting that the infectious agent is pure protein.
Prusiner coined the name prion for proteinaceous infectious
             Nine animal diseases now fall into this category, including the
"mad cow disease" that emerged in cattle in Great Britain in
1987. All nine are neurological diseases called spongiform
encephalopathies because large vacuoles develop in the brain
 The human diseases are kuru,
Creu tzfeldt- Jakob disease (ClD), Gerstmann-St riiussler-
Scheinker syndrome, and fatal familial insomnia.  These diseases run in families,
which indicates a possible genetic cause. However, they cannot
be purely inherited, because mad cow disease arose from
feeding scrapie-infected sheep meat to cattle, and the new
(bovine) variant  was transmitted to humans who ate under cooked
beef from infected cattle.
Additionally, CJD has been transmitted with transplanted nerve
tissue and contaminated  surgical instruments.

These diseases are caused by the conversion of a normal host
glycoprotein called Prpc (for cellular prion protein) into an infectious
fo rm called PrpSc (for scrapie protein). The gene for Prpc is
located on chromosome 20 in humans. Recent evidence suggests
that Prpc is involved in regulating cell death. One hypothesis for how an infectious agent  that lacks any nucleic acid can reproduce. The actual cause of cell damage is not known. Fragments of
PrpSc molecules accumulate in the brain, forming plaques; these
plaques are used for postmortem diagnosis, but they do not appear to be cause of cell damage. 
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Cited By Kamal Singh Khadka
Msc Microbiology, TU


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