MadSci Network: Medicine

Re: Why does Mad Cow Disease have such a long incubation period ?

Date: Fri Oct 27 00:20:46 2000
Posted By: Michel Ouellet, Grad student in Microbiology / Immunology
Area of science: Medicine
ID: 967059282.Me

Hey Matthew!

Diseases like the Mad Cow Disease, Kuru and Kreutzfeld-Jacob seem to be 
caused by a prion.  A prion is different from a virus in that it is only a 
protein.  While viruses have a genetic code, infect a cell and replicate 
themselves, prions are ONLY made of proteins, do not have a genetic 
material and does not need a cell to do their damage.  It is very resistant 
to known disinfectants and it is not yet known how to treat them.

How do prions do their damage is pretty impressive (well, I think).  In the 
case of the mad cow disease, the prion is a protein that is present in the 
normal brain but is mutated and has a different conformation (shape) than 
the normal one.  Let's call the normal protein Pc, and the mutated protein 
PrPc.  The PrPc protein seems to come from infected sheeps that were killed 
for their meat but their carcass (bones, brain, and any parts that are not 
selled to custommers) were crushed and powdered and added to the diet of 
cows as protein supplements.  Once it gets in a cow, the PrPc protein is 
taken up to the brain where it begins to change the normal protein Pc into 
a replicate of the mutated protein PrPc.  This action is very slow and it 
takes many years before the brain becomes encumbered with too much of these 
useless mutated proteins.  Even then, as the brain is a pretty powerful 
organ, it adapts to this newcomer and the symptoms are almost absent.  It's 
only after many, many years, when most of the space that was once used by 
the brain has been replaced by PrPc proteins that the symptoms begin.  The 
more the cows eat infected food, the faster the apparition of symptoms.

This is the most convincing hypotheses on the subject, and there is a lot 
of evidences that seems to confirm it.  How are prions transmitted?  well, 
contact with contaminated blood, food, and organs seems to be a risk 
factor.  There could be other factors for the development of these diseases 
 (so that even if you are contaminated with prions, you could never develop 
a fatal illness) like genetics, immunity, stress and environnement but 
since the disease is so slow, most people will die of other causes, having 
never experienced any symptoms.

These kind of illnesses are insidious, debilitating and frustrating and it 
is hoped that medical science will one day be able to cure or at least 
prevent them.

Thank you for your question, 

Michel Ouellet

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