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Carnivores and cold virus

Patrick O'Neil patrick at corona
Wed Mar 22 13:34:14 EST 1995



On Wed, 22 Mar 1995, Keith Sturts wrote:

>  
> What is the physiological, metabolic, or other mechanism operating that
> prohibits the common cold virus from effecting dogs, for instance.
>  

All viruses operate on the same principle on the most basic level:  They 
have specific proteins projecting from either their membrane  or from 
their protein capsid surrounding their genetic material.  These proteins 
are highly specific for particular proteins on the surface of host cells 
for which they have evolved.  A human rhinovirus (common cold) is 
specific to proteins that exist on the surface of cells lining the mucus 
membranes and these cell proteins are different from those on a dog, or a 
cat, or a bird, etc.  They often are homologous to varying extents, 
generally indicating the time since common ancestors diverged in 
evolution, but are different enough for very specific viral 
interactions.  
  The fact that proteins on cell surfaces are different between species 
is the reason that interspecific transplants are even more problematic 
than human - human transplants.  Even though the overall structure of 
such things as Major Histocompatibility Complex (MHC) proteins is 
evolutionarily conserved across species, they are different enough to 
allow a particular body to tell "self" from "non-self."  In some cases, 
as with humans and various types of simian, we are closely related 
enough that some viruses can "jump" species.  Generally, this is because 
the parts of cellular proteins used by a virus to enter a cell in species 
A is conserved and very much like the same parts in species B.  Once in, 
the cellular machinery and physiology must also be compatible enough to 
allow viral propagation.
  With the common cold, the receptors used by the virus (or the specific 
parts of the  receptors) are different enough between human and dog, for 
instance, to prevent interspecific transfer.  Of course, the virus for 
rabies is one that can cross many specific lines because its mode of 
entry and the cell physiology is similar enough to allow for it. 
  The bottom line is that viruses tend to co-evolve with specific hosts 
and, perhaps from mere chance circumstance, they might be exploiting a 
part of cellular machinery or structure that makes it possible for the 
virus to transfer into another host species when the opportunity arises 
(dog bites human, flea jumps from rat to dog or rat to human, hunter 
somehow exposes his blood to the blood of his prey through a cut or 
abrasion, human breaths dust containing virus - hanta virus with mouse 
urine and droppings containing virus which then aerosolize with the wind 
and enter the noses of nearby humans).

Patrick 



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