

New Republic Article: Virulence & Transmission Mode

Keith Robison robison1 at husc9.harvard.edu
Tue Feb 4 09:57:15 EST 1992

	In the October 7, 1991 issue of the New Republic, there is an 
article entitled "HIV's Weak Link".  I am excerpting parts of this
article because I found it thought provoking, and I would like to hear
comment as to whether the hypothesis in it is believable.

"Now one school of epidemiologists is saying that needle exchanges
 and condoms probably serve another function as well.  Not only
 will fewer people become infected with HIV, but the viruses in
 the population at large could well evolve into less harmful strains."

"Ten years ago, Paul Ewald, professor of biology at Amherst College...
 predicted that the virulence of a parasite is related to the degree to which
it relies on its host for mobility.  Parasites that depend heavily on their
human hosts for transportation, like rhinoviruses that cause the common
cold, need mobile, active hosts in order to survive and get passed along to
new hosts.  In contrast, a parasite that uses a carrier, or "vector", such
as a mosquito or tick, is transmitted effectively even if it incapacitates
its hosts.  These parasites are therefore free to reproduce extensively in 
the blood--sickening their hosts in the process--without endangering their
own long-term survival"

"The fastest replicators (and thus the most dangerous) of the countless viral
strains that live inside an HIV-infected person quickly outnumber their slower
neighbors, and thus are the most likely to be transmitted.  When transmission
rates drop, fast replication becomes a liability for the virus: fast 
replicators again come to dominate but, as a result, die with the host
without being transmitted.  Under these conditions, slow replicators are
more likely to make it into future generations"

Anyway, these quotes should give you the gist of the article.  My questions

1) How well does this transmission mode--virulence correlation really hold up?

2) Could we really expect to see the described evolutionary effect on the 

Any other rational comments would be appreciated.

Keith Robison
Harvard University
Program in Biochemistry, Molecular, Cellular, and Developmental Biology

robison at ribo.harvard.edu 

P.S. I realize that the ink is rather dry on this article.  It was sent to
     me by my brother, lost on my desk, found, lost during a move, and 
     then re-found.

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