Fitness vs virulence

Steve G. Kayes kayes at SUNGCG.USOUTHAL.EDU
Wed Dec 14 22:40:50 EST 1994


Having read Dr. Pappas's contribution regarding fitness, I realized that 
Eric's original question is rapidly being degraded.  I too, learned that 
virulence is synonamous with poor adaptation.  But the redirection of the 
thought process that was presented at the meetings in Ft. Collins, dealt 
specifically with virulence.  This, to my mind indicated that the 
question now was can increased virulence be considered a trait upon which 
natural selection can operate.  I preface my subsequent remarks by saying 
that I accept virtually all of Darwinian evolution.  However, we choose 
to use our logical minds to decide what the traits are that lead to 
survival, frame our experiments, and go on with life.  Virulence seems to 
be reasonable trait given the way it was presented at ASP.  That said, 
part of Peter Pappas's arguments are quite valid.  What is the effect of 
increased (a relative term) virulence on reproductive outcome.  If 
predators of a host are given more opportunities because of a highly 
virulent infection, then surely, a highly virulent organism could be said 
to be successful.  Each host parasite relationship should be considered 
in its entirity.  

	When I worked with Dr. Paul Beaver, he believed that a parasite 
could almost will a given histopathological response or put another way, 
the parasite is happier in a mononuclear cell-rich granulomatous response 
than say surrounded by fibroblasts.  At the time I tolerated his beliefs 
out of admiration.  Now as work with Th1 and Th2 T cell subsets expands, 
it is beginning to look like there was more than a grain of truth to what 
Dr. Beaver believed.  One subset (Th1) seems to favor resistance and all 
of the associated cellular responses necessary to go with it.  The other 
subset (Th2) favors susceptibility.  The big question today is, what 
property of the host-parasite interaction determines whether the 
resulting encounter goes to Th1 or to Th2.  It could well be argued that 
the parasite which is more virulent can manipulate the outcome or 
alternatively, live with the outcome.

	Others thoughts?

Steve

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