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eric jona wetzel wetzelej at WFU.EDU
Wed Dec 14 15:01:51 EST 1994

Dr. Peter Pappas writes:
..."Eric asks about the question of whether evolution of host-parasite
relationships will ultimately lead to decreased virulence; that's certainly
one of the basic tenets I learned when I took my first introductory
parasitology course (nearly 30 years ago!). Of course, I now know that there
are many "exceptions" to the rule.  Eric seems to assume that increased
virulence in a host will decrease the "fitness" of a parasite, and 
perhaps we need some clarification here.  When we speak of the fitness of 
a parasite we can not view fitness from the standpoint of the 
relationship between the parasite and a single host (unless it has a one-host
Allow me to clarify something (and please remember, don't kill the 
messenger!) -- when I said that the increased virulence would decrease 
fitness, I meant of the HOST, not the parasite.  I think we're assuming 
that the parasite will be maximizing its reproduction/fitness -- the 
question is whether this means it will be more, or less, virulent to the 

Pappas later writes:
..."Granted, increases in virulence might be expected to decrease the 
basic reproductive rate, but 
I can think of scenarios in which this might not be true.  For example, 
consider a hypothetical example of a tapeworm whose life-cycle involves 
an intermediate and definitive host.  Suppose the virulence of the 
infection in the definitive host increases, thus resulting in a decrease 
in the total number of eggs produced by the adult tapeworm (because the 
host is killed or dies sooner). But, suppose this increase in virulence 
is somehow linked to the parasite's ability to cause a behavioral change 
in the intermediate host so the intermediate host is more likely to be 
eaten by the definitive host.  (Note that the change in the intermediate 
host would not be cosidered a change in virulence.)"

This is a good example that I think gets to the point; that is, how is 
"virulence" being defined here?  Pathology?  The number of worms in the 
definitive host?  Some general 'harm'?  Also, what Ewald has said 
about the above scenario is that the parasite can afford to be harmful 
(virulent?) to the intermediate host, because it enhances transmission; 
indeed, this IS his alleged change in virulence in the intermed. host.  
Now whether this is valid is the question at hand.

Looks like a great start...


Eric Wetzel

Dept. of Biology
Wake Forest Univ.
POB 7325
Winston-Salem, NC 27109
wetzelej at wfu.edu

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