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