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Discussion: virulence

eric jona wetzel wetzelej at WFU.EDU
Wed Dec 14 10:33:05 EST 1994

Fellow Parasitologists ---

Motivated by the recent admonition of John Janovy, I would be interested 
to hear some discussion regarding the 'evolution of virulence' as it has 
recently been put forth by Dr. Paul Ewald.  

Many of you attended a talk by Ewald at the last ASP meetings, in which 
he presented many of his hypotheses (most of which are also in his 
recent book, Evolution of Infectious Disease) about the relationship (as 
he sees it) between "virulence" and transmission, or "dependence on host 
mobility" as he put it at the ASP meeting.  According to Ewald, part of 
his motivation for this is to challenge the "traditional" view that 
parasites always evolve to a less virulent (i.e., benign) state -- he 
states that this is the general view held by parasitologists.

His main premise is that, sometimes, parasites should evolve to a highly 
virulent state when it favors transmission, e.g., a parasite can afford 
to be highly virulent in an intermediate host (presumably causing
 reduced fitness) if compromising that host results in greater 
transmission to the definitive host.  Again, he ties much of this to host 
mobility; e.g., according to Ewald, malaria can 'afford' to be much more 
virulent in a human, even if it knocks that host down, since transmission 
is dependent on a mobile mosquito, in which (Ewald predicts) the parasite 
should necessarily be relatively benign (because of the need of 

So, what do folks think about this?  How, or should, we define virulence 
(since Ewald seems to change his definition depending on the scenario)?
It certainly has merit in generating hypotheses, but how does this fit 
with infections with protozoan parasites?  helminths?  etc?

It would be interesting to hear from a wide range of folks (ie., parasite 
ecologists, immunologists, etc.) on this, as this cuts across many of 
these areas.  Have at it!


Eric J. Wetzel
Dept. of Biology
Wake Forest Univ.
PO Box 7325
Winston-Salem, NC 27109
phone: 910-759-5577
fax: 910-759-6008
wetzelej at wfu.edu

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