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
wetzelej at wfu.edu