I'm afraid that I will have to take a step backward and state that I
agree with Dr. C.G. Clark that the terminology itself is not problematic
, it is just the current usage that is confounding. Dr. Paul Ewald in
his recent book appears to relate virulence with the reproductive
ratio of the parasite, referring to more benign strains as less harmful,
slower reproducing parasites. Intuitively I would think that the mechanisms
of host harm, whether mechanical, or caused by toxin production, would
require some expenditure of energy by the parasite, thus allowing less
pathenogenic (I know, I know...but I still like it) parasites an
avenue for increased reproductive output. A recent article by Ebert in
Parasitology gives a good review of studies that examined reproductive
ratios and host damage simultaneously, and found no correlation,
including his own work on a pathogen of Daphnia. In fact, a study by
Barbosa on Scistosoma mansoni in Biomphalaria snail intermediates showed
that the number of cercaria produced by infected snails was negatively
correlated with snail mortality. Ewald seems to argue that a parasite
should be as virulent as the situation allows in order to maximize the
number of offspring that will be produced, and therefore transmitted to a
new host. Of course, if all the parasites were identical, then increased
numbers would result in increased virulence, but natural selection
doesn't deal well with equivalence. It would seem to me that reducing
damage to the host and therby increasing both the duration of infection
and the reproductive output would be optimal. One other point that Ewalds
book seemed to miss is that a host population as a parasite resource is
finite under the conditions that would select for virulence. If virulence
increases to the point of high host mortality, the host resource will be
depleted, and selection will be again for less virulent forms.
Derek A. Zelmer
