virulence

derek a. zelmer zelmeda4 at WFU.EDU
Mon Dec 26 17:03:48 EST 1994


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



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