I'm afraid I let some of the original discussion on the topic expire
so I apologize for the lack of appropriate quotations from the earlier posts.
I took the opportunity to read Ewald's Sci American article on
the topic because I didn't hear the talk at ASP last year. The obvious thing
that struck me about his hypothesis was that most of the examples are based
on bacteria, viruses, and insect transmitted protozoa (Trypanasomes and
Plasmodium). In general, these are all characterized by simple lifecycles,
when compared to Trematodes with 2 intermediate hosts, indirect lifecycle
cestodes and nematodes, and coccidia like T. gondii which use a combination of
direct and indirect lifecycle stratagies to fullfill their Darwinian
obligations.
My question is: in which host (IH,DH or vector)does one look at virulence
as the defining characteristic of evolutionary fittness? For example we see
that not all mosquitos are suitable hosts for D. immitis, and some isolates of
T. gondii are particularily virulent in Goats, but well tolerated in mice or
other IHs. I guess I believe that all of these relationships need to be
considered as dynamic entities which are constantly evolving as the parasite
and its host(s) struggle in particular environmental contexts to survive
long enough to get their genes into the next generation. It seems to me that
the idea of parasite virulence associated with host switching still has some
validity as a concept for explaining adjustments between parasites and their
hosts in response to selective pressure.
This is a good topic for continuing discussion. Thanks Eric for
bringing it up. I'm looking forward to more debate on the issue.
**********************************
* Charles T. Faulkner * When you don't know where you're
* Univ of Tennessee, Knoxville * going any road will take you there.
* (ctfaulkn at utkvx.utk.edu) * Alice
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