I having been following the dialogue between Graham and Charles
back and forth over the basic definitions of virulence, pathogenicity and
other closely related points. Rather than watch this back and forth
conversation become a flame war, and agreeing that all must be using the
same definitions, I would like to see this thread get back to the
original question that Janovy posed. Simply stated, can a highly
virulent parasite be considered well adapted <in the Darwinian sense--my
insert>.
By permitting a free ranging discussion of this question all of
us in this newsgroup can brush up on our understanding of many aspects of
parasitology. Thus, I want to throw the following thought into the soup.
I think that virulence (a relative term) is a trait that natural
selection can act up. I do not believe for one minute that there is such
a thing as an avirulent pathogen. If you reduce to the lowest possible
unit, that no metabolic alteration occurs from being parasitized, then at
the the very least, the host must exert a miniscule amount of his own
energy to carry or move the parasite when the host moves. Failure to
find disease when infection occurs (and I use Graham's definition that
disease is a deviation from the normal) may mean that we just are looking
at the appropriate substrate. If medical test equipment similar to that
portrayed in Star Trek for testing every metabolic pathway in kinetic
real time were possible, I am sure that even the presence of a single
avirulent pathogen could be detected (i.e, the signal to ratio would
reflect the presence of the parasite and by extension, normal would have
been shifted). Therefore, natural selection can act on this host and
parasite system.
I would point out that the immunoinflammatory system requires an
incredible amount of caloric fuel to function. Even if the host does not
become sick, it has had to expend energy to overcome the presence of the
pathogen and this energy is then no longer available for other more
fundamental activities like reproducing which is the ultimate Darwinian
prize. What Ewald asked us to think about was a fundamental paradigm
shift (fundamental for us parasitologists) that for some, BUT NOT FOR
ALL, parasites that depend on vectors, a highly virulent state can
enhance the survival (and or dissemination) of the parasite. For those
parasites that choose this niche, it seems to work as Ewald suggested.
AIDS seems to be an example in that most who become infected will
succumb (the highly virulent aspect of the disease), but the fact that
human sexual drive is necessary for transmission would seem to me to be a
confounding variable. Common sense tells us how not to get this disease,
but hormones overcome our common sense. The fact that no treatment is at
hand for the treatment of AIDS says that this organism is pretty well
adapted to its host.
Enough from me for a while. Seasons' greetings to all in the group.
Steve
================================= :<) ===============================
/\ /\/ /######\ /#######\ Stephen G. Kayes, Ph.D.
/\/ /\/ / / / Structural & Cellular Biology
/\/ /\/ / / / University of South Alabama
/\/ /\/ /######/ /########/ Mobile AL 36688
/\/ /\/ / / / ******************************
\/ \/ / / / Office: (205) 460-6768
/#######/ /#####/ / / FAX: (205) 460-6771
=====================================================================
Without data, you are just another person with an opinion.
=====================================================================
