Virulence

Steve G. Kayes kayes at SUNGCG.USOUTHAL.EDU
Sat Dec 24 18:05:16 EST 1994


	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

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