Virulence

ctfaulkn at utkvx.utk.edu ctfaulkn at utkvx.utk.edu
Tue Dec 27 03:42:46 EST 1994


In Article <Pine.3.88.9412241651.A5923-0100000 at sungcg.usouthal.edu>
kayes at SUNGCG.USOUTHAL.EDU ("Steve G. Kayes") writes:
>
> I would like to see this thread get back to the 
>original question [snip, snip] can a highly 
>virulent parasite be considered well adapted <in the Darwinian sense--my 
>insert>.  
                  [more stuff deleted]

> 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. 

>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.
>
	In the Sci American paper Ewald distinguishes 2 different isolates of
HIV, one from Sengal, and another from the Ivory Coast.  The HIV from Sengal
is associated with low rates of AIDS, and the one from the Ivory Coast is 
associated with higher rates of AIDS.  He believes that the differences between
the 2 isolates are due to behavioral differences in the host population which 
relate to differences in the transmission potential of the agent. Sengal he
notes is characterized by cultural factors which favor reduced sexual partner
changes such as discouraging premarital and extramarital sex. The Ivory Coast
population, however, underwent major changes which resulted in the migration of
a large number of single males to urban centers for employment.  The
availablity of cash + large numbers of prostitutes allowed for high rates of
transmission between susceptible individuals.  Ewald predicts that 
increased interventions resulting in behavioral changes in the host population 
such as decreased needle sharing and increased fidelity between sexual partners
will result in selection for less virulent forms of HIV.  His example on
differences in the El Tor and and Classical biotypes of Vibrio Cholerae is
based on similar logic, high virulence can only be acomidated by high rates of
transmission.
                                                                  
	The malaria and Trypanasomes examples are interesting because high
virulence acts to immobilize the host population, making a sitting target
easier to feed on.  Another example which springs to mind is infection with
Myxobolus cerebralis the causative agent of whirling disease in Salmonid
fishes.  Presumably, the CNS involvment of the parasite makes infected hosts 
good prey items for other susceptible fish.  In our laboratory we have been
discussing the functional significance of this pattern of transmission  
with regard to other Myxobolus spp. found in various darter species. Insight
from those who are more knowledgeable in Myxobolus ecology is welcome. 

	It seems that the fundamental paradigm shift Ewald is advocating is
that changes in virulence between parasites and their hosts is based on
functional adjustments related to transmission potential, rather than the model
of benign coexistance we have grown up with.  In the benign coexistance model 
we have the parasite behaving teleologically to minimize host mortality and 
maximize parasite fecundity. In Ewald's model parasite reproduction and
transmission are the primary forcing functions and virulence is a side effect
or cost of high reproduction rates that is only tolerated in situations of high 
transmission potential.  I think the significance of this alternative view of
virulence is its explanatory potential.  Now we can hypothesize how changes in 
transmission will affect parasite virulence and use the predicitve power of
this paradigm to structure novel interventions.  Given the evolutionary success
of the parasitic way of life, I don't imagine we will ever erradicate the
parasites of major medical importance, but perhaps it will be possible to make
them more tolerable.


**********************************
*      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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