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virulence and selection

keas brian edward keasbe9 at WFU.EDU
Thu Dec 15 11:57:31 EST 1994

Peter Pappas writes:

> think of scenarios in which this might not be true.  For example, consider a 
> hypothetical example of a tapeworm whose life-cycle involves an intermediate 
> and definitive host.  Suppose the virulence of the infection in the definitive 
> host increases, thus resulting in a decrease in the total number of eggs 
> produced by the adult tapeworm (because the host is killed or dies sooner).  
> But, suppose this increase in virulence is somehow linked to the parasite's 
> ability to cause a behavioral change in the intermediate host so the
> intermediate host is more likely to be eaten by the definitive host.  (Note
This brings up an interesting question regarding selection for virulence 
or any other trait in parasites with complex life cycles.  Is the 
genetic expression in one stage (e.g. adult virulence) linked to that in 
another stage (e.g. host behavioral changes by cysticercoids)?  If this 
is so, then selection on the genetic expression in one stage will affect 
another.  Does anyone have any information regarding this?
My first thoughts would be that this "linking" is not very common.  For 
example, many adult trematodes are considerably different (at least 
allowing me to distinguish among them) whereas their metacercariae are 
morphologically indistinguishable (and thus requiring some in vitro or in 
vivo transformation to adults to identify).  To me, this suggests that 
selection in the adults has been completely separated from selection in 
intermediate stages (and that different genes are activated in each 

Back to virulence....one more thought.

Graham Clark writes:

>all organisms, individual parasites that leave the greatest number of
>'offspring' will be the most successful over time. The optimum level
>of virulence becomes a balance between parasite transmission and host
>mortality, since virulence is often directly linked to reproduction of
>the parasite. This often leads to a low level of virulence, but not

I think that this is Ewald's point exactly, except that Ewald would 
stress "...but not always."

Brian Keas
keasbe9 at wfu.edu
Wake Forest University
Winston-Salem, NC 27109
(910) 759-5957

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