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another defini post

derek a. zelmer zelmeda4 at WFU.EDU
Fri Mar 31 08:41:49 EST 1995

On Thu, 30 Mar 1995 ctfaulkn at UTKVX.UTCC.UTK.EDU wrote:

> 	In an earlier post I commented on the sometimes "fine-line"
> between being a commensal and a "parasite (pathogen)".  I think Jeff's
> comments go a long way toward resolving the problems inherent in equating
> parasite with pathogen.  Consider the pinworms as an example. As a general 
> rule they cause no harm to tortises, and some have suggested that they play 
> an important role in breaking down the hemicellulose in their diets, yet on
> other occasions an excessive number of parasites coupled with inadequate 
> nutrition results in serious disease, and even death.  What do you call these
> parasites, facultative comensals (or pathogens).  

	I am not sure that it is a fine line, it seems more like a large 
grey area to me. You cannot differentiate between parasitism and 
commensalism based solely on metabolic dependence, you must look at the 
overall effect on the host. If there is no benefit to having the 
endosymbiont it is definitely a parasite because it is definitely causing 
a negative effect. It is taking up nutrients that must be supplied by the 
host, it is causing an immune response which requires energy, and is 
likely to have an effect on other bodily functions.

> Another question, how long must a parasite (pathogen) be associated 
> with a host before it becomes a comensal, and what sort of selective 
> preassures favor this transition?

This question is complicated by the numbers game you mentioned earlier, 
for example, you or I could harbor 1-10 Ascaris with no noticable 
effects, a six year old could die from 50-100 worms. I would venture to 
say that, on the whole, most metazoan parasites cause little damage in 
small infections (although I would not call them commensals), but do 
cause severe damage or death in larger infections. Because parasites are 
overdispersed, only a small number of infected hosts will be killed by 
heavy infections (a la Crofton). So how do we determine the level of 
pathogenicity associated with this parasite? It is a difficult question. If
you are interested in the "Evolution of Virulence" you may want to read Paul 
Ewalds work, although I wasn't very impressed with it. I do think that a 
given species of parasite has an inherent ability to cause a given degree 
of harm in a given situation, but I think that the mortality and 
morbidity associated with a given infection in a population will be 
determined by more proximal, ecological causes, most often associated 
with transmission.
				Derek A. Zelmer

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