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Brian E. Keas keasbe9 at WFU.EDU
Wed Feb 15 10:51:13 EST 1995

On 10 Feb 1995, derek a. zelmer wrote:

> Due to recent (sometimes violent) discussions I have been involved in, I 
> would like to get some opinions on a question of basic philosophy...what 
> is a parasite? Are they always overdispersed? Does there have to be a 
> nutritional dependence of the adult on the host? Should parasitoids be 
> included, or does obligate host death preclude parasitism? etc, etc....I am 
> curious as to how narrow or broad the current viewpoints are, for example 
> a colleague of mine considers grazing herbivores to be parasitic. I think 
> he's daft. Touch gloves and come out swinging.
> 			Derek A. Zelmer
Other resonses to this post have suggested that there are no 
all-encompassing definitions of a parasite.  I for one am a little uneasy 
with that suggestion (after all, how can anyone say they study parasites 
if they can't even define what a parasite is).  Maybe breaking down bits 
of the definition will help.  For starters:

Derek asks, "Does there have to be a nutritional dependence of the adult 
on the host?"

Not purely nutritional, but...
Yes, for any organism to be considered a parasite it must have a net 
negative effect on host resources.  I will qualify that by adding that 
for any stage (not just the adult) to be considered parasitic, it must 
have a net negative effect on host resources. This means that each stage 
in a complex life cycle should be considered separately.  For digenetic 

	Adults:  parasitic (all? using host resources for eggs)
	Eggs/Miracidia:  free living (all?)
	Sporocyts/Rediae:  parasitic (all? using host resources for cerc.)
	Cercariae:  free living (almost all? some are ingested while 
		still in the molluscan host)
	Unencysted Meta-/Mesocercariae:  parasitic (all? using host 
		resources for movement, etc?)
	Encysted Metacercariae (host response): parasitic (all? 
		host losing resources by building cyst wall)
	Encysted Metacercariae ("parasite" response): free living 
		(all? e.g. encysting on plants [or plastic in the lab])  

Read the ( ) as "all parasitic?" or "all free living?"  For digenetic 
trematodes, I believe that these stages are considered one or the other 
but am not 100% sure, hence the ?.

1. Thus, if an organism (or life-cycle stage)  causes a net negative 
effect on host resources it COULD be a parasite.  {If it causes no net 
negative effect, it CANNOT be a parasite (e.g. flagellates in a termite 
gut use host resources but make cellulose available to the termite and 
so there is a positive effect on host resources; not a parasite).}

2. An organism (or life-cycle stage) that ABSOLUTLEY CAN ONLY use one host 
at a time to obtain host resources COULD be a parasite.  {An organism that 
could POSSIBLY obtain resources from two hosts simultaneously CANNOT be a 
parasite (a cow could possibly obtain resources from two separate plants 
(grasses) in the same mouthful; not a parsite).}

3. Host is defined here as any living organism (and must be living if 
acting as a host to parasites). 

4.  The combination of #1 and #2 with the definition in #3 is an 
all-encompassing definition of a parasite.

When I first started writing this respnse, I was planning only to answer 
the question about nutrition, but look what it evolved into.  As much as 
my mind will work right now, I can't think of exceptions to this 
definition for what are classically considered parasites.  However, I am 
pretty sure there will be some - which is why I put this out for your 

Brian Keas
keasbe9 at wfu.edu
Wake Forest University
Winston-Salem, NC 27109

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