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another defini post

Jeffrey Lotz jlotz at medea.gp.usm.edu
Wed Mar 1 13:23:32 EST 1995


Two types of responses have been posted in reference to the 
call for a discussion of a definition of a "parasite". Those 
that suggest there are no such things as parasites and 
therefore there is no reason for such a discussion. And those 
that suggest there are such things as parasites and that we can 
learn something about parasites through a discussion of what 
the extention of the word "parasite" is. I am in the second 
group and think that we can discover something about what a 
parasite is empirically. 

To do this we start with those organisms that are generally 
agreed to be parasites: cestodes, digeneans, apicomplexans, 
ascaridoids, ergasilids et c. We should test any definition (or 
concept) of parasite against this empirical set. We will not 
necessarily end up with only those organisms but any definition 
has to include, at a minimum, those organisms. It seems to me 
that we are attempting to develop a notion of parasite that is 
an abstraction of the characteristics of the initial set of 
organisms.

Three attributes have been proposed that would characterize the 
organisms that parasitologists study.

1) Use of a host (one individual) as habitat

2) Some type of dependence of the parasite on the host That is 
a parasite is tied to its host in some manner.

3) Some harmful effect on the host by the parasite.

I think that 1 and 2 get to the meat of what a distinct kind of 
life history is  (although I am not completely convinced that 2 
is necessary). These two attributes identify the set of all 
organisms studied by parasitologists, i.e., no parasite is 
free-living. This is the attribute that holds parasites 
together as a class (group). Although many species mix 
free-living stages with parasitic stages they are of interest 
to parasitologists because of their non-free-living stages. To 
be a parasite it is necessary and sufficient to be 
not-free-living.

Attribute 3 is not important and does not apply to all or 
perhaps even most, organisms that are generally considered to 
be parasitic. This character essentially equates "parasite" 
with pathogen and not all parasites are pathogenic. Parasite 
may be a synonym of infectious agent but it is not a synonym of 
pathogen.  I think parasitologists study parasites not because 
they are harmful but because they live in or on other organisms.

Even on sematic (rather than empirical) grounds, I would argue 
against the restriction of parasite to pathogen. If we restrict 
the use of parasite to pathogen then we are left without a word 
to apply to organisms that are not-free-living. Typically 
symbiosis is used to encorporate interspecific relationships 
including mutualist-commensal-parasite. However, symbionts 
usually include nest-parasites, cleaner fish, cattle egrets and 
other organisms that live in close association with another 
organism but are essentially free-living. Given this situation 
we need a word that means not-free-living, that world should be 
"parasite". We already have a word for a not-free-living 
organism that does harm to its habitat, that word is pathogen.

Whether one decides to restrict the use of the word "parasite" 
to pathogens is really not important. 

Rather, I would like to see some continued discussion of what 
it means to be not-free-living, rather than what it means to be 
harmful. Where are the difficulties in distinguishing 
free-living from not-free-living organisms? Restrict the 
discussion to stages (adult, metacercariea et c.) rather than 
species.

--
Jeffrey M. Lotz                     Phone (601) 872-4247
Gulf Coast Research Lab             Fax   (601) 872-4204
P.O. Box 7000                       Internet: jLotz at medea.gp.usm.edu
Ocean Springs, MS 39566-7000 USA



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