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
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
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
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