definition

Brian E. Keas keasbe9 at WFU.EDU
Fri Feb 17 12:40:51 EST 1995


On Wed, 15 Feb 1995, derek a. zelmer wrote:

> Brian Keas stated the following requirement for an organism to be 
> considered parasitic:
> 
> > 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).}
> 
> Consider, if you will, the question asked of J. Holmes by N.A. Croll: 
> "Are predators alway idiots?" in the context of parasites that alter 
> intermediate host behavior in order to complete their life cycle by 
> increasing the probability of the predator eating an infected 
> intermediate. The question implyed that there must be selective pressure 
> against eating infected intermediates if the resulting infection has a 
> negative effect. Holmes response was that the tradeoff must be favorable 
> to the host; i.e. the energy gained from the easier capture and ingestion 
> of the prey must be greater than the energy consumed by the parasite that 
> establishes within the host. In other words there is a net benefit to the 
> host as a result of the infection. According to your definition these are 
> not parasites. This intermediate host manipulation is characteristic of 
> all acanthocephalans, and I would personally mourn the loss of this group 
> from my area of study.

Perhaps the first part of the definition should be "...an organism (or 
life-cycle stage) DIRECTLY causes..."  which would account for your 
example and still define acanthocephalans as parasites. The 
definition would then exclude indirect benefits of parasitic organisms 
(if there are any).

As to the answer by Holmes, if I understood correctly, his answer did not 
include any experimental evidence and was also based on a group 
(population) selection argument. IMHO a more convincing case for this 
indirect benefit in parasitized prey-predator interactions was presented 
by K. Lafferty (1992.  Foraging on prey that are modified by parasites.  
Am. Nat. 140:854-867) where he used modeling to suggest that "predators 
may actually benefit from their parasites if energetic costs of 
parasitism are moderate and prey capture is facilitated by parasites."  
He also makes the argument that these are still parasites and should not 
be considered mutualists, although that also depends on your definition 
of mutualists (I'll wait on that for some other rainy day).
In his model, several assumptions had to be made which are absolutlely 
necessary and these assumptions are described clearly.  As he noted these 
assumptions may be violated in nature and I think that most of these will 
tend to decrease the liklihood of predators benefiting from parasitized 
prey. 

	IMHO most of the time parasites will end up costing their hosts 
more than the benefits provided by ingesting intermediate hosts.  In 
acanthocephalans,  attachment of the worms to the intestine may cause 
an inflammatory response, deposition of collagen, etc.  Penetration 
through the intestine which occurs sometimes causes severe pathology.  
Can't forget the energy required to grow from a cystacanth to an adult 
that may be 30 cm long in some species as well as production of 
thousands to tens of thousands of eggs/day per female in an infected 
host.  Even in hosts in which little pathology apparently occurs,  it has 
been shown in fish that heavy or repeated infections cause a doubling in 
the number of goblet cells.  Not surprising then that mucus production is 
increased (possibly doubled?) which represents a large energy drain.

Again most of this discussion is based on little experimental evidence, 
so if anyone knows of references detailing energetic costs/benefits for 
predators ingesting parasitized prey please send them my way.  To prevent 
clutter in the group, you can send them directly to me and I will post 
summary of those I receive.

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




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