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Paratenic host?

derek a. zelmer zelmeda4 at WFU.EDU
Wed Nov 13 20:15:13 EST 1996

On Tue, 12 Nov 1996, Omar O. Barriga wrote:

> At 01:12 PM 11/12/96 -0500, you wrote:
>         Derek, I am not sure that I like your definition. On one hand, it
> unnecessarily complicates
> the concept. On the other, as soon as you say "is . . . necessary to
> complete the life cycle" you 
> start having problems with the differentiation with intermediate hosts. I
> agree that a paratenic
> host may focus the parasite toward the host (they are often part of the same
> alimentary chain) 
> and that may protect the parasite from the vicissitudes of the free-living
> life but it has to 
> have some advantage, otherwise it would not exist. I still believe that the
> paratenic host is 
> only an accident in the life of the parasite that happens to be convenient.
> And because it is 
> convenient it persists in nature.

Well, you are forcing me to give away my ASP talk, but here goes. I work 
on Halipegus occidualis, a trematode that lives in the mouth of frogs and 
passes eggs into the pond where they are eaten by a snail. The snail 
sheds cystophorous cercariae that "inject" the cercariae into an 
ostracod. The ostracod is eaten by an odonate niad, which in turn is 
eaten by the frog. The frogs do not eat ostracods, and become infected as 
adults by eating dragonflys, so infection is not through the ingestion of 
ostracods by tadpoles. All the population dynamics indicate that 
infection is through the odonate...BUT in the lab, I can infect frogs 
with infected ostracods. With no odonates, the cycle would not continue 
in the pond, but physiologically the odonate is not necessary for the 
development of the parasite. Thus the odonate bridges the ecological gap 
between the ostracod and the definitive host with no development by the 
parasite, and is therefore a paratenic host.

Derek Zelmer

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