On Wed, 13 Nov 1996, Charles T. Faulkner wrote:
> In other cases, extreme dependence on the
> paratenic way of life has resulted in the elimination of freeliving and/or
> environmental stages (e.g. Trichinella spiralis).
ohno second: [def] unit of time between pressing the send key and the
realization that you really want the electronic post back for editing.
oops! bad example with T. spiralis, and perhaps C. hepatica too!
Returning to consideration of paratenisis as a convenient accident
that occurs and persists in nature.......
Adult dogs (> 1 yr) rarely harbor adult stages of Toxocara canis, yet
vertical (transplacental) transmission of arrested L3s account for almost
100% prevalence of the parasite in newborn pups. When patent infections
are allowed to develop in the pups, the environment is contaminated with
parasite progeny, the bitch accidentaly ingests the embryonated eggs
during grooming or some other dog activity and replenishes the reservoir
of infective stage parasites. The bitch is a paratenic host because the
L3s never undergo development and remain poised to cross the placenta to
infect another litter of pups. It is hard to accept a biological
pattern with this degree of fecundity as a convenient accident of nature.
Although the issue has not been resolved experimentally, the incidence of
neonatal Isospora infections in nursery pigs, kennels and catteries
circumstanially suggests vertical transmission (either prenatally or
perinatally) of arrested sporozoites from the tissues of the mother (sow,
dog, cat) to her offspring. Again, the adult is a paratenic host because
sporulated oocysts ingested from the environment, replenish the
sporozoites in tissues without further development. Standard disclaimer
applies.....this is my imagination gone wild......Perhaps someone
anyone actually pursuing this problem would like to contribute, or set me
In short...the paratenic way of life confers a systematic advantage of
extending the temporal and spatial viability of parasite progeny for
transmission to other definitve hosts. If it were simply an accidental
occurence that was convenient it would not persist with the degree of
fidelity observed in various parasite lifecycles.