Introduction to Parasitology
Information from Life Cycles
The key to understanding Parasitology lies in a thorough
knowledge of the life cycles whether the parasites are nematodes, cestodes,
trematodes, arthropods or protozoa. However, these life cycles are extremely
varied, ranging from the simple to the very complex. Learning them can be
daunting to many veterinary students but it helps to distill them down to a few
common questions as listed below.
How does an animal host acquire a parasitic infection? For
example, does the host become infected by direct ingestion of an infective
stage, by ingestion of an intermediate host or transport host
containing infective stages, by skin penetration of an infective
stage, by direct contact with an infected host, or via the bite of an
intermediate host serving as a vector. An important question in the case of
many parasites is whether the host can be infected by transmission from
mother to fetus across the placenta or to nursing offspring via colostrum or
The second question asks what the predilection site of
the parasite is in its definitive host. The predilection site is the place
in the host where adult males and females are found. In those parasites
without recognizable male and female forms, it's the site(s) where sexual
and/or asexual reproduction occurs.
A related third question asks how the parasite reaches the
predilections site since this may involve extensive migration through the
body of a host.
The fourth questions asks how a parasite leaves its
definitive host to return either to the environment or to its intermediate
The fifth and final question asks for the identity two
life cycle stages;
the (infective) stage entering the host and the stage leaving the host. The
latter is often called the diagnostic stage if it is valuable in diagnosing
The following tables answer these questions for eight different parasites
across the spectrum of Parasitology.
Table 1 - Toxocara canis,
Dirofilaria immitis, Fasciola hepatica, Haemonchus contortus
Table 2 - Dipylidium caninum, Isospora suis, Hematopinus
suis, Babesia species
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