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.

  1. 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 milk.

  2. 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. 

  3. A related third question asks how the parasite reaches the predilections site since this may involve extensive migration through the body of a host.

  4. The fourth questions asks how a parasite leaves its definitive host to return either to the environment or to its intermediate host.  

  5. 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 parasitic infections.

    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

If you go to these tables, please use the back button on your browser to return to this page.




Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000