Introduction to Parasitology

Parasite Life Cycles

Despite their diversity, parasites do have some common features: all of them have unique life cycles and each major parasite group (arthropods, nematodes, cestodes, trematodes and protozoa) includes a set of well-defined and recognizable stages unique to each group.

The multicellular nematodes, for example, have life cycles which include adult males and females. They reproduce sexually and eggs are laid by the female. Larvae develop, in these eggs, hatch and progress through a series of developing larval stages until they reach adulthood as sexually mature males and females and the cycle begins again. Most nematodes follow this basic life cycle pattern, although some may also include variations and additional complexities.

Click here to view the life cycle of the small strongyles of horses, an uncomplicated cycle. Remember to use the back button of your browser to return to this page.

Click here to view the life cycle of Dirofilaria immitis, the heartworm of dogs. This pattern is complicated by the inclusion of required intermediate hosts, mosquitos. Use the back button of your browser to return directly to this page.

The protozoa are unicellular organisms. Most of them have a phase of sexual reproduction in their life cycles and also include an asexual reproductive phase. They lack recognizable male and female adult stages as we see in the nematodes but they do have male and female stages that are analogous to eggs and sperm. For example, the coccidia have the following stages in their life cycles: oocysts, sporozoites, schizonts, merozoites, microgamets, macrogametes, zygotes. In a simple coccidian life cycle, a host is infecte by an oocyst containgin sporozoites which leave the oocyst and infect host cells. Asexual multiplication occurs resulting in an intracellular schizont containing many merozoites.  Asexual reproduction (schizogony) may be repeated several times and ends with the beginning of sexual reproduction. Merozoites transform into micro or macro gametocytes.  Macrogametocytes are female and grow to occupy a single host cell when they are usually called macrogametes.  Microgametocytes are male and divide to produce microgametes which leave the host cell in search of a macrogamete. The latter is penetrated by a microgamete, fusion of their nuclei occurs and a zygote is formed. A cyst wall is formed around the zygote resulting in an oocyst which sporulates, forming sporozoites. This sporulated occyst is the infective form for the host. Click here to view the life cycle of Eimeria bovis, a coccidian of cattle. (Use the back button of your browser to return to this page). 

The arthropods have different life cycles but like the others they also have distinct and recognizable life cycle stages. 


A female louse lays eggs which attach to the hair or feathers of a host A miniature adult called a nymph hatches from each egg and grows to an adult. The whole life cycle takes approximately one month.


Fleas are also arthropods and although they have distinct life cycle stages they are different to those of their lice cousins. Female fleas lay eggs from which larvae hatch. Each larva metamorphoses first into a  pupa which in turn transforms into an adult.





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