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Parasitism is the relationship between two organisms where one is living at the expense of the other. Parasites are highly specialised, have nutritional dependence on the host and can avoid an immunological attack. They have evolved to integrate their life cycle with that of their host species. The two main groups of parasites are the microparasites and the macroparasites. Protozoa are microparasites as they multiply within the host. Macroparasites on the other hand, establish themselves in or on the host but produce eggs or larvae which are dispersed to the environment. Macroparasites include helminths and arthropods. The number of macroparasites in the body reflects the number which invaded the animal.

Useful Resources

Sea Lice - Copyright Joseph G. Kunkel at The Kunkel Fish & Aquatic Invert Site
Mites parasitising a harvestman - Wikimedia Commons

Information on the diagnostic tests, collecting of samples and identification of parasites


A glossary of the important concepts and terms in parasitology


This section relies heavily on lecture notes and input from:

  • Professor Dennis Jacobs
  • Dr Mark T Fox BVetMed PhD HFEA DipEVPC MRCVS

Other sources

  • Wall and Shearer: Veterinary Entomology
  • Taylor, Coop and Wall: Veterinary Parasitology, third edition