Introduction to Parasitology

Key definitions

  1. Veterinary Parasitology

    Veterinary Parasitology is the science that deals with the parasites of domestic animals. More specifically, it is the science that deals with the interactions between a host and the population of parasites that are found on or in that host. A more encompassing point of view, from an epidemiological perspective, would define Veterinary Parasitology as the science that deals with the interactions between host populations and the parasites that infect them. This broad definition means that Veterinary Parasitology covers many aspects of parasites of domestic animals and their hosts including: the morphology, biochemistry, physiology and life cycles of parasites, the immunological,   pathological and clinical responses of the host to the presence of parasites, all aspects of  treatment and control of parasitic infections and diseases and the public health aspects of parasites of domestic animals that may also infect humans.
  2. Parasitism
    The term parasitism may be defined as a two-species association in which one species, the parasite, lives on or in a second species, the host, for a significant period of its life and obtains nourishment from it. This is a commonly accepted working definition of parasitism and using it we can emphasize several important features of the host-parasite relationship.
  1. Parasitism always involves two species, the parasite and the host.
  2. Many of these parasitic associations produce pathological changes in hosts that may result in disease.
  3. Successful treatment and control of parasitic diseases requires not only comprehensive information  about the parasite itself but also a good understanding of the nature of parasites' interactions with their hosts.
  4. The parasite is always the beneficiary and the host is always the provider in any host-parasite relationship.

This definition of parasitism is a general one but it tells us nothing about parasites themselves. It does not address which particular infectious organisms of domestic animals we might  include in the realm of parasitology.  The protozoa, arthropods and helminths are traditionally defined as parasites. However, there are members of the scientific community who designate all infectious agents of animals as parasites including viruses, bacteria and fungi. This broader definition of parasites includes viruses, bacteria and fungi as well as the arthropods, helminths and protozoa. Within this broad definition, parasites are further divided into microparasites and macroparasites. The following table summarizes  their salient characteristics.



Bacteria, Viruses, Fungi, Protozoa

Arthropods, Helminths

Unicellular or acellular organisms Multicellular organisms
Usually multiply in the host so that a few infecting organisms may give rise to many in a non-immune host. Rarely multiply in a host
Short generation time - hours or days Long generation time - usually weeks or months
Acute infections  most commonly seen. Infected animals may succumb,  may recover and show significant protective immunity or the infection may, in some cases revert to a chronic state  Chronic infections are most commonly seen but acute infections may be seen in young, susceptible animals. Recovery from acute infections does not necessarily confer immune protection on the host.

However, the consensus among parasitologists is to view the subjects of the discipline as including only the arthropods, helminths and protozoa.




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