Introduction to 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.
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
- Parasitism always involves two species,
the parasite and the host.
- Many of these parasitic associations
produce pathological changes in hosts that may result in disease.
- 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.
- 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,
|Unicellular or acellular 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 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
However, the consensus among parasitologists is to view the
subjects of the discipline as including only the arthropods, helminths and