PARASITOLOGY
within their host to which they have become adapted. There are very few areas of the
vertebrate body that have not been invaded by parasites adapted to survive in that par-
ticular microenvironment.
Parasites are often described according to which site they inhabit, such as:n Intracellular or inside the membrane of host cells — intracellular parasites are more
or less restricted to the size of these cells and are mostly microscopic.
n Extracellular or outside the cell, but within body fluids or in the ground substance or
matrix of tissues and organs. They range is size from micro- to macroscopic.One of the major problems that a parasite has to overcome is that the environment
(the host) in which it lives also happens to be its source of food, but reacts against it. The
presence of the parasite stimulates the host to try and destroy the parasite. In order to
survive, the parasite has to try and avoid this reaction (see Box 1.1).
The majority of the parasites studied by modern parasitologists cause disease in
humans or domestic animals. They are also known as parasites of economic importance.
Nearly all of these parasites are invertebrates belonging to one of the following phyla or
groups: the Protozoa, the Cestoda, the Trematoda, the Nematoda, the Acanthocephola
or the Arthropoda. It is within these phyla that most parasites of economic importance
are found.
It is almost certain that all parasites evolved from free-living forms. While certain species
were evolving into larger aquatic and terrestrial creatures, others that had not evolved
to the same extent chose to invade the larger forms. The successful invaders soon estab-
lished their habitat within the larger organism. Over millions of years of evolutionary time
both the parasite and its host adapted to (on the part of the parasite) and tolerated (on
side of the host) each other. The younger the host–parasite relationship is in evolution-
ary terms the less tolerant the host will be of the invader. The host tries its utmost to
exclude the invader for the simple reason that presence of parasites causes varying
degrees of pathological damage to the host (see Box 1.2).n 1.1.1 METHODS OF INVASION AND ESCAPE
In order to survive as species the parasites must have some means of locating and invad-
ing new hosts. After development and maturation within the host the next generation
must be able to escape from the host (see Fig. 1.1).n BOX 1.
Parasitology as a discipline has to investigate all aspects of the following:nThe biology of the parasite.
nThe variations in life-cycle of the parasites.
nMethods of invasion of the host.
nMigration and maturation within the host.
nThe effect of the parasite upon the host.
nThe host reaction and response to the parasite.
nMethods of escape from the host.
nDistribution of the parasite.