PARASITOLOGY

The majority of established parasites have exploited their hosts to their maximum
potential by making their hosts their environment. Parasites have adapted to almost
every part of the vertebrate body, with some becoming so specialised that they will sur-
vive only in one particular niche within the host.
The majority of the parasites studied by parasitologists are endoparasites. They live
entirely within the host, as opposed to ectoparasites which live on the surface of the
host. Most ectoparasites are arthropods and tend to studied by people who specialise in
animals from that phylum.
The parasites of economic importance can effect the health and life of humans and do-
mestic animals as well as the food supply. There are two main reasons why man and
domestic animals tend to suffer from parasitic infection:

n Both man and domestic animals are gregarious, ie they live in relatively large communities

close to one another. This favours the distribution of parasites.
n In evolutionary terms both man and domestic animals are very young.

n 1. 3PARASITES’ EFFECT UPON THE HOST
The details of the pathological effects of selected parasites upon their host are covered
in a later section. In this section only general considerations will be presented. In gen-
eral the causes and types of damage done by parasites to their hosts are:

n Mechanical damage mainly due to blockages.
n Migration through tissues, penetration into cells.
n Nutrition; depleting the host of nutrients or competing with the host for essential

nutrients.
n Toxins; very often the metabolic waste products of the parasite’s metabolism accumulate

in the host tissues and become toxic to the host.
n Immunosuppression; most parasites are relatively long-lived and continually present

an antigenic challenge to the host, reducing the effectiveness of the immune response.
n Parasites in the ‘wrong host’ are in general more pathogenic. Most endoparasites have

a preferred site within the host. This is not normally at the point of entry, hence the

INTRODUCTION TO PARASITOLOGY

A

Parasite Host

Nutrition

Inhibition

Parasite Host

Parasite Host

B

C

There is a pathological effect

The parasite is damaged

• Figure 1.2The survival
  of a parasite depends
  upon living in equilibrium
  with its host. The stability
  of the relationship is a
  balance between the
  feeding of the parasite
  and the host’s ability to
  inhibit the parasite.
  A: the equilibrium state.
  B: pathological damage
  to the host resulting from
  the parasite’s feeding.
  C: damage to the parasite
  caused by the host’s
  resistance.

Ectoparasites live on the

surface of their hosts.