INTRODUCTION TO PARASITOLOGY
n 1.1 PARASITES AND PARASITISM
The word ‘parasite’ is derived from the Greek words para(meaning beside) and sitos
(meaning food). Parasites can be described as living organisms that are associated with
food for all or part of their life-cycle. The organism providing the food is generally
called the host. A parasite has at least one host per life-cycle. If there is more than one
host per life-cycle, the host in which sexual maturity occurs is referred to as the
definitive host and the other host/s are known as intermediate hosts. The study of
parasites invariably involves firstly the biology of the parasite and secondly the biology
of the host — the parasite’s environment. Hence the following comments can be made
regarding parasitology:
n Parasitology can be considered to be a specialised branch of ecology.
n Parasitology is the study of organisms living within a specialised environment.
The problems related to survival for parasites are almost the opposite of those faced
by free-living animals. Parasites are surrounded by (they live within or on) their food and
do not need to spend energy to find food; whereas free living animals are continuously
searching for food. Free living animals have fewer problems than parasites in reproduc-
tion and distribution. During the distributive phase of its life-cycle, the probability of a
parasite making contact with a new host is relatively low.
The evolution of successful methods of invasion and escape is essential for the
survival of a parasite. Parasites have evolved mechanisms to ensure distribution and
making contact with a new host. Similar adaptive strategies that follow a basic ‘parasitic’
plan/design have evolved within different taxonomic groups. Parasites have to adapt to
two basic environments:
n the micro-environment — the habitat within the host immediately surrounding the
parasite;
n the macro-environment — the habitat of the host.
Like many free-living animals, parasites have become equipped to survive within
a certain habitat. The majority of parasites occupy a specific or predetermined site