PARASITOLOGY

n Larvae actively penetrate the definitive host, mature into adults and then produce

eggs which exit via the faeces eg Ancylostomaspp (hookworms), Schistosoma mansoni.
n Larvae enter into the circulation via an arthropod vector and mature into males and

females. The females release live larvae which enter the blood circulation and are
transferred to another host via blood-feeding arthropods (vector hosts) eg Wuchereria
bancrofti,Brugia pahangi.
n Larvae are introduced by a vector but do not enter the circulation. They mature in a

subcutaneous location where they either actively escape, eg Dracunculus medinensis, or
are removed by a feeding arthropod eg Onchocercaspp.
n A cyst containing several spores is swallowed and the host’s digestive enzymes break

down the wall or shell of the cyst to release the spores. These then invade host cells
and undergo asexual multiplicative phases. After one or more asexual cycles, selected
individuals develop into male and female gametes. The male fertilises the female
ovum to produce a zygote and the nucleus of the zygote undergoes several nuclear divi-
sions to produce spores. The outer layers of the zygote produce a protective covering
membrane or shell and the structure now becomes a cyst which passes out of the host
eg Eimeriaspp.
n A vegetative stage such as a trophozoite is inoculated via a blood-feeding arthropod.

Host cells are invaded, followed by an asexual multiplicative phase and then the pro-
duction of male and female gametes. Fertilisation occurs when the gametes are taken
up by a feeding vector eg Plasmodiumspp.
n A vegetative stage such as an epimastigote is introduced during vector feeding. Host

cells are invaded followed by an asexual phase but no sexual multiplication eg
Trypanosomaspp.
n Eggs are swallowed by an intermediate host and the larvae hatch out in the gut. The

larvae migrate through the gut wall into the body tissues. They then undergo growth
and development into advanced larval stages. They become quiescent within the
intermediate host tissues and wait to be consumed by the definitive host eg Taenia
solium.
n Eggs are swallowed and develop as above but undergo a process of asexual multipli-

cation within the intermediate host eg Echinococcus granulosus.

n 1.2 THE NATURE OF PARASITISM
Parasites and in particular endoparasites have adapted to living in specific parts of the host.
The more specific the site the greater is the probability that the host–parasite relation-
ship is stable and old in terms of evolution. Although the host still attempts to remove
the parasite, the latter has evolved a whole strategy of avoidance mechanisms and, as long
as the parasite can survive to reproduce, the effect of its presence upon the host is of no
consequence.
Migration within the host, other than moving to a predetermined site, is usually an
indication that either the parasite has found itself in the wrong host or this is a relatively
new host–parasite relationship. Host reactions toward the parasite are a strong enough
stimulus to force the parasite to move, trying to find a protected site. Often the chosen
site is one where the host’s immune response is comparatively weak or non-existent, such
as the brain or eye.
Humans, in evolutionary terms, are the youngest of the mammals that inhabit the
earth. Domestic animals have come into existence mainly due to man selectively breed-
ing certain species and they too could be considered very young from an evolutionary
point of view.

INTRODUCTION TO PARASITOLOGY

Endoparasites have

become adapted to living

within hosts.