PARASITE EXAMPLES GROUPED ACCORDING
TO LIFE-CYCLE
Nearly every vertebrate is infected with a parasite at some stage of its life. Many
of the parasites are host-specific. The parasites of man and domestic animals tend to
be the most well known and the most widely distributed and are the parasites mainly
investigated by parasitologists. In this section there are descriptions of some of these
parasites, described in relation to the number of hosts that they infect during their life-
cycle. The parasites using only one host during their life-cycle are considered to have
a direct life-cycle; and those with more than one host are described as having indirect
life-cycles.
n 4.1 DIRECT LIFE-CYCLES
The parasites invade only one host during their life-cycle. This occurs in both protozoan
and helminth parasites.
n 4.1.1ENTEROBIUS VERMICULARIS
A gut-dwelling nematode commonly known as the human pinworm. This parasite has
worldwide distribution and almost all humans (mainly during childhood) are infected for
a brief spell with this parasite.
n The adult females are 8–13 mm long; and the males are 2–5 mm long and 0.2–0.6 mm
wide.
n Embryonated eggs — the infective stage — are swallowed and the larvae hatch in the
gut and then migrate to the lower regions of the intestine, mainly the colon. The
larvae undergo at least four moults before maturing into adults. Fertile females move
down the colon and ‘crawl’ out of the anus onto the perianal region where they then
deposit their eggs. The females usually die or return to the colon (see Fig. 4.1).The most reliable method of diagnosis is to place a strip of cellotape along the anal
region. Any eggs deposited will stick to the tape. The tape is placed on a microscope slide
and then examined under a microscope for eggs.
The presence of worms and eggs around the anal region can be the cause of a cer-
tain amount of irritability and scratching. From the parasite’s point of view this helps
with dispersal. The eggs have a resistant shell and can withstand a certain amount of