is found wherever there is intensive sheep farming and relatively damp conditions.
Sheep is the dominant definitive host but man and other herbivores can also become
infected. The adults, like the majority of trematodes, are hermaphrodite.
n The adult flukes inhabit the bile duct and have a flattened leaf-like shape. They grow
to about 35 ×15 mm. A mature individual produces eggs which pass into the bile duct,
then into the intestine and finally out of the host with the faeces. If the eggs are
deposited in fresh water, a free-swimming miracidium hatches out after 9 days.
The eggs have an operculum (a lid-like structure) through which the miracidium
emerges.
n The ciliated miracidium swims until it comes across Lymnaeaspp — a freshwater snail
— and penetrates through the snail’s soft tissues into the hepatopancreas. Once set-
tled, the germ cells within the miracidium develop into sporocysts and usually pro-
duce a second generation of sporocysts. From the germ cells within the sporocysts the
next generation forms — the redia. Within the redia the germ cells develop into cer-
cariae. The cercariae exit from the redia and eventually from the snail and settle on
grass. Once on a blade of grass they encyst and are now referred to as metacercariae.
n If a metacercaria is eaten the cercariae emerge from their cyst penetrate the wall of the
host’s gut and grow into juvenile flukes. The juveniles migrate through the abdominal
cavity over the visceral organs until they reach the liver. They actively penetrate the
liver and migrate through the hepatic tissues until they reach the bile duct.n SUMMARY
Endoparasites either have a direct life-cycle, that is involving only one host, or they have
an indirect life-cycle, ie more than one host. Almost all the parasites have a sexual mutli-
plicative phase (exceptions are some of the zooflagellates) which occurs in the definitive
host. In addition many parasites have an asexual multiplicative phase that may take place
in either the definitive or the intermediate host.
The distributive phases can occur when the parasite passes out of the host a larva or is
transmitted by an arthropod vector or if the intermediate host is eaten by the definitive host.End of chapter questions
Question 4.1 Name the different phases that can occur during a the life-cycle of
a parasite.
Question 4.2 What aspect of the parasite’s life-cycle determines the description of the
host?
Question 4.3 What is the difference between direct and indirect life-cycle?
Question 4.4 What does ‘internal accumulation’ mean with regard to parasite life-cycles?
Question 4.5 Give examples of parasites that have only one host and no asexual
reproduction.
Question 4.6 Name examples of parasites that reproduce asexually; and for which no
sexual reproductive phase has been observed.
Question 4.7 Which group of parasites have both asexual and sexual reproduction within
the same host?
Question 4.8 Using named examples explain the terms homogonic and heterogonic.
PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE