n 3.11.4 LIFE-CYCLE
The eggs hatch into a first stage larva. This moults into a second stage larva. There are
usually four moults and the fifth stage usually develops into the adult.n BOX 3.1 EXAMPLES OF NEMATODES
Enterobius vermicularisOnly one host during its life-cycle. The adult worms live in the gut.
The female migrates to the perianal region at night to lay eggs. The eggs are swallowed
(oral infection).
Ascaris lumbricoidesOnly one host during its life-cyle. The adult lives in the gut and the
cycle begins with eggs being swallowed. Larvae hatch out of the eggs in the small intes-
tine and migrate through the gut wall to enter the blood circulation. They migrate to the
heart and lungs then back to the alimentary canal. During migration the larva moults and
matures into the adult form within the gut.
Trichinella spiralisAdult male and female live in the gut. The female produces live
larvae which then burrow into the intestinal wall and migrate to the striated muscle where
they encyst. Both male and female adults are then expelled from the gut after 14 days. The
larvae remain until the host is eaten and then they excyst and mature within the new host.
Wuchereria bancroftiThe adult worms occur in tightly coiled masses in the lymphatic
ducts. Female produces live larvae which enter into the blood circulation and are then trans-
mitted to a new host by a mosquito.
Toxocara canisPregnant infected bitch reactivates dormant L 3 larvae by hormonal
changes during pregnancy. In a lactating bitch the larvae are passed to puppies via the
milk. In bitch and puppies the larvae migrate from gut to liver, lungs and trachea. Coughed
up and swallowed, they develop into L 4 larvae and adults and then release eggs.n SUMMARY
The characteristics and an outline classification of the Phylum Platyhelminthes are presented.
The basic morphology and physiology of a typical cestode, trematode and nematode
are described. The general plan of the life-cycles of each group is outlined. The cestodes
have several different types of metacestode (larval stages) which occur within the inter-
mediate hosts; and it is some of these that are the cause of severe pathology in humans
and domestic animals.
The trematodes have an even more complex life-cycle but the intermediate host is always
a species of mollusc. The basic life-cycle and variations that occur are outlined. The nema-
todes have a more regular pattern to their life-cycles. Normally the larvae moult four times
into the next stage before reaching sexual maturity.End of chapter questions
PLATYHELMINTHS
Question 3.1 What are the distinguishing features of the phylum Platyhelminthes?
Question 3.2 How many body layers are there in platyhelminths and what are the layers
called?PARASITOLOGY