PARASITOLOGY

n The infective stage larvae are released into the wound as the fly feeds. Once in the host’s

subcutaneous tissues they moult and develop into adults and remain in the host for up

to 4 years.

n 4.5.4DRACUNCULUS MEDINENSIS
D. medinensis(a nematode) is distributed throughout India, the Middle East, Central
Africa and S. America. It is a zoonotic parasite and apart from man can live in monkeys,
horses, dogs, raccoons, foxes and cattle. The intermediate host is a freshwater copepod,
Cyclopsspp, Mesocyclopsspp and Thermocyclopsspp.

n The adult worms usually live in the lower limbs of the definitive host. The female is

the larger with a length of about 100 cm ×1.5 mm and the male is 40 × 0.4 mm.
n The female in the lower limbs positions itself with the oral cavity facing downwards.

Around the mouth region the host tissue becomes ulcerated. As the female matures the
uterus undergoes a ‘prolapse’ with the result that the uterine opening lies close to the
oral cavity. An ulcer forms on the host’s skin around the anterior end of the worm.
When the limb comes into contact with water the ulcer ‘bursts’ and motile larvae are
released from the uterus into the oral cavity and these then escape into the water.
n If the larvae are eaten by a copepod they develop, after about 3 weeks, in the cope-

pod’s haemocoel into an infective L 3 larva.
n When the copepod is swallowed by the definitive host the larvae are released into the

lumen of the host’s intestine. The larvae migrate through the gut wall into the con-

nective tissues and eventually to the subcutaneous tissues of the limbs. The migration

takes about a year and during this period there are two further moults.

n 4.6 PARASITES WITH TWO HOSTS AND FREE-LIVING
DISTRIBUTIVE PHASES
All of the Eucestoda (with a few exceptions) use two hosts to complete their life-cycle.

n 4.6.1TAENIASPP
Taenia saginata(the beef tapeworm) — also known as Taeniarhynchus saginatus— and Taenia
solium(the pork tapeworm) adults normally live only in the human gut. Both parasites
are long-lived and have a worldwide distribution. T. saginatacan grow to the length of
10 m and T. solium6 m and both are normally found within the small intestine.

n A scolex of a mature adult becomes embedded within the gut mucosa and is anchored

by four suckers. T. soliumhas a ring of hooks, the rostellum, at the tip of the scolex.
T. saginatadoes not have a rostellum. Behind the scolex is the ‘neck’ region from
which the proglottids originate. The youngest proglottid is nearest to the neck region
and the oldest furthest away. Each proglottid contains a set of male and female organs.
n It has been speculated that cross-fertilisation occurs between different proglottids.

The gravid proglottids, ie those containing only fertilised eggs, are shed into the gut
lumen and pass out via the faeces. In some cases the proglottids are shed as ‘ribbons’
out of the anus. Some shed proglottids can ‘wriggle’ across the grass where they are
ingested by the intermediate host.
n If the proglottids are eaten, the embryonated eggs are released in the gut of the inter-

mediate host. Alternatively the eggs are swallowed via the vegetation.
n The shelled eggs contain a hexacanth embryo (the oncosphere). The gut environment

stimulates the oncosphere to hatch out of the egg. The oncosphere is motile and

PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE