migrates from the gut mucosa into and through the epithelium to be distributed round
the body. Skeletal muscle is the common destination for the larva and once on or
within the muscle, it develops into a cysticercus (bladderworm), the metacestode
stage.
n A cysticercus is a round fluid-filled cyst and folded in from the outer tegument into
the fluid is an invaginated scolex. The cysticercus remains in an inactivated state
within the muscle until eaten by the definitive host. The scolex evaginates in the gut
of the definitive host and attaches to the gut epithelium; and development of the adult
stage then commences.n 4.6.2DIPHYLLOBOTHRIUM LATUM
The adult tapeworm lives in the gut of humans as well as fish-eating carnivores. In
humans the adult worm can reach a length of 10 m.n The scolex has two attachment organs called bothria and no hooks.
n Mature proglottids have a centrally located uterine pore and the eggs are shed into the
gut lumen and pass out with the faeces. If the eggs reach fresh water, a free-swimming
coracidium larva hatches out.
n The coracidium is eaten by the first intermediate host Diaptomas(a small crustacean)
and develops into the first metacestode stage — the procercoid — in the body cavity
of Diaptomas.
n The crustacean is eaten by a fish — the second intermediate host — and the procer-
coid burrows through the intestinal wall and into the muscle where it develops into a
plerocercoid — the second metacestode stage.
n The fish is eaten by man or a fish-eating mammal and the plerocercoid matures in the
small intestine into an adult worm. Should the fish be eaten by another predator fish
the plerocercoid remains in the muscle of the predator fish which simply acts as a
carrier host.n 4.7 CESTODES WITH AN ASEXUAL MULTIPLICATIVE PHASE
IN THE INTERMEDIATE HOST
4.7.1ECHINOCCUS GRANULOSUS
In contrast to the tapeworms already described the adult of E. granulosusis the smallest
tapeworm (see Fig. 4.8). The metacestode stage is known as a hydatid cyst and when fully
developed is one of the largest of the metacestodes.n The definitive host is usually a carnivore, with the domestic dog the most frequently
detected host. The adult tapeworm consists of a scolex, with at most three proglot-
tids and is only 5 mm long. An infected dog can harbour 1,000+adult worms.
However the total biomass is probably less than one adult T. saginata.
n After fertilisation the end proglottid becomes filled with embryonated eggs (gravid).
It then detaches and fertilised eggs are released into the gut lumen which escape from
host via the faeces.
n Man and several different types of herbivore are potential intermediate hosts. The
eggs are swallowed and the digestion processes of the host help the oncosphere to
hatch.
n The oncosphere moves into the gut mucosa and then penetrates through the gut
epithelium into the lamina propria. Once inside the host’s tissue the larva migrates
via the blood circulation until it reaches one of the larger organs, such as the liverPARASITOLOGY