Page 134 - Medical Parasitology_ A Textbook ( PDFDrive )

Cyclophyllidean Tapeworms 127

Taenia saginata asiatica

Taenia saginata asiatica is closely related to T. saginata and is found mainly in
Asia. It is morphologically similar to T. saginata except it is smaller in size. Pig is
its intermediate host and its cysticerci are located primarily in liver of pig. Clinical
features, diagnosis and treatment are similar to that of T. saginata.

Echinococcus granulosus

Common name Dog tapeworm

Distribution
The hydatid disease caused by E. granulosus is prevalent in most parts of the world
and is most extensive in the sheep and cattle rearing countries (Australia, Africa and
South America).

Habitat
The adult worm lives in the small intestine of dogs and other canine. The larval
stage (hydatid cyst) is found in humans and herbivorous animals (sheep, goat, cattle
and horse).

Morphology
Adult worm measures 3–6 mm in length. It consists of a scolex, neck and strobila.
The scolex is pyriform, with 4 suckers and a rostellum bearing 2 circular rows of
hooklets. The strobila is composed of 3 proglottids; immature, mature and gravid
proglottids. The gravid proglottid contains branched uterus filled with eggs. The
lifespan of the adult worm is 6–30 months.
The eggs of Echinococcus are indistinguishable from those of Taenia species.
The larval form (hydatid cyst) develops in various organs of the intermediate
host (Fig. 11.8). At the site of deposition, the embryo slowly develops into a
hollow bladder or cyst filled with fluid. This becomes the hydatid cyst. It
enlarges slowly and reaches a diameter of 0.5–1 cm in about 6 months. The
growing cyst evokes host tissue reaction leading to the formation of fibrous
capsule around it. The cyst wall secreted by the embryo consists of 3 layers;
pericyst, ectocyst and endocyst. The cyst is filled with fluid which is antigenic
and its release into circulation may cause anaphylaxis.
From the germinal layer of the endocyst, small knobs protrude into the lumen
of the cyst. These enlarge, become vacuolated and are filled with fluid. These are
brood capsules which are initially attached to the germinal layer by stalks, but
later are detached and released into the fluid filled cyst cavity. From the inner wall
of the brood capsules, protoscolices develop. Inside mature hydatid cysts, further