PARASITOLOGY
n 7.6. 3BIOLOGY OF E. GRANULOSUS
This cestode is associated with domestic animals and humans; and has a worldwide dis-
tribution. The adult E. granulosusconsists of only three proglottids, with a total length of
5 mm; and inhabits the small intestine of the canine family, dogs, jackals and hyenas. The
metacestodes, known as hydatid cysts, develop from hatched eggs within intermediate
hosts, which are mainly herbivorous animals and man. The most common intermediate
hosts are sheep, horses and cattle.
In a mature adult worm eggs accumulate in the posterior segment/proglottid which
when ripe (gravid) disintegrates, releasing the eggs which pass out of the host via the
faeces. The eggs containing the oncosphere larva are swallowed by grazing animals. The
oncosphere hatches out into the small intestine, burrows into the gut wall and migrates
round the body via the blood circulation. The liver and lungs are the most common
organs in which the larvae settle, however they sometimes find their way into other
organs.
Once the larva has reached a suitable site, it settles and secretes a surrounding hyaline
membrane. The membrane undergoes a process of development and differentiates
into an inner germinal layer and an outer acellular laminate layer. This structure, which
has an outer appearance of a small bladder, becomes fluid-filled and over a period of
time enlarges into a hydatid cyst. The innermost layer has two functions: it forms the
germinal layer which produces the protoscoleces; and secretes more of the outer layer.
As the cyst increases in size the germinal layer loops inwards to form brood capsules
and, in each of the brood capsules, the germinal membrane continues to produce pro-
toscoleces. Eventually a large fluid-filled cyst develops containing numerous brood
capsules.
It can take several months for a cyst to mature and they vary in size, 2–20 cm. Once
a hydatid cyst is eaten by the definitive host, the numerous protoscoleces are released
into the host’s intestine, become attached to the gut mucosa by means of hooks and
suckers and each grows into an adult tapeworm.
Adult tapeworms appear to have little or no obvious effect upon the host. However
in the intermediate host the hydatid cyst increases in size and its presence causes the dis-
ease known as hydatidosis. Should the cyst rupture or be damaged in any way, the
released brood capsules can establish themselves in new tissues and form into secondary
cysts.
A growing hydatid cyst is first surrounded by inflammatory tissue and the tissue in which
it is embedded often becomes soft, tender and necrotic. Eventually the inflammatory cells
are replaced by fibrous/scar tissue and the infected area hardens.
In the lungs, the pulmonary tissue adjacent to the cysts is replaced by inflammatory
tissue and thickens into layers of collagen and fibrogen. Histological observations shown BOX 7.5
Hydatidosis is the disease caused by this parasite and has been known for a long time.
Hydatid cysts recovered form humans were first described by Pallas in 1766. In 1782
protoscolices were first described by Goeze who found them within a cyst. Adult worms
recovered from dogs were first described in 1853 by von Siebold and this was followed by
the 1863 by Nauyn who fed dogs with the contents of hydatid cysts obtained from humans
and recovered adult worms.