| The parasitic
illnesses that cause disease in man are produced
by Protozoa and Helminths. Protozoa are unicellular
eukaryotic organisms. Helminths are multicellular
worms and are divided into round worms (nematodes),
tenias (cestodes) and staves (trematodes).
Equinococosis
or hydatid disease is caused by the cestode type
in its larval or cystic stage. Four species of
parasite have been identified: Echinococcus granulosus
(by far the most frequent), and E. obligarthrus,
E. Patagonicus and E. Multilocularis. The commonest
host is the dog and other carnivores. The intermediate
hosts affected are sheep and cows, but man and
other mammals can be infected through contact
with dog wastes and consumption of the eggs contained
in them. Many of the larvae are destroyed but
some are encysted and grow.
The most frequent
locations of the hydatid cyst are the liver (two
thirds of patients), lung and peritoneum, but
they can exist in many locations including spleen,
bone, soft parts, heart etc. When the hydatid
cyst is viable, skin tests and complement fixation
are often positive and eosinophilia is common.
The death of the parasite leads to collapse of
the cyst, wall necrosis and calcification. Laboratory
diagnosis can be done using hydatid serology
and confirmed or established with ultrasonography
or computerized tomography. In liver equinococosis
frequently there is communication between the
hydatid cyst and the biliary tract with superinfection.
Cysts wchich burst in the peritoneal cavity can
produce a fatal anaphylactic reaction or lead
to the formation of numerous granulomas and multiple
cysts. In pulmonary equinococosis communication
of the cyst with the bronchial tree may give
rise to expulsion of hydatid vesicles through
the mouth in the form of vomiting.
Macroscopically
hydatid cysts are solitary in about two thirds
of cases, with a size ranging from 1 to 7cm. (Fig.1).
Viable cysts are full of transparent clear liquid
that contains daughter vesicles with a scolex
(head) in their interior that can be seen like
a stone in the hydatid liquid.
Microscopically
the cysts show an external chitinous rolling
layer, characteristically anuclear, and an internal
nuclear layer or germinative layer. Initially
the cysts appear like small projections of the
germinative layer that are transformed into vesicles
(Fig. 2) with the scolex of the worm interiorly
(Fig.3). The hydatid cyst can be surrounded by
a weaving of granulation tissue or a fibrous
layer (called pericystic) that represents the
inflammatory reaction of the host. The fibrous
layer may be calcified. The parenchyma often
shows signs of atrophy by compression and chronic
inflammatory infiltrate with the presence of
abundant eosinophils.
Author:
Tomás Castiella Muruzábal
Servicio de Anatomía Patológica
Hospital Clínico Universitario “Lozano
Blesa”
Zaragoza
Translation:
Kelly Watt
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