[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 10 The Cestodes
Laboratory Diagnosis
E. granulosus may be diagnosed in several ways.
Hydatid cyst fluid may be examined on biopsy
samples for the presence of scolices, daughter
cysts, brood capsules, or hydatid sand. Care
must be used when choosing this method of diagnosis
because infected patients may suffer from
anaphylaxis if fluid escapes from the hydatid cyst
during specimen collection. Serologic tests such
as ELISA, indirect hemagglutination, and the
Western blot test are available. Detection of the
hydatid cyst may be accomplished using radiography,
computed tomography (CT), or ultrasound
scan techniques.
Life Cycle Notes
Humans serve as accidental intermediate hosts
for E. granulosus. The typical intermediate host
is sheep, although other herbivores may also
serve in this role. Human infection begins following
the ingestion of Echinococcus eggs obtained
by contact with contaminated dog feces. Larvae
from the eggs penetrate the intestine and migrate
via the bloodstream to a number of tissue sites,
particularly the lung and liver. A hydatid cyst
develops in the infected tissue. Humans are considered
to be dead-end hosts because the Echinococcus
life cycle ceases in human tissue.
When the sheep serves as the intermediate
host, the life cycle may be completed. The hydatid
cyst forms in the viscera (soft parts and internal
organs of major cavities in the body) of the
sheep. The definitive host, the dog or wild canine,
contracts the parasite by consuming infected
sheep viscera. On ingestion of the cyst, each
scolex develops into an adult worm. The adult
worms reside in the definitive host’s intestine.
Eggs are produced and passed into the environment
via the feces, in which they are capable of
initiating a new cycle.
Epidemiology
E. granulosus is primarily found in areas in
which sheep or other herbivores are raised and
are in close contact with dogs or wild canines.
Other criteria for areas at risk include those in
which close contact between canines and humans
occurs. These areas include Great Britain, parts
of South America, Australia, parts of Africa,
Asia, and China, and select portions of the
Middle East. There have been several cases
reported in the United States, particularly in
Alaska, as well as in the West and Southwest.
Clinical Symptoms
Echinococcosis: Hydatid Cyst, Hydatid Disease,
Hydatidosis. The extent to which infected patients
may experience discomfort varies, depending on
the size and location of the hydatid cyst. In
general, patients experience little if any symptoms
for approximately one year or more following
ingestion of the Echinococcus eggs. As the
cyst continues to enlarge, necrosis of the infected
tissues, accompanied by a buildup of pressure on
these tissues, usually results. Death may also
occur. Rupture of a cyst may occur naturally or
during the process of obtaining a biopsy for
microscopic examination. Patients may suffer
from anaphylactic shock, eosinophilia, and allergic
reactions, or even death. The cyst fluid that
emerges from the rupturing cyst, under the right
conditions, can spread to other sites and form
new cysts.
Patients suffering from E. granulosus lung
infection may develop chest pain, coughing, and
shortness of breath. Liver involvement may result
in obstructive jaundice. Symptoms related to cyst
development in other body organs are sitespecific.
Further discussion of these symptoms is
beyond the scope of this chapter.
Treatment
Surgical removal of the hydatid cyst, when
located in a suitable area for surgery, has historically
been considered as the treatment of choice
for Echinococcus. However, the advent of antiparasitic
medications has offered an alternative
to surgery, if appropriate. The medications
mebendazole, albendazole, and praziquantel