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Invasive Tissue Parasites 205

cysts by eating meat infected by E. granulosus is well size of a large orange and has a laminated outer layer and
researched and accepted. an inner germinal layer of tissue capable of nourishing
and supporting the daughter cysts and brood capsules.
Morphology These smaller cysts bud from the germinal layer and
contain loose pieces of germinal tissues as well as
E. granulosus is well established as a hydatid tapeworm scolices, which are known as hydatid sand. The cyst is
of dogs, and the infection commonly causes neurologi- also filled with fluid. Humans become infected when
cal disorders in dogs whose nervous systems contain E. granulosus eggs are ingested accidentally, chiefly by
hydatid cysts. One old definition associated with the hand-to-mouth contact with infected dog feces. The
presence of hydatid cysts in dogs described the dogs as human is a dead-end host, so the life cycle is halted
suffering from “running fits.” The eggs of E. granulosus at this stage. Therefore, humans who suffer from this
are almost never found in fecal specimens but when they disease do not spread the organisms to other mammals.
are recovered they appear almost identical to those of the
Taenia spp. (described earlier). The worm of E. granulo- Disease Transmission
sus is only about 4 mm in length and consists of a scolex
with four suckers, numerous hooks, and only three pro- Transmission of the disease to humans, a dead-end host,
glottids, unlike other cestodes; for example, T. solium, as occurs when the eggs of E. granulosus are ingested by
well as a number of other tapeworm species, may include humans from contact with infected dog feces. These eggs
up to 1000 proglottids. then are transported to body tissues throughout the body,
where they produce hydatid cysts. Those who work with
Symptoms herds or flocks of animals and where herd dogs are used
are especially vulnerable to infection with this organism.
Humans are not definitive hosts, so the adult tapeworm Hand-to-mouth contact while eating in the field and
does not infect humans but does inhabit the intestines while working with animals lends itself greatly to trans-
of members of the canid family (dogs, foxes, wolves, mission of the infection.
etc.). In humans a hydatid cyst may develop in one
or more tissues and organs of the body, producing Laboratory Diagnosis
symptoms related to the organ affected. Cysts may
develop in any area of the body but the lungs and liver Since E. granulosus eggs are not often found in humans,
are most frequently impacted, followed by organs of the radiographic and serological studies are primarily used
central nervous system. Pulmonary symptoms may also for diagnosis of echinococcosis. A demonstration of the
arise, with coughing accompanied by chest pain and presence of hydatid sand in the cyst(s) is also a diagnos-
lung infection. The presence of hydatid cyst disease is tic laboratory tool that may be employed for a definitive
also capable of eliciting anaphylactic shock if the cyst diagnosis and to ensure eradication of the parasite when
ruptures. no more cysts are present in the body.

Life Cycle Treatment and Prevention

Herbivores such as sheep and other cattle act as Surgical removal of hydatid cysts may be necessary, and
intermediate hosts after ingesting ova from pastures several anti-helminthic medications are also available for
where the ground and the pasture grass are contaminated treatment. Good personal hygiene is effective in prevent-
by feces from dogs or other canids. When carnivores ing hand-to-mouth transmission of eggs from dog feces
ingest the tissues of these animals, which contain the to humans. But the most effective manner for control-
infective cysts, they in turn become infected. Each ling the transmission of the disease can be accomplished
cyst contains a scolex that can develop into an adult by preventing dogs and other carnivorous animals from
tapeworm. The tapeworm itself resides in the small eating sheep viscera and other organs. Routine and
intestines of dogs and related species, but does not regular treatment of working dogs for parasites is quite
primarily infect humans. A hydatid cyst can become the effective.