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206 CHAPTER 9

OTHER COMMON
TISSUE-INVASIVE PARASITES
Echinococcal Organisms

The second species of the genus Echinococcus that forms
hydatid cysts that invade and reproduce in various tis- Source: Centers for Disease Control and Prevention (CDC)
sues of the human body is E. multilocularis. As with so
many other parasite infections, the course of an Echi-
nococcus infection is complex. This worm has a life cycle
that requires two types of hosts: a definitive host and an
intermediate host. Typically the definitive host is a car-
nivore such as a dog or a wolf. The intermediate host
FIGURE 9-6 Morphology of an adult cestode,
is usually an herbivore such as sheep or cattle, but the Echinococcus granulosus
disease is most prevalent in sheep. As in infections with
E. granulosus, humans function as accidental hosts and
are usually known as a dead-end host, because humans
do not pass the organisms to other species or individuals
during the infection and reproductive cycles. This is pri-
marily due to the fact that most humans who are infected
seldom or never defecate in the woods or fields but in
restroom facilities. Source: Centers for Disease Control and Prevention (CDC)
Echinococcus granulosus is tapeworm that chiefly
infests dogs and other carnivores. The larval form of
this organism is called a hydatid and develops in other
mammals. The disease cycle begins when the adult tape-
worm gains entry and attaches to the gut of the definitive
hosts. The definitive host is usually a carnivore and can
FIGURE 9-7 Scolex of Echinococcus granulosus from
either be of the canine (dog) or the feline (cat) lineage.
hydatid cyst
The adult tapeworm then produces eggs in the fecal
waste from the host and the eggs are eliminated from the
body of the host into the soil. Then an intermediate host consist of a scolex and only three proglottids, unlike the
such as a cow, goat, or sheep becomes infected by eating lengthy number of proglottids exhibited by other species
the grass where the eggs of the parasite may be found. of cestodes. The third (terminal) proglottid is the only
Prepared foods contaminated by feces containing the gravid link and is longer than it is wide as can be seen
E. granulosus eggs may, on rare occasions, also be a in the following image. The scolex contains four suckers
source of infection for a human. The following illustra- and a rostellum with 25–50 hooks (Figure 9-7).
tion reveals the typical structural morphology found in
an adult cestode, Echinococcus granulosus, which was re- Life Cycle
covered from the bodies of dogs (Figure 9-6).
Because dogs and other canids such as foxes and wolves
Morphology are definitive hosts for Echinococcus spp., humans are
only infected by the larvae after ingesting either food
E. granulosus causes the disease known as cystic containing these larvae or by drinking contaminated
echinococcosis. As dogs and other canids are the only water or contact with fomites (something to which feces
definitive hosts for Echinococcus, adult forms of the or- may cling), which is also contaminated with dog feces.
ganism are not expected to be found in the human host. After the human host ingests the eggs, the oncospheres
Adult worms range from 3 to 6 mm in length and usually move from the intestinal lumen to other areas of the body,