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Invasive Tissue Parasites 207

where development begins into hydatid cysts. These protoscolices, producing hydatid sand, which is what is
cysts may be found in many parts of the body but are fre- depicted in Figure 9-9.
quently found in the liver, lung, and particularly in the The second echinococcal organism, Echinococ-
central nervous system. cus multilocularis, invades the tissues of humans who
Inside the intermediate host of an herbivore, or serve as dead-end hosts. It produces an alveolar-type
typical grass eater, which has eaten infected vegetation, cyst, which contains an inner, nucleated germinal layer
eggs hatch and release very small embryonic forms called that gives rise to brood capsules. As Echinococcus mul-
oncospheres that contain hooks and travel through the tilocularis infections progress, larger and larger cysts de-
circulatory system. These oncospheres eventually gain velop in intermediate hosts. Symptoms of infection with
a hold in the organs of the body, and are chiefly found E. multilocularis arise as the cysts progressively grow
in the liver, kidneys, and lungs, where they develop into larger and begin eroding the surrounding tissue and
hydatid cysts. These cysts are a hollow bladder-like perhaps placing pressure on blood vessels and other
structures where brood (breeding) capsules are formed, organs. Small secondary parasitic cysts, usually a de-
and are sometimes attached to a mother cyst, sometimes rivative of a hydatid cyst called a mother cyst, contain
called a unilocular type of hydatid (Figure 9-8). brood chambers which increase the number of organ-
The scolex (plural is scolices) is the headlike struc- isms (Figure 9-10). Large cysts may eventually induce
ture of tapeworm organisms, and these newly formed anaphylactic shock, which may be fatal if the cysts do
organisms grow inside the brood capsules. Inside these rupture.
cysts grow thousands of tapeworm larvae called hydatid
sand, which is the next stage in the life cycle of the para-
site. When a predator eats the intermediate host, the lar-
vae that are eaten develop into adult tapeworms in the
intestine and the infection cycle restarts. Again, because
humans do not ordinarily eat carnivores, humans do
not spread the tapeworm for this species in the way that
meat-eating animals do. During their life cycle within Source: Centers for Disease Control and Prevention (CDC)
the host, these tapeworms pass through an egg stage
followed by an oncosphere stage. The next step of the
life cycle is the formation of a cystic stage which under-
goes enlargement, thereby producing protoscolices and
daughter cysts. Rupture of these cysts releases the larval

FIGURE 9-9 Rupture of cysts release larval
Source: Centers for Disease Control and Prevention (CDC) Source: Centers for Disease Control and Prevention (CDC)
protoscolices, resulting in hydatid sand

FIGURE 9-8 Dissected (pronounced dis– sected, not
di– sected, as there are two s’s) rat showing evidence of
echinococcosis due to Echinococcus multilocularis in
organs at 45 days FIGURE 9-10 Membrane and hydatid excised cyst