[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 10 The Cestodes
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those of Taenia spp. (see Fig. 10-2). Fortunately,
the diagnostic stage of E. granulosus is that of
the larval stage described later.
Hydatid Cysts. The hydatid cyst larval stage
of E. granulosus, found in human tissue, consists
of several structures (Table 10-11). These
structures overlap somewhat in their definitions,
resulting in a confusion of the terms and
making a clear and concise description of them
a challenge. The following description should
be clear.
The entire structure is termed a hydatid cyst.
Within the cyst, miniaturizations of the entire
hydatid cyst may occur; these are referred to as
TABLE 10-11
Parameter
Protective coverings
Basic cyst makeup
Structures that arise
from inner germinal layer
Other possible structures
present
Ecbhinococcus granulosus
Hydatid Cyst: Typical
Characteristics at
a Glance
Description
Cyst wall; multiple
laminated germinal
tissue layers
Fluid-filled bladder
Daughter cysts
Brood capsules
Hydatid sand
daughter cysts. Both types of cysts are surrounded
by a protective cyst wall and laminated
layers of germinal tissue. In addition, brood
capsules, which lack a protective cyst wall, form
from the inner germinal layer. Developing scolices
are found within these structures. Each
scolex, once fully developed, has the capability
of developing into an adult worm when present
in the definitive host. A hydatid sand, defined
as components found in the fluid of older E.
granulosus cysts that typically include daughter
cysts, free scolices, hooklets, and miscellaneous
nondescript material may evolve, as shown in
Figure 10-17.
Some hydatid cysts fail to produce all or some
of these structures. In this case, some hydatid
cysts may start out by developing such structures
but, because of the onset of secondary bacterial
infections, may result in sterility or death of the
cyst, with subsequent calcification.
Adults. The average adult E. granulosus is
small, relatively speaking, measuring only
4.5 mm in length. The worm consists of a scolex,
small neck, and three proglottids, one at each
developmental stage—immature, mature, and
gravid. The scolex contains four suckers and has
approximately 36 hooks. This form is not typically
seen in humans but is commonly found in
canines, which serve as definitive hosts.
Laminated layers
of germinal tissue
and outer covering
Cyst contains
hyatid sand
Daughter cyst
Developing
scolex
Brood
capsule
Brood capsule
containing
developing
scolex
Cyst wall
Germinal layer
FIGURE 10-17 Echinococcus granulosus hydatid cyst.