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CHAPTER 10 The Cestodes
TABLE 10-5
Hymenolepis nana Egg:
Typical Characteristics at
a Glance
Parameter Description
Average size 45 by 38 µm
Hooklets
Three pairs; hexacanth embryo
Polar thickenings Present
Polar filaments Present
Embryophore Present; colorless
TABLE 10-6
Parameter
Scolex
Number of suckers
Rostellum
Hooks
Gravid Proglottid
Size
Appearance
Hymenolepis nana Adult:
Typical Characteristics at
a Glance
Description
Four
Present; short
Present; one row
Twice as wide as long
Saclike uterus filled with eggs
Short rostellum
with hooks
Suckers
FIGURE 10-11 Hymenolepis nana scolex.
H. diminuta. A colorless embryophore serves as
the outermost egg layer.
Scolices. Like the other cestode scolices discussed
thus far, the H. nana scolex contains four
suckers (Fig. 10-11; Table 10-6). The presence of
a short rostellum with one row of hooks helps
distinguish it from that of H. diminuta, which
has no hooks at all.
Proglottids. The mature and gravid proglottids
of H. nana resemble those of H. diminuta
both in size and appearance (see Fig. 10-9 and
Table 10-6). A brief description may be found in
the H. diminuta proglottid morphology section.
Laboratory Diagnosis
Laboratory diagnosis of H. nana is accomplished
by examining stool samples for the characteristic
eggs.
Life Cycle Notes
Consumption of an infective egg initiates human
H. nana infection. Development of the cysticercoid
larva occurs in the intestine. On further
maturation, the scolex emerges and attaches to
the intestinal mucosa. The resulting adult worm
resides in the intestine, in which it is capable of
self-reproduction. Disintegration of gravid proglottids
releases numerous eggs. At this point, a
resulting egg may take one of two routes. The
egg may be passed outside the body via the feces
or initiate autoreinfection. An egg released into
the outside environment is in the infective stage.
No intermediate host is required to complete the
cycle. Such an egg, when ingested by a new
human host, initiates a new cycle. An egg that
remains inside the human may hatch in the gastrointestinal
tract and develop into an adult,
never leaving the human host and thus initiating
a new cycle.
As noted, H. nana does not require an intermediate
host to complete its life cycle. However,
this parasite may exist in a number of other
animal transport hosts, such as fleas, beetles,
rats, and house mice. It is interesting to note that
the cysticercoid larval stage may develop in these
hosts; when this occurs; such hosts are infective
to both humans and rodents.
Epidemiology
H. nana is considered to be the most common
tapeworm recovered in the United States, particularly
in the southeastern part of the country.