[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 10 The Cestodes
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presence of six small hooks (called hooklets),
arranged in pairs, that are believed to pierce the
intestinal wall of the infected host. In the intestinal
tapeworm life cycle, human ingestion of an
egg or larval stage results in an adult worm eventually
emerging in the intestine. An intermediate
host is required for the development of the larval
form in certain life cycles. It is important to note
that under no known circumstances is the larval
form seen in human specimens. Adult tapeworms
resemble a ribbon in appearance and range in
length from several millimeters up to an impressive
15 to 20 m. These worms are very primitive
in that they absorb nutrients and excrete waste
products through their outer surface, called a
tegument. Although this group of adult parasites
possess a reproductive system, they lack sophisticated
body parts and systems, such as a mouth,
digestive tract, and internal means of excretion.
Three distinct features common to all adult
tapeworms are the presence of a scolex (a defined
anterior end), a neck region, and a series of proglottids
(individual segments that in their mature
form are equipped with both male and female
reproductive organs), referred to as the strobila.
The typical scolex contains four cup-shaped
structures, known as suckers, that provide the
worm with the ability to attach to the intestinal
mucosa of the infected host. Some species have
a fleshy extension of the scolex known as a rostellum,
from which one or two rows of hooks
might be present.
The internal structures that are visible vary
with the age of the proglottid. Because all
tapeworms are self-fertilizing (hermaphroditic),
both male and female reproductive organs are
present in the mature proglottid. Following selffertilization,
the resulting pregnant (gravid) proglottids
each consist of a uterus filled with eggs.
The gravid proglottids rupture when these eggs
are released into the intestine. The resulting
eggs are usually passed into the outside environment
via the stool.
There are several other life cycle notes of
importance. Because tapeworms are hermaphroditic,
human ingestion of a single egg will usually
initiate a new life cycle. Autoreinfection is known
to occur in the life cycle of Hymenolepis nana.
Most cestodes require at least one intermediate
host for their life cycles to continue. Development
of a cyst in tissue occurs in the intestinalextraintestinal
cestode species Echinococcus
granulosus. Specifics of both organisms’ morphology,
as well as life cycle notes, are described
individually.
Quick Quiz! 10-1
The cestode morphologic form characterized by a
segmented appearance that houses male and female
reproductive structures is referred to as a(an): (Objective
10-1)
A. Scolex
B. Proglottid
C. Egg
D. Cyst
LABORATORY DIAGNOSIS
The primary specimen necessary to recover and
identify intestinal tapeworms is stool. These
samples are generally examined for the presence
of eggs and, occasionally, gravid proglottids, which
may be partially degenerated. Rarely, a scolex may
be recovered, particularly following treatment. A
biopsy of tissue presumed to be infected with the
atrial cestode E. granulosus may be examined for
the presence of organisms. In addition, serologic
tests are available for select organisms. Representative
laboratory diagnosis methodologies are
located in Chapter 2 as well as in each individual
parasite discussion, as appropriate.
Quick Quiz! 10-2
Characteristics of the cestodes include all the following
except: (Objective 10-9)
A. They are hermaphroditic.
B. They generally require intermediate host(s).
C. Their laboratory diagnosis consists of finding
larvae in feces.
D. Their anatomic regions include the scolex, neck,
and strobila.