[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 8 The Nematodes
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TABLE 8-14
Parameter
Description
Average length 690 µm
Length of esophagus Long
Tail
Notched
TABLE 8-15
Parameter
Approximate size
Other features
Adult Female. Only the female adult S. stercoralis
has been described (Table 8-15). Measuring
approximately 2 by 0.4 mm, this small female
worm is equipped with a short buccal cavity as
well as a long and slender esophagus. The colorless
body appears almost transparent. A finely
striated cuticle covers the body. Probably because
no adult male S. stercoralis is known to exist, the
adult female is considered as parthenogenic,
because there are no obvious morphologic structures
to indicate that a male is not required for
fertilization.
Laboratory Diagnosis
Strongyloides stercoralis
Filariform Larva: Typical
Characteristics at a Glance
Strongyloides stercoralis
Adult Female: Typical
Characteristics at a Glance
Description
2 by 0.4 mm
Colorless, transparent body;
finely striated cuticle;
short buccal cavity;
Long and slender
esophagus
Laboratory diagnosis of threadworm may be
accomplished in several ways. Diagnostic eggs,
often indistinguishable from those of the hookworm,
may be present in stool samples from
patients suffering from severe diarrhea. Stool
concentration with zinc sulfate has successfully
recovered these eggs. Diagnostic rhabditiform
larvae may be recovered in fresh stool samples
and duodenal aspirates. It is important to note
that careful screening of feces is necessary to
distinguish the rhabditiform larvae of the hookworm
from those of S. stercoralis. Furthermore,
the Enterotest has proven successful in obtaining
the desired larvae and the hookworm-like eggs.
Sputum samples have also yielded S. stercoralis
larvae in patients suffering from disseminated
disease. It is important to note that threadworm
larvae have a typically higher recovery rate in
concentrated specimens than those in which a
flotation technique has been used. In addition to
these methods, several serologic tests, including
ELISA, have been developed.
Life Cycle Notes
There are three possible routes threadworms may
take in their life cycles—direct, similar in most
respects to that of the hookworm, indirect, and
autoinfection. Unlike in the hookworm life cycle,
in which eggs are the primary morphologic form
seen in feces, life cycle rhabditiform larvae in the
threadworm are usually passed in the feces. Eggs
are only occasionally found in such samples.
These rhabditiform larvae develop directly into
the third-stage infective filariform larvae in
warm, moist soil. The remaining phases of the
threadworm life cycle basically mimic those of
the hookworm life cycle.
In the indirect cycle, threadworm rhabditiform
larvae are passed into the outside environment
(soil) and mature into free-living adults that
are nonparasitic. These adult females produce
eggs that develop into rhabditiform larvae. These
larvae mature and transform into the filariform
stage, at which time they may initiate a new
indirect cycle or become the infective stage for a
human host and begin a direct cycle.
Autoinfection occurs when the rhabditiform
larvae develop into the filariform stage inside
the intestine of the human host. The resulting
infective larvae may then enter the lymphatic
system or bloodstream and initiate a new cycle of
infection.
Epidemiology
Similar to the hookworm, Strongyloides is found
predominantly in the tropical and subtropical
regions of the world. In addition to being present