Hematology and Clinical Microscopy Glossary - College of American ...

Pinworm Preparations
Helminths (Includes Pinworm)
Humans are a common host for Enterobius
vermicularis (pinworm), and the number of human
infections is estimated at 209 million cases worldwide,
with the highest prevalence of infestation in children
ages five to 14 in temperate, rather than tropical,
zones. Adult pinworms inhabit the human appendix,
cecum, and ascending colon without invasion of the
intestinal mucosa. The gravid female descends the
human colon nocturnally, emerging from the anus
and crawling over the perianal/perineal/vaginal
areas to deposit her eggs; each female worm harbors
about 11,000 eggs. The eggs are not usually shed
within the lumen of the human intestine, in contrast
with those of other parasites; thus, the standard “O&P”
stool exam is unlikely to reveal pinworm eggs.
Ova are laid in the perianal region of the human host
by the gravid female pinworm and embryonate to
the infective first stage within four to six hours.
Infection is usually by direct transmission of eggs to
mouth by hands or through fomites (dust particles
containing infective eggs). As anal pruritus is a
common symptom due to migration of the egg-laying
female worm through the anus, and since children are
the most common hosts, scratching with subsequent
finger-sucking produces autoinfection. Some eggs
may hatch in the perianal region, with these larvae
reentering the rectum and maturing into adults
(retroinfection).
Egg morphology is highly characteristic for Enterobius.
They are elongate or ovoid, with a thick, colorless
shell, 50 to 60 μm long and 20 to 32 μm wide. Typically,
they are conspicuously flattened on one side, which
helps distinguish them from hookworm eggs, which
also have thinner shells. The egg of the whipworm
(Trichuris trichiura), another human colonic nematode,
is about the same size as a pinworm egg, but is
barrel-shaped with a transparent plug at each end.
Specimen collection is by cellophane tape or
Graham technique (adhesive cellophane tape is
firmly applied to the uncleansed perianal area in
the morning). The tape is then applied to a glass
microslide on which a small amount of toluidine has
been placed to partially clear the tape and eliminate
distracting air bubbles. Alternatively, there is an anal
swab technique using paraffin/petroleum jelly-coated
cotton swabs, or the surface of stool specimens may
be gently scraped to remove adherent Enterobius
Clinical Microscopy Miscellaneous Cell
eggs. Multiple samples over several days may be
necessary to establish the diagnosis.
Strongyloides stercoralis (rhabditiform larva) is a tiny
intestinal nematode where the mature form and eggs
are rarely seen. However, the rhabditiform larvae can
be found in the duodenal contents and stool and
comprise the diagnostic form. The larva is small and
slender, measuring about 225 by 16 μm. The head has
a short buccal cavity, distinguishing it from hookworm
larva which have long buccal cavities. The tail is
notched, in contrast to the pointed tail of hookworm
larvae.
References
Smith JW, Gutierrez Y. Medical parasitology. In:
Henry JB, ed. Clinical Diagnosis by Laboratory
Methods. 20th ed. Philadelphia, PA: WB Saunders;
2001.
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