[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

CHAPTER 8 The Nematodes
193
Uterus
Intestine
Oviduct
Anus
Ovary
Vagina
Vulva
Size range: 7-14 mm long by up to 0.5 mm wide
FIGURE 8-3 Enterobius vermicularis adult female.
Esophagus
Mouth
TABLE 8-2
common name pinworm. The rarely seen adult
male worms are also yellowish-white in color
and typically are smaller in size than the females,
ranging from 2 to 4 mm long by no more than
0.3 mm wide.
Laboratory Diagnosis
Enterobius vermicularis
Adults: Typical
Characteristics at a Glance
Characteristic Adult Female Adult Male*
Length 7-14 mm 2-4 mm
Width Up to 0.5 mm ≤0.3 mm
Color Yellowish-white Yellowish-white
Tail
Pointed;
resembles
pinhead
*Adult males are only rarely seen.
The specimen of choice for the recovery of E.
vermicularis is a cellophane tape preparation collected
from the perianal region of the person
suspected of infection. Although eggs are the
primary morphologic form seen, adult females
may also be present if the sample is collected
when the females enter the perianal region to lay
their eggs. It is important to note that multiple
samples may be required to confirm the presence
of a light infection as well as to determine that a
patient is free of infection. On rare occasions,
eggs and/or adult females may be recovered in
stool samples.
Life Cycle Notes
Humans are the only known host of E. vermicularis.
Pinworm infection, which is usually selflimiting,
is initiated following the ingestion of
the infective eggs. The eggs migrate through the
digestive tract into the small intestine, where
they hatch and release young larvae. The resulting
larvae continue to grow and mature, ultimately
transforming into adult worms. The adult
worms reside in the colon. Following mating of
select worms (copulation), including roundworms,
the resulting pregnant (gravid) female
worm migrates outside the body to the perianal
region, where she may deposit up to 15,000
eggs. Following 4 to 6 hours incubation, the
developing eggs achieve infective status. These
infective eggs may then become dislodged from
the body, caused at least in part by intense
scratching of the anal area by the infected
person. Once apart from the host, the infective
eggs may take up residence in a number of
locations, including dust, sandboxes, linens,
and clothing. In addition, the eggs may become
airborne. The infective eggs may survive for a
few days up to several weeks under suitable
environmental conditions. The ideal surroundings
for thriving infective eggs consist of a moderate
temperature accompanied by high humidity.
Ingestion of these infective eggs initiates a
new cycle.
A retroinfection, defined in pinworm-specific
terms as infective pinworm eggs that migrate
back into the host body, develop and reproduce