Infection occurs by ingestion of embryonated eggs excreted in
the stool of infected persons and may occur by hand-to-mouth transmission or
by oral contact with infected fomites, such as toys, bedding, or
clothing. The eggs average 55 μm by 35 μm
and appear flattened on one side and convex on the other. They are
fully mature and infective 3 to 8 hours after being deposited, but
at normal room temperature, less than 10% of eggs live
for 48 hours. Ingested eggs with first-stage larvae hatch in the duodenum,
and the larvae develop into adults in the cecum, where they mate.
The gravid female detaches from the cecal mucosa and migrates down
the large bowel, usually passing out the anus onto the perianal
and perineal skin, leaving a trail of eggs on the surface of the
skin. Yellow-white female adult pinworms measuring 8 to 13 mm may
be seen emerging from the rectum of infected children, most often
around 10 or 11 pm. In approximately 5% of patients, eggs
are deposited in the bowel and may be found in feces. Generally,
the worm dies after ovipositing is completed, so repeated infections
are the result of autoinfection or
reinfection from other environmental sources. There is no good evidence
that retrograde infection occurs.1