Causal Agent:
The nematode (roundworm) Enterobius vermicularis (previously Oxyuris vermicularis) also called human pinworm.  (Adult females: 8 to 13 mm, adult male: 2 to 5 mm.)  Humans are considered to be the only hosts of E. vermicularis.  A second species, Enterobius gregorii, has been described and reported from Europe, Africa, and Asia.  For all practical purposes, the morphology, life cycle, clinical presentation, and treatment of E. gregorii is identical to E. vermicularis.

Life Cycle:

Eggs are deposited on perianal folds .  Self-infection occurs by transferring infective eggs to the mouth with hands that have scratched the perianal area .  Person-to-person transmission can also occur through handling of contaminated clothes or bed linens.  Enterobiasis may also be acquired through surfaces in the environment that are contaminated with pinworm eggs (e.g., curtains, carpeting).  Some small number of eggs may become airborne and inhaled.  These would be swallowed and follow the same development as ingested eggs.  Following ingestion of infective eggs, the larvae hatch in the small intestine and the adults establish themselves in the colon .  The time interval from ingestion of infective eggs to oviposition by the adult females is about one month.  The life span of the adults is about two months.  Gravid females migrate nocturnally outside the anus and oviposit while crawling on the skin of the perianal area .  The larvae contained inside the eggs develop (the eggs become infective) in 4 to 6 hours under optimal conditions .  Retroinfection, or the migration of newly hatched larvae from the anal skin back into the rectum, may occur but the frequency with which this happens is unknown.

Geographic Distribution:
Worldwide, with infections more frequent in school- or preschool-children and in crowded conditions.  Enterobiasis appears to be more common in temperate than tropical countries.  The most common helminthic infection in the United States (an estimated 40 million persons infected).