The pinworm (species Enterobius vermicularis), also known as threadworm (in the United Kingdom, Australia and New Zealand) or seatworm, is a parasitic worm.

[23] The emerging pinworm larvae grow rapidly to a size of 140 to 150 μm,[20] and migrate through the small intestine towards the colon.

[21] The egg-laying process begins about five weeks after initial ingestion of pinworm eggs by the human host.

[19] The gravid female pinworms migrate through the colon towards the rectum at a rate of 12 to 14 cm per hour.

[19] They emerge from the anus, and while moving on the skin near the anus, the female pinworms deposit eggs either through (1) contracting and expelling the eggs, (2) dying and then disintegrating, or (3) bodily rupture due to the host scratching the worm.

[24] E. vermicularis causes the medical condition pinworm infection also known as enterobiasis, whose primary symptom is itching in the anal area.

[26] Extraintestinal disease is rare and most commonly involves the female reproductive tract,[27] but spleen abscess has also been reported.

[29][30] The pinworm has a worldwide distribution,[25] and is the cause of the most common helminthiasis (parasitic worm infection) in the United States, western Europe, and Oceania.

[21] In the United States, a study by the Center of Disease Control reported an overall incidence rate of 11.4% among children.

[21] Pinworms are particularly common in children, with prevalence rates in this age group having been reported as high as 61% in India, 50% in England, 39% in Thailand, 37% in Sweden, and 29% in Denmark.

[21] Finger sucking has been shown to increase both incidence and relapse rates,[21] and nail biting has been similarly associated.