MODEL RELEASED. Child with threadworms. Young girl wearing a nightdress, scratching the skin near her anus to relieve the symptoms of threadworm infection (enterobiasis). Enterobiasis is caused by parasitic nematode worms of the genus Enterobius. The adult worms inhabit the large intestine, where they grow to about 12mm long. They pass out of the anus at night-time to lay eggs in the skin around the anus, causing itching. The infective eggs are transferred to the fingers by scratching, from where they can spread to other children. Enterobiasis is very common, affecting 1 in 5 children in temperate climates. Treatment includes ointments and drugs.