Pinworm, also called threadworm, is a small roundworm. Pinworms are parasites. That is, they live in the body of other animals. They are about 3/8 inch (9 millimeters) long and have white bodies and pointed tails. Some pinworms infect horses and rabbits. One type, Enterobius vermicularis, commonly infects humans.
The young worms live in the upper part of the large intestine. When the females are ready to lay eggs, they crawl down the rectum and out the intestinal opening called the anus, usually at night. They lay eggs on the surrounding skin. This movement causes skin swellings and severe itching. The eggs fall off onto the bedding or clothing, or may be picked up under fingernails in scratching. If the eggs are swallowed, they reach the intestine and become adult pinworms.
Pinworms are not very harmful unless they are present in large numbers. But their eggs may infect other people or reinfect the original carrier. In some areas, 10 to 60 per cent of the children may have pinworms at some time. Doctors use drugs to treat infection caused by pinworms.
