Threadworms (Pinworms, Enterobius vermicularis)

It is estimated that threadworms infect more than 400,000,000 people throughout the world (10% of humans), and in many areas of the world (e.g., North America and Europe) it is the most common nematode parasite of humans.

Adult threadworms live in the large intestines; males and females are about 5 mm and 10 mm long, respectively. After copulation the males die. When the female is ready to lay eggs she crawls out of the anus and deposits the eggs on the surrounding perianal skin; a single female can produce more than 10,000 eggs. After laying her eggs, the female also dies. At body temperature the eggs develop quickly and are infective in a few hours. When ingested by another person the eggs hatch in the small intestine, and the juvenile worms grow into adult, sexually mature worms in about a month.

The commonest symptom associated with threadworm infections is an itchy bottom (Pruritus ani). Scratching of the perianal skin to relieve the itching can lead to bacterial infections. This cycle can result in a situation where the infected person becomes very uncomfortable. In women, the adult threadworms can enter the vagina and cause additional irritation.

Threadworms are highly contagious. The eggs are infective within a few hours of being laid, and they are deposited directly on the perianal skin. Bed linen, clothing, carpets, etc., can be contaminated with eggs. The infected person's hands will, invariably, be contaminated with eggs, providing a route for reinfection and egg dispersal. It is for this reason that if one member of a family is infected for threadworms, the whole family is treated.

Threadworm infections are detected by finding the eggs or worms on the perianal skin. If the perianal skin is examined using a flashlight the worms can be seen; they may glow under the light. Eggs on the perianal skin can be detected by using a piece of sticky tape attached to a wooden applicator stick, sticky side out. The tape is then pressed against the perianal skin and later examined for worms and eggs. This is best done before getting out of bed.

Your pharmacist will be able to give you treatment and advice. Mebendazole is the drug of choice for patients of all ages over 2 years and is taken as a single dose, because reinfection is so common a second dose may be given after 2-3 weeks. Piperazine is an alternative preparation.

The affected person and the rest of the family should:

  • Keep nails cut short
  • Wear pyjamas or pants in bed
  • Have a bath each morning, making sure the bottom is thoroughly washed
  • Don't share towels
  • Change and wash underwear and pyjamas daily
  • Wash hands and scrub nails after each visit to the toilet
  • Wash hands and scrub nails before each meal
  • Disinfect toilet seat, toilet handle, and door handle regularly
  • Damp dust and vacuum clean bedrooms daily.
To conclude threadworms are a nuisance but generally harmless.

Further information

This article published on
12 December 2005

Next review date 12/1/2013


Embarrasing stuff

Areaof the body

Stomach and digestion

Male or female?


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