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Microbiology · Nematodes

Enterobius vermicularis !

  • Parasite
  • Exam priority: high

A tiny white worm that lays eggs around the anus at night and causes itching. Very common in children; it spreads at home and at school.

Updated 2026-11-01

Also called: pinworm, threadworm (UK)

At a glance

Shape
Thin white worm; the female has a long pointed tail
Family
Oxyuridae
Type
Intestinal nematode (pinworm), not soil-transmitted
Size
Female 8–13 mm; male 2–5 mm

Morphology

  • Oval egg, 50–60 × 20–30 µm, flattened on one side (D-shaped).
  • Transparent, sticky shell; passed partly embryonated.
  • Sticks to clothing, sheets, fingernails and surfaces.

Transmission

  • Anus-hand-mouth autoinfection from scratching.
  • Eggs on bedding, underwear, toys and surfaces.
  • Spread within households, day-care centres and schools; rarely by inhaled eggs.

Reservoir: Humans only

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. Eggs are swallowed from hands, nails, bedding or surfaces.
  2. Larvae hatch in the small bowel and adults settle in the caecum.
  3. At night the gravid female crawls out of the anus and lays eggs on the perianal skin.
  4. The eggs become infective within hours; scratching carries them to fingernails and mouth.
  5. Retroinfection: larvae hatching on the perianal skin may crawl back into the rectum.
  6. About a month passes from ingestion to new egg-laying.

Virulence factors

  • The female's night-time migration causes intense anal itching.
  • In girls it can migrate to the vulva and cause vulvovaginitis.
  • Scratching causes excoriation and bacterial superinfection.
  • It is occasionally found in a removed appendix.

Diseases it causes

  • Enterobiasis (pinworm infection)
  • Vulvovaginitis in girls from pinworm migration

Clinical picture

  • Night-time anal itching.
  • Restless sleep, irritability.
  • Perianal scratch marks.
  • Several household members with symptoms.

Diagnosis

  • Clear adhesive tape test in the morning, before bathing or a bowel movement; repeated on several days.
  • Stool exams rarely show the eggs.
  • Adult worms seen in the perianal area.

Treatment

Benzimidazoles (mebendazole or albendazole) or pyrantel pamoate; treatment is repeated because the drugs do not kill eggs, and the whole household is treated at once.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Handwashing and short fingernails.
  • Morning bath and daily change of underwear.
  • Wash bedding and pyjamas without shaking them.
  • Treat all household members at the same time.

Test yourself

Think of the answer, then tap the question to see it.

What is the test of choice for pinworm?

The clear adhesive tape test on the perianal skin in the morning, before bathing.

Why is pinworm itching worse at night?

Because the gravid female crawls out of the anus at night to lay eggs on the perianal skin.

Why is pinworm treatment repeated?

Because the drugs kill the worms but not the eggs, which can start a new infection.

What does the Enterobius vermicularis egg look like?

Oval, transparent and flattened on one side, D-shaped.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Enterobiasis (CDC, 2019); About Pinworm Infection | Pinworm Infection | CDC (CDC, 2024); Pinworms: MedlinePlus (MedlinePlus (NLM), 2024).

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Representative model, for educational purposes only.