Ascaris lumbricoides !

Parasite
Exam priority: high

The largest human gut worm. People swallow its eggs from soil; in children with many worms it can block the bowel.

Updated 2026-11-01

Also called: large roundworm, giant intestinal roundworm

At a glance

Shape: Pinkish-white cylindrical worm; mouth with three lips
Family: Ascarididae
Type: Intestinal nematode (soil-transmitted helminth)
Size: Female 20–35 cm; male somewhat smaller

Morphology

Fertile egg, round, 45–75 µm, with a thick mammillated outer coat stained brown by bile.
Unfertilized egg more elongated (up to about 90 µm), thin-walled and full of refractile granules.
Decorticated eggs: lacking the mammillated outer layer.

Transmission

Swallowing embryonated eggs in soil contaminated with human faeces.
Poorly washed produce, unsafe water and dirty hands to the mouth.
No direct person-to-person spread: the egg needs weeks in soil to become infective.

Reservoir: Humans; Faecally contaminated soil

Isolation precautions

What the care team adds to standard precautions.

Standard

Life cycle

Adults live in the small bowel; the female lays thousands of eggs that leave in stool.
In moist, shaded soil the egg embryonates within weeks and becomes infective.
Once swallowed, the larva hatches, crosses the gut wall and travels in the blood to the liver and lung.
In the lung it matures for about 10–14 days, enters the alveoli, climbs the airway and is swallowed.
Back in the small bowel it becomes an adult; from egg swallowed to new eggs takes 2–3 months.

Virulence factors

Larval lung migration causes transient eosinophilic pneumonitis (Löffler syndrome).
A bolus of adults can block the ileum, especially in heavily infected children.
Adults wander to ectopic sites (common bile duct, pancreatic duct, appendix), especially with fever or anaesthesia.
They compete for nutrients and add to malnutrition and stunting.

Diseases it causes

Intestinal ascariasis Soil-transmitted helminthiasis (ascariasis, hookworm, trichuriasis) →
Bowel obstruction by a worm bolus in children Small bowel obstruction →
Cholangitis from Ascaris migrating into the bile duct Acute cholangitis →
Acute pancreatitis from Ascaris in the pancreatic duct Acute pancreatitis →

Clinical picture

Many infections are asymptomatic.
Lung phase: cough, wheeze, low fever and eosinophilia.
Abdominal pain, bloating and worms passed in stool, mouth or nose.
Right upper quadrant pain and jaundice if it migrates into the bile duct.

Diagnosis

Eggs on stool microscopy, ideally after formalin-ethyl acetate concentration; Kato-Katz measures the burden.
Larvae in sputum or gastric aspirate during the lung phase.
A passed adult worm, recognisable by its three lips.
Ultrasound for Ascaris in the biliary tree or bowel.

Treatment

Benzimidazoles (albendazole or mebendazole); pyrantel pamoate is an alternative. Obstruction is first managed conservatively, then by endoscopy or surgery if that fails.

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

In Pharmacology:

Prevention

Basic sanitation: latrines or sewerage and safe water.
Handwashing and careful washing of fruit and vegetables.
Periodic deworming of school-age and preschool children in at-risk areas (WHO preventive chemotherapy).
Health education in schools and homes.

Test yourself

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

Which lung syndrome does Ascaris larval migration cause?

Löffler syndrome: cough, wheeze and transient lung infiltrates with eosinophilia.

What does a fertile Ascaris egg look like?

Round, 45–75 µm, with a thick mammillated outer coat stained brown.

What is the typical mechanical complication in children with heavy Ascaris loads?

Bowel obstruction by a bolus of worms, usually in the ileum.

Why is Ascaris not spread directly from person to person?

Because its eggs need weeks in soil to embryonate and become infective.

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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 - Ascariasis (CDC, 2019); Soil-transmitted helminth infections (WHO, 2023); Ascariasis: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024); Geohelmintiasis (Ministerio de Salud y Protección Social de Colombia, 2014).

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