Question
Pathogenesis and complications of ascariasis
Answer
Pathogenesis of ascariasis (caused by Ascaris lumbricoides, the largest intestinal nematode infecting humans):
Transmission: faecal-oral ingestion of embryonated eggs from soil contaminated with human faeces (a soil-transmitted helminth).
Life cycle and pathogenesis: ingested embryonated eggs hatch in the small intestine, releasing larvae that penetrate the intestinal wall and enter the bloodstream/lymphatics, undergoing a hepato-pulmonary migration — first passing through the liver, then reaching the lungs via the bloodstream. In the lungs, larvae break out of pulmonary capillaries into the alveoli, ascend the bronchial tree and trachea, and are swallowed, returning to the small intestine, where they mature into adult worms over about 2–3 months. Adult worms live free in the intestinal lumen (not attached to the mucosa), feeding on intestinal contents, and female worms produce enormous numbers of eggs (up to 200,000/day) that are passed in faeces to continue the cycle.
Complications:
- Pulmonary (Löffler’s syndrome) — during larval lung migration, causes cough, dyspnoea, wheeze, and eosinophilic pneumonitis, with patchy pulmonary infiltrates on chest X-ray and marked peripheral eosinophilia — a hypersensitivity-type reaction to migrating larvae.
- Intestinal obstruction — heavy worm burden (a bolus of tangled adult worms) can cause mechanical obstruction of the intestinal lumen, particularly in children with heavy infection — a significant cause of surgical emergency in endemic areas.
- Biliary/pancreatic duct obstruction — adult worms can migrate into and obstruct the common bile duct or pancreatic duct, causing biliary colic, cholangitis, or acute pancreatitis.
- Appendicitis — worm migration into the appendiceal lumen can precipitate obstruction and appendicitis.
- Nutritional impairment — chronic heavy infection can contribute to malnutrition and impaired growth/cognitive development in children, due to competition for nutrients and reduced appetite.
- Migration to unusual sites — worms can migrate aberrantly (e.g., emerging through the mouth/nose, particularly triggered by fever, anaesthesia, or certain drugs), reflecting the adult worm’s characteristic restlessness in a suboptimal environment.
Laboratory diagnosis: stool microscopy for characteristic fertilized (mammillated, oval, thick-shelled) or unfertilized eggs; occasionally, adult worms are passed spontaneously or seen on imaging (as filling defects on barium studies).