Ascaris lumbricoides (roundworm)
This pale yellow nematode is 20–35 cm long. Humans are infected by eating food contaminated with mature ova. Ascaris larvae hatch in the duodenum, migrate through the lungs, ascend the bronchial tree, are swallowed and mature in the small intestine. This tissu migration can provoke both local and general hypersensitivity reactions, with pneumonitis, eosinophilic granulomas, bronchial asthma and urticaria.
Intestinal ascariasis causes symptoms ranging from occasional vague abdominal pain through to malnutrition. The large size of the adult worm and its tendency to aggregate and migrate can result in obstructive complications. Tropical and subtropical areas are endemic for ascariasis, and in these areas it causes up to 35% of all intestinal obstructions, most commonly in the terminal ileum. Obstruction can be complicated further by intussusception, volvulus, haemorrhagic infarction and perforation. Other complications include blockage of the bile or pancreatic duct and obstruction of the appendix by adult worms.
The diagnosis is made microscopically by finding ova in the faeces. Adult worms are frequently expelled rectally or orally. Occasionally, the worms are demonstrated radiographically by a barium examination. There is eosinophilia.
A single dose of albendazole (400 mg), pyrantel pamoate (11 mg/kg; maximum 1 g), ivermectin (150–200 μg/kg) or mebendazole (100 mg twice daily for 3 days) is effective for intestinal ascariasis. Patients should be warned that they might expel numerous whole, large worms. Obstruction due to ascariasis should be treated with nasogastric suction, piperazine and intravenous fluids.
Community chemotherapy programmes have been used to reduce Ascaris infection. The whole community can be treated every 3 months for several years. Alternatively, schoolchildren can be targeted; treating them lowers the prevalence of ascariasis in the community.