An adult Ascaris lumbricoides. This nematode parasite inhabits the small intestine and can measure up to 30cm in length and 0.8 cm in diameter. About 1,000,000,000 people are believed to harbor these parasites. Ascaris lumbricoides is an intestinal roundworm.
Infection occurs by ingestion of embryonated eggs either in fluids or food.
The larvae emerge in the duodenum and migrate to the lungs via the portal circulation. About two weeks after ingestion symptoms of pneumitis develop with fever, sputum production and radiographic infiltrates. The larvae migrate through the alveoli in the bronchopulmonary tree, and arrive in the pharynx where they are swallowed. Ascaris lumbricoides has a worldwide distribution. Infection rates are particularly high in warm humid tropical regions.