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.
Ascaris is a large roundworm more than 20 cm. in length. They are seen most commonly in the small intestine, or secondarily in the appendis. Rarely they migrate to the bile ducts. Multiple worms may be seen and these can cause obstructing balls. The worms may elicit little reaction, but can cause in inflammatory reaction with reddening and edema of the wall.