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Ascaris life cycle.
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1. Adult worms live in the lumen of the small bowel.
2. A female may produce about 200,000 eggs/day; the eggs are excreted with feces. Unfertilized eggs may be ingested but are not infective.
3. In the environment, fertile eggs, depending on the conditions, embryonate and become infective after 18 days to several weeks; optimal conditions are moist, warm, shaded soil.
4. Infective eggs are swallowed.
5. In the small bowel, the eggs hatch into larvae.
6. Larvae penetrate the wall of the small bowel and migrate via the portal circulation through the liver, then via the systemic circulation to the lungs, where they mature further (in 10 to 14 days).
7. The larvae penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed. They then return to the small bowel, where they develop into adult worms.
Image from the Centers for Disease Control and Prevention Image Library.