Merck Manual

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Life Cycle of Paragonimus westermani
Life Cycle of   Paragonimus westermani
Life Cycle of Paragonimus westermani
  • 1. Eggs are excreted in sputum or stool of infected people.

  • 2–3. In the environment, the eggs develop, hatch into an immature form (called miracidia), and are ingested by snails.

  • 4. Inside the snail, the miracidia go through several stages to develop into a form that can swim (cercariae).

  • 5.The cercariae infect crabs or crayfish and form cysts (called metacercariae).

  • 6. People are infected when they swallow cysts in raw, undercooked, or pickled freshwater crabs or crayfish.

  • 7. In the intestine, the larvae leave the cyst.

  • 8. The larvae penetrate the wall of the intestine, pass through the diaphragm, and invade the lungs. There, they develop into adults and produce eggs, which are passed in sputum that is coughed up and spit out or swallowed and passed in stool.

Image from the Centers for Disease Control and Prevention Image Library.