Disease
Human paragonimiasis is acquired through ingestion of raw or undercooked crabs or crayfish, and is usually a lung infection. After ingestion, metacercariae excyst in the small intestine and release larvae that penetrate the duodenal wall and enter the peritoneal cavity. The larvae migrate for approximately 1 week, then penetrate the diaphragm, enter the pleural cavity, and migrate directly through lung tissue to reach the bronchi. There they form cystic cavities and develop into adult worms in 5-6 weeks. The adult parasites are reddish brown and ovoid, measuring 7.5-12 mm by 4-6 mm. Adult worms induce an inflammatory response in the lungs, generating a fibrous cyst that contains a purulent, bloody effusion and eggs released by the flukes which are passed into the environment via expectoration, or may be swallowed and passed with feces. When deposited in fresh water, eggs hatch to release miracidiae, which then invade specific snail hosts. Thousands of cercariae are later released from the infected snail, which encyst (as metacercariae) in the gills, muscles, legs, and viscera of freshwater crustaceans (crabs or crayfish).