Identification of the transmission routes of the trematode parasite Paragonimus miyazakii into different intermediate hosts would help to explain the natural distribution of the parasite. The behavior of P. miyazakii cercariae released from snails into water and in the presence of a living host or a whole crab leg was observed by stereoscopic or light microscopy at various times after exposure started. On encountering a crab leg or cheliped, the cercariae became entangled with the host via mucoid strands arising from the cercariae. Within 3 hr, most cercariae were attached to the host; cuticular penetration took between 5 and 6 hr, after which cercariae were found in the cavity of the leg. Crabs examined 102–149 days after exposure to the cercariae contained fully developed metacercariae. The metacercariae were fed to 2 rats, and the rats were killed 83 or 111 days later. Some of the metacercariae had reached maturity in the rats. That the cercariae were not ingested by the crabs but penetrated the crabs percutaneously (through hard as well as soft tissue) means that transmission can occur even in areas in which crabs and the host snails do not coexist, as they would if the usual route were oral (when the crabs ate infected snails).

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