Paragonimus westermani: life cycle, intermediate hosts, transmission to man and geographical distribution in Korea.

Since discovery of this lung fluke in 1878 by Kerbert, Paragonimus westermani has been found to occur in many parts of Asia. Recently, however, it was detected that there exists a bisexual type and also a parthenogenetic type of the lung fluke which has been called P. westermani for a century. The natural definitive hosts other than man include a wide range of mammals. Adult worms are localized in the lungs. The eggs excreted with sputum and reaching water develop into miracidia in water. Miracidia hatch and finally enter molluscan hosts and mature to microcercous cercariae. Then, cercariae invade crustacean hosts and mature to metacercariae. When metacercariae are ingested by the definitive host, they excyst in the small intestine and migrate to the lungs via the peritoneal cavity. The circuitous route of migration allows the worms to lodge and mature in ectopic locations, too. Intermediate hosts are fairly restricted: in Korea, molluscan hosts are Semisulcospira libertina and other species of Semisulcospira depending on the endemic locality. Common crustacean hosts are Eriocheir japonicus, E. sinensis and Cambaroides similis. Human infection may result from consumption of parasitized raw freshwater crabs or crayfish, often soaked in soy bean sauce to improve taste. Recently, paragonimiasis was found to occur in individuals eating slices of wild boar meat harbouring immature P. westermani. Pigs are important paratenic hosts of P. westermani. P. westermani occurs usually in hilly and mountainous stream valleys. The parthenogenetic type is common in Korea. The bisexual type is found rarely and only in the southern part of Korea.