The natural first intermediate host of Paragonimus siamensis (Miyazaki and Wykoff, 1965) in Thailand.

The first intermediate host of six-known the Paragonimus species in Thailand had not been found until the Filopaludina (Siamopaludina) martensi martensi snail was discovered to maintain the cercariae of a Paragonimus species. An extensive study examined cercarial development through to adult worms by infecting 3 genera of 7 crab species with penetration of cercariae and feeding of snails containing such cercariae. These crabs provided many metacercariae which were fed to cats and bandicoots. The animals gave many Paragonimus adult worms which were characterized as Paragonimus siamensis by the following criteria: 6-lobed ovary and cuticular spines in groups. It is concluded that the Filopaludina martensi martensi snail is a susceptable natural first intermediate host of P. siamensis. Second intermediate hosts Somanniathelphusa brandti, S. sexpunctatum and S. bangkokensis were experimentally infected; prior to this study only S. germaini and S. dugasti had ever been naturally infected with metacercariae of this species.