When cercariae of Centrocestus formosanus were placed together with the carp Cyprinus carpio, all of the cercariae disappeared in 30 min, while the cercariae of Haplorchis pumilio totally disappeared in 10 min. However, only 45.2% of C. formosanus and 42.6% of H. pumilio cercariae were later recovered from C. carpio as metacercarial cysts at 1 month postinfection. The lower than expected cyst recovery rates could be explained by the trapping of many C. formosanus cercariae in the mucus secreted by the gills, and by the difficulty with which H. pumilio cercariae could penetrate the scale-covered parts of the fish. Toxic effects of mucus secreted from the body surface of the fish may be partly responsible for the demise of cercariae. The cysts of C. formosanus were encapsulated by layers of cells and fibrous material of host origin, indicating that the gills can elicit a strong tissue reaction against these parasites.
Heterophyidae, Cercarial penetration, Metacercaria
