Ultrastructural observations and radiometric assay on cercarial penetration and migration of the digenean Diplostomum spathaceum in the rainbow trout Oncorhynchus mykiss.

The entrance into host tissues by the cercaria of the digenean Diplostomum spathaceum and the parasite's migration to the eyes of the fish host (post-penetration larvae) were studied by scanning and transmission electron microscopy at the ultrastructural level and by a radiometric assay in experimentally infected rainbow trout (Oncorhynchus mykiss). It was demonstrated that cercariae penetrated the host surface at several sites, although the major site involved the gill region. This observation was verified by the measurement of radioactivity in different regions of fish at intervals following their exposure to cercariae that had been radiolabeled with selenium in vivo. The migratory routes of the post-penetration-labeled cercariae (diplostomulum) were followed along with the distribution of radioactivity, which was altered with increasing time after exposure in the different regions of the host. A marked increase in radioactivity was noted, particularly in the eyes of the host, whereas the radiolabel decreased elsewhere. The results support the view that the cercariae of D. spathaceum contact the host by chance. The finding that the parasite mainly penetrates the host through the gills may be connected with fish ventilation hydrodynamics. The total amount of radioactivity retained in the fish gradually decreased with time, which may suggest that only a portion of the primarily attached cercariae actually penetrated the fish. The majority of the penetrating parasites reached the eyes of the host, where they became established as metacercariae within 24 h, thus indicating migration directed to the eye region. This is facilitated by the functional morphology of the migrating stage of D. spathaceum.