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.

The epidemiology of the metacercariae of Diplostomum baeri and D. spathaceum in perch (Perca fluviatilis) from the warm water effluent of a nuclear power station

Thermal effects on the dynamics of infection with metacercariae of Diplostomum baeri and D. spathaceum were monitored between May and September, in a year-class of perch Perca fluviatilis, in an artificial lake receiving warm water discharges from a nuclear power station and in an unheated reference site, for a two year period. In the heated area the prevalence of infection of the retinal form, D. baeri, was always 100%, whereas in the unheated site there was an increase from 93% in May to 100% in June 1986. The relative density of D. baeri was found to increase gradually during the first summer in both areas, although the accumulation rate of metacercariae was significantly increased in the heated area. In August of the first year the relative density of D. baeri peaked in the heated area, whereas it continued to increase in the unheated control. However, in September of the following year, the relative densities were at the same level in both thermal regimes. A concomitant decline in the degree of overdispersion of metacercariae within the host population was observed in the heated area as the population density of metacercariae decreased, whereas the index of dispersion remained at the same level throughout the study in the unheated area. However, it was not possible to sample perch in the unheated area between October 1986 and May 1987 and changes in the parasite population could have occurred during this period. The prevalence and relative density of D. spathaceum, the lens form, was on the contrary low, especially in the unheated site where it was recorded only occasionally. The infection of D. baeri exhibited a convex pattern in both thermal regimens, although the peak infection was noticed earlier in the heated area. Regulation of the parasite infrapopulation may have been achieved by the combined effects of a decreased transmission rate of cercariae with increasing age of the host, the mortality of metacercariae as a natural termination of the life span and to selective predation of heavily infected hosts. However, regardless of an increased accumulation rate of metacercariae in the heated area, the relative densities of D. baeri became equal in both thermal regimes at the end of the study. The mortality rate of metacercariae in the heated area was therefore presumably increased as compared with the unheated reference site.