Putative immune response of rainbow trout, Salmo gairdneri, to Diplostomum spathaceum infections

The immune response of rainbow trout to infection with the cercariae of the digenean parasite Diplostomum spathaceum was investigated. Rainbow trout infected with cercariae at weekly intervals, or injected with a suspension of dead cercariae, did not produce a specific humoral response against D. spathaceum cercariae, as tested using immunoperoxidase, agglutination and cytotoxicity assays. However, a significant difference occurred in the rate of infection of rainbow trout given weekly exposure to cercariae of D. spathaceum in winter and summer. Rainbow trout injected with a suspension of dead cercariae acquired significantly fewer metacercariae than controls when exposed to a challenge infection. This suggested a specific immune response by the host and is the first example of a reduction in the infection rate of rainbow trout immunized against a digenean parasite, when exposed to a challenge infection.     

Cadmium toxicity and snail-digenean interactions in a population of Lymnaea spp

The toxicity of cadmium to a population of Lymnaea peregra and L. stagnalis naturally infected with a range of digeneans and collected from a number of sites in the lower Thames Valley, UK was investigated. Lymnaeid snails were exposed to 100 microg l-1 cadmium and the effects on host survival and emergence of cercariae recorded. Overwintered L. peregra, but not L. stagnalis, showed significantly reduced survival compared to seasonally infected snails, i.e. snails which have acquired an infection during the spring or summer. A significant increase in survival with increasing snail size was demonstrated for L. stagnalis and for seasonally infected L. peregra only. Only L. stagnalis infected with Diplostomum spathaceum and L. peregra infected individually with D. spathaceum, Sanguinicola inermis, Echinoparyphium recurvatum and Notocotylus attenuatus demonstrated a significantly reduced survival compared to laboratory-bred controls. The exposure of L. stagnalis to cadmium resulted in a significant reduction in the emergence of D. spathaceum over a 5-day period but cadmium-exposed L. peregra showed no difference in the emergence of E. recurvatum cercariae over a 3-day exposure period. The mechanisms and importance of metal toxicity to snail-digenean interactions are discussed.     

Diplostomum spathaceum cercariae respond to a unique profile of cues during recognition of their fish host

During its normal life cycle, Diplostomum spathaceum cercariae attach to and invade fish intermediate hosts. They are also known to attach to various other aquatic animals in response to water currents, touch and carbon dioxide. The purpose of this study was to identify the specific stimuli used by D. spathaceum cercariae to recognise the appropriate fish host. We characterised the host cues which stimulate them to remain on the host (enduring contact) and to penetrate the skin. Cercariae were exposed to animal skin tissues and fish skin surface mucus, their extracts and chemical modifications integrated into agar or offered via membrane filters. Enduring contact was stimulated by hydrophilic extracts Mr<3kDa, which were sensitive to oxidation of carbohydrates. The stimulating cues are probably small molecular carbohydrates, as monosaccharides stimulated enduring contacts, but amino acids, urea, electrolytes and peptides did not. Penetration was stimulated by hydrophilic macromolecules, Mr>30kDa, and by lipids. The hydrophilic stimuli were protease resistant and precipitable with Alcian blue and they were sensitive to alkaline cleavage, to digestion with lysozyme and neuraminidase as well as to oxidation of sialic acids. They were considered to be glycoproteins with O-glycosidically linked carbohydrate chains and bound sialic acids as signal structures. The lipophilic penetration stimuli were contained exclusively in the fatty acid fractions, and the stimulating characteristics of these fatty acids resembled the stimulating penetrations in other cercarial species. Diplostomum spathaceum cercariae respond to a unique profile of cues in their sequence of host-recognition phases. These cues differ from those used in other fish parasites studied to date and underline the diversity of fish recognition strategies.     

Cercariae and diplostomules of Diplostomum spathaceum (Digenea) elicit an immune response in rainbow trout, Salmo gairdneri Richardson

The immune response of rainbow trout to cercariae and diplostomules of Diplostomum spathaceum was investigated. Following immunization, serum was tested for the presence of antibody, using an enzyme-linked immunosorbent assay (ELISA) and immunofluorescence. Circulating antibody was detected to both cercariae and diplostomules by 6–7 weeks postimmunization and the sera were tested for their cross-reactivity with the other larval stage, using an ELISA.     

Diplostomum spathaceum (Rud. 1819): effect of miracidial age and lifespan on miracidial infectivity

Experiments were performed to study the effect of age and lifespan on the infection success of Diplostomum spathaceum miracidia. Specimens of Lymnaea peregra and L. stagnalis were individually exposed to single miracidia of D. spathaceum of different ages under conditions of constant illumination at 20 degrees C. The life-span of the free-swimming miracidium of D. spathaceum is 24 h at 20 degrees C. It was observed that as the miracidium ages, its ability to successfully infect snails declines rapidly.     

Phenotypic variation in infectivity of Diplostomum spathaceum cercariae within a population

The present study examined phenotypic variation in infectivity of Diplostomum spathaceum (Trematoda) cercariae within a natural population. Twelve infected Lymnaea stagnalis were collected from the field, and the infectivity of cercariae from individual snails was assessed under constant laboratory conditions. At a water temperature of 16.3 C, the mean infectivity of cercariae from the snails varied between 55.5% and 87.5%. Depending on the source of variation, this may have important ecological and evolutionary implications for both natural parasite populations and those occurring in aquaculture.     

Changes in survival characteristics of Diplostomum spathaceum cercariae emerged from cadmium-exposed Lymnaea stagnalis

The effect of exposing Lymnaea stagnalis (Gastropoda: Pulmonata), infected with Diplostomum spathaceum (Trematoda: Diplostomatidae), to 100 microg l(-1) cadmium for 7 days on survival characteristics (survival, tail loss, decaudized cercarial life-span) of emerged cercariae was investigated. Exposure of L. stagnalis to cadmium resulted in significantly increased D. spathaceum cercarial survival and an inhibited tail loss compared to controls. The normal parallel relationship which exists over time between decreasing cercarial survival and increasing tail loss in controls was changed in cercariae from cadmium-exposed hosts with an increased proportion of cercarial deaths occurring without tail loss. The decaudized cercarial life-span over the survival period of the cercarial population did not significantly change. However comparisons between individuals decaudized during the initial 24 h time period with those which were decaudized during the final period of cercarial survival showed a significantly altered life span which did not occur in the control population. As a potential indicator of penetration 'fitness' comparisons were also undertaken between control and exposed cercariae decaudized during the initial 24 h time period, which revealed that the decaudized cercarial life-span from the exposed hosts was significantly different from controls. This may have important implications for the ability of cercariae to migrate through the tissues of their target host. The importance and relevance of these results to parasite transmission are discussed.     

Observations on the occurrence of Diplostomum spathaceum and Schistocephalus sp. in ninespine sticklebacks (Pungitius pungitius) from the Belcher Islands, Northwest Territories, Canada

All 175 ninespine sticklebacks, Pungitius pungitius (Linné), collected from the Belcher Islands were parasitized by Diplostomum spathaceum (Rudolphi) and 43% by Schistocephalus sp. D. spathaceum metacercariae were mostly confined to a dorso-ventral band encircling the lens of the eye, and were concentrated in the anterio-dorsal sector of this band. The central area of the lens was thus relatively free of diplostomula, probably minimizing interference with the vision of the fish. The frequency distribution of D. spathaceum abundance in P. pungitius was closely approximated by a negative binomial, while that for Schistocephalus was best fitted by a Poisson. None of the fish condition factors examined appeared related to the intensity of the parasitic infections.     

Toxicity of cadmium and zinc to Diplostomum spathaceum (Trematoda: Diplostomidae) cercarial survival

The toxicity of cadmium and zinc at concentrations ranging from 0.1 to 10000 microg/l was investigated with cercariae of Diplostomum spathaceum at three temperatures (12, 20, 25 degrees C) and three levels of water hardness (distilled water, soft water, hard water). Under most environmental conditions survival of cercariae was reduced by increasing metal concentration. Increasing water hardness and decreasing water temperature caused an increase in the survival of both control and metal exposed cercariae. However, with decreasing temperature increased survival of metal-exposed cercariae above the control occurred at a number of low metal concentrations. Differences in the relative toxicity of cadmium and zinc to cercariae was dependent on the environmental conditions of exposure. The two heavy metals demonstrated little toxicity to cercariae during the period of maximum cercarial infectivity (0-5 h).The mechanisms of metal toxicity and their effects on cercarial survival are discussed.     

Studies on the infectivity of Diplostomum spathaceum in rainbow trout (Oncorhynchus mykiss)

The infectivity of Diplostomum spathaceum (Digenea: Trematoda) cercariae to rainbow trout and the efficacy of the diplostomule migration to the lens following different routes of administration was examined. The optimum age of infectivity for cercariae was between 0-5 h after liberation from the snail and for intraperitoneally injected diplostomules, 5 h post-transformation in vitro through fish skin. After exposure of the entire fish body or head to cercariae, metacercariae first appeared in the lens at 5 h and their numbers gradually increased until 22 h. Following exposure of the tail region of rainbow trout to cercariae, metacercariae first appeared in the lens at 14 h. Significantly more metacercariae established in the lens of fish following exposure of the fish head compared with the tail region; 40% of penetrating cercariae reached the lens of fish following exposure of the head or entire body, 20% of cercariae or diplostomules injected either intraperitoneally, intramuscularly or intracardially reached the lens while only 5% of cercariae established as metacercariae following exposure of the tail region.     

Interactions among co-infecting parasite species: a mechanism maintaining genetic variation in parasites?

Individuals of free-living organisms are often infected simultaneously by a community of parasites. If the co-infecting parasites interact, then this can add significantly to the diversity of host genotypexparasite genotype interactions. However, interactions between parasite species are usually not examined considering potential variation in interactions between different strain combinations of co-infecting parasites. Here, we examined the importance of interactions between strains of fish eye flukes Diplostomum spathaceum and Diplostomum gasterostei on their infectivity in naive fish hosts. We assessed the infection success of strains of both species in single-strain exposures and in co-exposures with a random strain of the other species. Parasite infection success did not consistently increase or decrease in the co-exposure treatment, but depended on the combinations of co-infecting parasite strains. This disrupted the relative infectivity of D. spathaceum strains observed in single-strain exposures. The infection success of D. gasterostei strains was independent of exposure type. These results suggest that interactions among parasite species may be strain specific and potentially promote maintenance of genetic polymorphism in parasite populations.     

Selection of the host's habitat by cercariae: from laboratory experiments to the field

The distribution of cercariae was studied in 18-cm-sized cuvettes under different lighting conditions, in Plexiglas cylinders (80 cm high) vertically placed in a pond, and when swimming freely in a pond. The vertical distribution and the effect of light intensity on it were relatively similar in the cuvettes, in the cylinders, and in the pond. Each of the species (Schistosoma mansoni, Diplostomum spathaceum, Echinostoma caproni, and Pseudechinoparyphium echinatum) showed its individual distribution within the water column, with distinct changes during the time after shedding. We hypothesize that the species-specific distributions in the water reflect behavioral adaptations to increase the chances of encountering the host spectra.     

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.     

Infectivity of Diplostomum spp. in Arctic charr: aspects of exposure duration and cercariae morphology

The diplostomid flukes, Diplostomum spp., infect fish and cause cataract opacities in the eye lens. The effect of exposure dose on abundance of Diplostomum spp. eye flukes in fish is known, but the effect of the duration of cercariae exposure has not been studied. However, under natural conditions, the temporal window for a successful cercaria attachment on fish is very short and, consequently, differences in infectivity of eye fluke cercariae, in the short-exposure durations of a few seconds, are probably biologically the most meaningful. We investigated infectivity of Diplostomum spp. cercariae originating from snail hosts in 3 lakes (3 Lymnaea stagnalis populations and 1 Radix balthica population) in 6 exposure times, ranging from 5 sec to 15 min, in young-of-the-year Arctic charr Salvelinus alpinus. In addition, we compared the infectivity to the cross-morphology of the cercariae. In the long-exposure duration, i.e., > or = 5 min, infectivity of Diplostomum spp. did not vary between the snail host species (L. stagnalis and R. balthica) of the same lake or across the L. stagnalis populations of 3 different lakes. However, in the short-exposure duration, i.e., < or = 60 sec, Diplostomum spp. cercariae shed from L. stagnalis had higher infectivity than did cercariae from R. balthica of the same lake. This indicates that that there is an interaction between length of cercariae exposure and origin of Diplostomum spp., and that the duration of exposure may influence the results when fish are experimentally infected. Within a lake, cercariae shed from L. stagnalis were also smaller than cercariae shed from R. balthica.     

Toxicity of cadmium and zinc mixtures to the decaudized cercarial life span of Diplostomum spathaceum

The effects of cadmium and zinc mixtures at concentrations ranging from 0.1 to 10,000 microg l(-1) on the life-span of decaudized cercarial bodies (cercariae that have shed their tails) of Diplostomum spathaceum (Trematoda: Diplostomatidae) was investigated. Cercariae were exposed to metal mixtures of equal and unequal concentrations, and a low-dose pre-treatment followed by a high-dose exposure mixtures. Metal mixtures demonstrated variable effects on decaudized cercariae either by increasing or reducing their life-span compared to single metal exposures dependent on concentration and the type of mixed metal treatment. Prolonged exposure to equal metal mixtures at low concentrations (0.1-100 microg l(-1)) resulted in a reduction in the life-span of decaudized cercariae at 0.1 and 100 microg l(-1) in those individuals decaudized during the initial 24 h exposure period compared with those decaudized during the final 24 h period of cercarial survival, whilst in controls there was no significant life-span change between the two time periods. Decaudized cercariae which were exposed to low concentrations (0.1-100 microg l(-1)) of equal metal mixtures were also evaluated for their role as an indicator of larval 'fitness' for migrating through the tissues of their target fish host for those individuals decaudized during the initial 24 h exposure period, and demonstrated only a limited change in their life-span compared to control and single metal exposures. The importance of metal mixtures in parasite establishment in the fish host is discussed.     

Feasibility of protecting rainbow trout, Salmo gairdneri Richardson, by immunizing against the eye fluke, Diplostomum spathaceum

Rainbow trout, Salmo gairdneri , were immunized with sonicated metacercarial antigens from the eye fluke, Diplostomum spathaceum . Immunization increased survival times in rainbow trout by about 4 months when compared with unimmunized controls. Survival time increased with both size of the fish and strength of the antigen dose.     

Chromosomal apparatus of trematodes of the general Diplostomum and Tylodelphys (Strigeidida, Diplostomatidae) and its taxonomic significance

The chromosomal apparatus of three species of trematodes of Diplostomum (D. spathaceum, D. indistinctum, D. mergi) and one species Tylodelphys (T. clavata) is described. It has been suggested that karyological investigations can be used for taxonomic purposes. The opinion has been expressed that the number of chromosomes in the karyotype of these trematodes plays no taxonomic role. Of great taxonomic significance are the size and morphology of chromosomes. Differences in karyotypes of trematodes belonging to different species and genera were revealed by these characters. The absence of close dependence between morphology of adult and specific peculiarities of karyotypes in species of the same genus enables in some cases the use of chromosomal apparatus as a most reliable criterion for a differentiation of close species.     

Seasonal dynamics of diurnal production and rhythmic of diurnal production of the cercaria Diplostomum volvens Nordmann, 1832 in molluscs of the genus Lymnaea from lakes of the Karasuk water system

Infection rate of the cercariae Diplostomum volvens in molluscs of the genus Lymnaea in natural conditions (Western Siberia) was examined in 1990-1994. Experimental studies have shown that light and water temperatire are main factors regulating rhythmic of diurnal production of cercariae by molluscs.     

Parasite fauna of the European river lamprey Lampetra fluviatilis (L.) from Lake Onega

Data on the parasite fauna of the adult European river lamprey Lampetra fluviatilis (L.) from Lake Onega are reported. Ten parasite species are found, including trematodes Diplostomum petromyzifluviatilis and D. spathaceum (metacercariae), cestode Proteocephalus longicollis, nematodes Cucullanus truttae and Raphidascaris acus, acanthocephalan Echinorhynchus salmonis, ectoparasitic infusoria Chilodonella hexastica, Trichodina tenuidens, and Trichodinella epizootica, and fungus Saprolegnia parasitica. Three species are found to be dominate, namely D. petromyzifluviatilis, Cucullanus truttae, and P. longicollis. Comparative analysis of the parasite faunas of the lampreys from other basins is carried out. Some similarity in the parasite faunas of lampreys and salmonids is discovered.     

Skin-penetrating parasites and the release of alarm substances in juvenile rainbow trout

Juvenile trout Oncorhynchus mykiss did not react to the odours of Diplostomum sp. Cercariae alone, indicating that they were incapable of detecting the parasites directly. However, they increased the number of random darts as well as the amount of time spent motionless when exposed to the odours of a conspecific that was being infected by Diplostomum cercariae. These results suggest that even the minor damage inflicted by the cercariae to the fish's skin was enough to cause the release of alarm substances. The effectiveness of the fish's response with respect to the avoidance of parasites remains to be demonstrated.