Eye fluke-induced cataracts in natural fish populations: is there potential for host manipulation?

In-feed Praziquantel treatment of grass carp (Ctenopharyngodon idella) infected with eye flukes

Praziquantel (PZQ) is a drug effective against several parasitic diseases of fish caused by Platyhelminthes. The study assesses the efficacy of selected in-feed PZQ doses in eliminating metacercariae Diplostomum pseudospathaceum in grass carp. The fish were infected with D. pseudospathaceum in laboratory conditions and subsequently treated with PZQ at daily doses of 30 (PZQ1), 50 (PZQ2), and 100 (PZQ3) mg/kg bw for seven days. The treatment trial was followed by a 23-day observation period during which the fish were parasitologically examined on days 0, 2, 8 and 23. Smaller fish were infected with a significantly higher (P < 0.05) number of metacercariae than bigger fish among three weight groups as follows: up to 7 g > 7 to 12 g > 12 g. The in-feed PZQ at doses of 30, 50 and 100 mg/kg bw for seven days was 47%, 84% and 88% effective against metacercariae D. pseudospathaceum, respectively. A significant decrease (P < 0.01) in moving and an increase (P < 0.01) in the immotile metacercariae in the eye's lens were recorded in all the trial groups on each sampling day. An increasing proportion of motile metacercariae in the post-treatment observation period was seen in PZQ2 and PZQ3, indicating a decreasing trend in the actual efficacy in an environment with no reinfection possibility.

Potential multidimensional behavioural impacts of differential infection in two fish populations

Parasites can influence the behaviour of their hosts, however investigations on how they may shape multiple personality traits are uncommon. The flatworm parasite Tylodelphys darbyi resides in the eyes of common bully, Gobiomorphus cotidianus, a locally common fish host to a range of other parasites that could also influence their behaviour. Here we assess how parasitism may be related to personality traits of two G. cotidianus populations; one where T. darbyi is highly abundant and one where the parasite is absent. We hypothesized that T. darbyi would have a stronger association with the different personality traits than other parasite taxa, and that the effects of infection on personality traits would vary between populations. Our results demonstrate that T. darbyi infections correlate with boldness, exploration, and activity within and among individuals. Further, we show that the relationship and therefore possible influence of other parasites, e.g., Apatemon sp., on personality traits vary between two host populations. Our study has revealed potential patterns highlighting how parasitism may differentially contribute to behavioural and ecological divergence among host populations.

Evidence for enemy release in invasive common dace Leuciscus leuciscus in Ireland: a helminth community survey and systematic review

Invasive species lose parasites in the process of invasion and tend to be less parasitized than conspecifics in the native range and sympatric native species in the invasive range (enemy release). We evaluated enemy release in an invasive freshwater fish in Ireland, common dace Leuciscus leuciscus, using helminth parasite community surveys at the core and front of the invasive range of common dace. Furthermore, we undertook a systematic literature review of helminth infection in common dace across its native range in Great Britain and Europe and invasive range in Ireland. The helminth parasite community survey revealed that invasive common dace were infected with fewer helminth species at the invasion front than at the core. Four helminth taxa - Acanthocephala, Monogenea, Digenea and Nematoda - were present in dace at the invasion core compared to only a single helminth species (Pomphorhynchus tereticollis) at the front. The systematic review revealed that invasive common dace in Ireland hosted fewer species of helminths than common dace in the native range. We report a total of three helminth species in common dace in Ireland compared to 24 in Great Britain and 84 in Continental Europe. Our results support the hypotheses that invasive populations are less parasitized than native populations and that more recently established populations host fewer parasites. However, we demonstrate that invasive species may continue to experience release from parasites long after initial invasion.

Resolution of the identity of three species of Diplostomum (Digenea: Diplostomidae) parasitising freshwater fishes in South Africa, combining molecular and morphological evidence

Reliable data on the diversity of the genus Diplostomum (Digenea: Diplostomidae) parasitising freshwater fishes in South Africa, as well as in Africa, is almost non-existent. Most of the morphology-based identifications of species within this genus reported from Africa require critical revision. The aim of the present study was to determine the diversity of Diplostomum metacercariae in South African fishes applying molecular and traditional morphological techniques. To achieve this aim, a total of 216 fishes belonging to 21 species collected in the Rivers Phongolo, Riet, Usuthu and Mooi in three provinces of South Africa were examined. Metacercariae of Diplostomum were recovered from the eye lenses of 38 fishes belonging to five species of the families Anguillidae, Cichilidae and Mochokidae, with an overall low prevalence of infection (18%). Metacercariae were subjected to morphological study and molecular sequencing of the partial mithochondrial cox1 and ribosomal 28S rDNA genes as well as of ribosomal ITS1-5.8S-ITS2 region. Morphological and phylogenetic analyses revealed the presence of three species which matched those previously reported from Nigeria, Iraq and China, therefore those from Tilapia sparrmanii and Synodontis zambezensis were named Diplostomum sp.; those from Anguilla labiata, Oreochromis mossambicus and S. zambezensis were named Diplostomum sp. 14; and those from Pseudocrenilabrus philander were named Diplostomum sp. 16. Geographic distribution of several species of Diplostomum appeared to be wider than expected. Morphological description and novel sequence data generated during this study will contribute to the elucidation of the life cycles of Diplostomum sp., Diplostomum sp. 14 and Diplostomum sp. 16 and advance further research of diplostomids in Africa.

An experimental test of how parasites of predators can influence trophic cascades and ecosystem functioning

Parasites can shape the structure and function of ecosystems by influencing both the density and traits of their hosts. Such changes in ecosystems are particularly likely when the host is a predator that mediates the dynamics of trophic cascades. Here, we experimentally tested how parasite load of a small predatory fish, the threespine stickleback, can affect the occurrence and strength of trophic cascades and ecosystem functioning. In a factorial mesocosm experiment, we manipulated the density of stickleback (low vs. high), and the level of parasite load (natural vs. reduced). In addition, we used two stickleback populations from different lineages: an eastern European lineage with a more pelagic phenotype (Lake Constance) and a western European lineage with a more benthic phenotype (Lake Geneva). We found that stickleback caused trophic cascades in the pelagic but not the benthic food chain. Evidence for pelagic trophic cascades was stronger in treatments where parasite load of stickleback was reduced with an antihelmintic medication, and where fish originated from Lake Constance (i.e., the more pelagic lineage). A structural equation model revealed that differences in stickleback lineage and parasite load were most likely to impact trophic cascades via changes in the composition, rather than overall biomass, of zooplankton communities. Overall, our results provide experimental evidence that parasites of predators can influence the cascading effects of fish on lower trophic levels with consequences on ecosystem functioning.

Consequences of eye fluke infection on anti-predator behaviours in invasive round gobies in Kalmar Sound

Larvae of the eye fluke, Diplostomum, emerge from snails and infect fish by penetrating skin or gills, then move to the lens where they may impair the vision of the fish. For the fluke to reproduce, a bird must eat the infected fish, and it has been suggested that they therefore actively manipulate the fish's behaviour to increase the risk of predation. We found that round gobies Neogobius melanostomus, a species that was recently introduced to the Kalmar Sound of the Baltic Sea, had an eye fluke prevalence of 90-100%. We investigated how the infection related to behavioural variation in round gobies. Our results showed that the more intense the parasite-induced cataract, the weaker the host's response was to simulated avian attack. The eye flukes did not impair other potentially important anti-predator behaviours, such as shelter use, boldness and the preference for shade. Our results are in accordance with the suggestion that parasites induce changes in host behaviour that will facilitate transfer to their final host.

Parasites and a host's sense of smell: reduced chemosensory performance of fathead minnows (Pimephales promelas) infected with a monogenean parasite

Parasites residing within the central nervous system of their hosts have the potential to reduce various components of host performance, but such effects are rarely evaluated. We assessed the olfactory acuity of fathead minnows (Pimephales promelas) infected experimentally with the monogenean Dactylogyrus olfactorius, the adults of which live within the host's olfactory chambers. Olfactory acuity was compared between infected and uninfected hosts by assessing electro-olfactography (EOG) neural responses to chemical stimuli that indicate the presence of food (L-alanine) or the presence of conspecifics (taurocholic acid). We also compared differences in gross morphology of the olfactory epithelium in infected and uninfected minnows. Differences in EOG responses between infected and uninfected minnows to both cue types were non-significant at 30 days post-exposure. By days 60 and 90, coincident with a two times increase in parasite intensity in the olfactory chambers, the EOG responses of infected minnows were 70-90% lower than controls. When infected fish were treated with a parasiticide (Prazipro), olfactory acuity returned to control levels by day 7 post-treatment. The observed reduction in olfactory acuity is best explained by the reduced density of cilia covering the olfactory chambers of infected fish, or by the concomitant increase in the density of mucous cells that cover the olfactory chambers. These morphological changes are likely due to the direct effects of attachment and feeding by individual worms or by indirect effects associated with host responses. Our results show that infection of a commonly occurring monogenean in fathead minnows reduces olfactory acuity. Parasite-induced interference with olfactory performance may reduce a fish's ability to detect, or respond to, chemical cues originating from food, predators, competitors or mates.

Life History, Systematics and Evolution of the Diplostomoidea Poirier, 1886: Progress, Promises and Challenges Emerging From Molecular Studies

Members of the Diplostomoidea mature in amniotes and employ vertebrates, annelids and molluscs as second intermediate hosts. Diplostomoid life cycles generally follow a three-host pattern typical of digeneans, but novelties have arisen in some species, including obligate four-host life cycles, vertical transmission, and intracellular parasitism. In this review, we summarize the basic biology of diplostomoids with reference to molecular studies, and present challenges, gaps and areas where molecular data could address long-standing questions. Our analysis of published studies revealed that most molecular surveys find more diplostomoid species than expected, but this tendency is influenced by how much effort goes into examining specimens morphologically and the number of sequenced worms. To date, molecular work has concentrated disproportionately on intraspecific or species-level diversity of larval stages in the Diplostomidae in temperate northern regions. Although the higher taxonomy of the superfamily is recognized to be in need of revision, little molecular work has been conducted at this level. Our phylogenetic analysis indicates several families and subfamilies require reconsideration, and that larval morphotypes are more reflective of evolutionary relationships than definitive hosts. The host associations of adult diplostomoids result from host-switching processes, whereas molecular surveys indicate that larval diplostomoid metacercariae have narrow ranges of second intermediate hosts, consistent with coevolution. Molecular data are often used to link diplostomoid developmental stages, and we provide data from adult Neodiplostomum and Mesoophorodiplostomum that correct earlier misidentifications of their larval stages and propose alternatives to collecting definitive hosts.

Possible mechanism of host manipulation resulting from a diel behaviour pattern of eye-dwelling parasites?

Parasitic infection often results in alterations to the host's phenotype, and may modify selection pressures for host populations. Elucidating the mechanisms underlying these changes is essential to understand the evolution of host–parasite interactions. A variety of mechanisms may result in changes in the host's behavioural phenotype, ranging from simple by-products of infection to chemicals directly released by the parasite to alter behaviour. Another possibility may involve parasites freely moving to certain sites within tissues, at specific times of the day to induce behavioural changes in the host. We tested the hypothesis that parasites shift to certain sites within the host by quantifying the location and activity of the trematode Tylodelphys sp., whose mobile metacercarial stages remain unencysted in the eyes of the second intermediate fish host, the common bully (Gobiomorphus cotidianus). This parasite's definitive host is a piscivorous bird feeding exclusively during daytime. Ocular obstruction and metacercarial activity were assessed within the sedated host's eye at three time points 24 h−1 period, using video captured via an ophthalmoscope. Although observed metacercarial activity did not change between time periods, ocular obstruction was significantly reduced at night. Increased visual obstruction specifically during the foraging time of the parasite's definitive host strongly suggests that the parasite's activity pattern is adaptive.

Interactions among bacterial strains and fluke genotypes shape virulence of co-infection

Most studies of virulence of infection focus on pairwise host-parasite interactions. However, hosts are almost universally co-infected by several parasite strains and/or genotypes of the same or different species. While theory predicts that co-infection favours more virulent parasite genotypes through intensified competition for host resources, knowledge of the effects of genotype by genotype (G × G) interactions between unrelated parasite species on virulence of co-infection is limited. Here, we tested such a relationship by challenging rainbow trout with replicated bacterial strains and fluke genotypes both singly and in all possible pairwise combinations. We found that virulence (host mortality) was higher in co-infections compared with single infections. Importantly, we also found that the overall virulence was dependent on the genetic identity of the co-infecting partners so that the outcome of co-infection could not be predicted from the respective virulence of single infections. Our results imply that G × G interactions among co-infecting parasites may significantly affect host health, add to variance in parasite fitness and thus influence evolutionary dynamics and ecology of disease in unexpected ways.

Complete mitochondrial genomes and nuclear ribosomal RNA operons of two species of Diplostomum (Platyhelminthes: Trematoda): a molecular resource for taxonomy and molecular epidemiology of important fish pathogens

BACKGROUND

The genus Diplostomum (Platyhelminthes: Trematoda: Diplostomidae) is a diverse group of freshwater parasites with complex life-cycles and global distribution. The larval stages are important pathogens causing eye fluke disease implicated in substantial impacts on natural fish populations and losses in aquaculture. However, the problematic species delimitation and difficulties in the identification of larval stages hamper the assessment of the distributional and host ranges of Diplostomum spp. and their transmission ecology.

METHODS

Total genomic DNA was isolated from adult worms and shotgun sequenced using Illumina MiSeq technology. Mitochondrial (mt) genomes and nuclear ribosomal RNA (rRNA) operons were assembled using established bioinformatic tools and fully annotated. Mt protein-coding genes and nuclear rRNA genes were subjected to phylogenetic analysis by maximum likelihood and the resulting topologies compared.

RESULTS

We characterised novel complete mt genomes and nuclear rRNA operons of two closely related species, Diplostomum spathaceum and D. pseudospathaceum. Comparative mt genome assessment revealed that the cox1 gene and its 'barcode' region used for molecular identification are the most conserved regions; instead, nad4 and nad5 genes were identified as most promising molecular diagnostic markers. Using the novel data, we provide the first genome wide estimation of the phylogenetic relationships of the order Diplostomida, one of the two fundamental lineages of the Digenea. Analyses of the mitogenomic data invariably recovered the Diplostomidae as a sister lineage of the order Plagiorchiida rather than as a basal lineage of the Diplostomida as inferred in rDNA phylogenies; this was concordant with the mt gene order of Diplostomum spp. exhibiting closer match to the conserved gene order of the Plagiorchiida.

CONCLUSIONS

Complete sequences of the mt genome and rRNA operon of two species of Diplostomum provide a valuable resource for novel genetic markers for species delineation and large-scale molecular epidemiology and disease ecology studies based on the most accessible life-cycle stages of eye flukes.

Increased ventilation by fish leads to a higher risk of parasitism

Background

Fish are common intermediate hosts of trematode cercariae and their gills can potentially serve as important sites of penetration by these larval stages. We experimentally tested the hypothesis that volume of ventilation flow across the gills contributes to acquisition of these parasites by fish. We manipulated the intensity of ventilation by using different oxygen concentrations.

Methods

Juvenile Oncorhynchus mykiss were individually exposed for 10 minutes to a standard dose of Diplostomum pseudospathaceum cercariae at three levels of oxygen concentration, 30, 60 and 90%. Ventilation amplitude (measured as a distance between left and right operculum), operculum beat rate, and the number of cercariae established in the eyes of fish were recorded.

Results

Fish reacted to low oxygen concentration with wider expansion of opercula (but not with increasing beat rate), leading to an increase in ventilation volume. As expected, the intensity of infection increased with decreasing oxygen saturation—probably due to a higher exposure to cercariae caused by increased ventilation under low oxygen concentrations. The number of cercariae acquired by an individual fish was positively correlated with ventilation amplitude and with ventilation volume, but not with operculum beat rate. However, even though the infection rate increased under these circumstances, the proportion of larval trematodes successfully establishing in fish eyes decreased with increasing ventilation volume, suggesting that the high flow velocity, although increasing host exposure to cercarial parasites, may interfere with the ability of these parasites to penetrate their hosts. There was no difference in the behaviour of trematode cercariae exposed to low and high oxygen concentrations.

Conclusion

A reduction in oxygen saturation resulted in an increase in ventilation volume across the gills and in doing so an increase in the exposure of fish to cercariae. A significant correlation between ventilation volume and parasitism represents the first experimental evidence that this physiological mechanism generates variation in transmission of parasites to fish hosts. Other factors that modify ventilation flow, e.g. physiological or social stressors, are expected to produce similar effects on the transmission success of the parasites penetrating fish hosts using the gills.

Differences in susceptibility and immune responses of three-spined sticklebacks (Gasterosteus aculeatus) from lake and river ecotypes to sequential infections with the eye fluke Diplostomum pseudospathaceum

BACKGROUND

The eye fluke Diplostomum pseudospathaceum is a frequent parasite of many fresh-water fish species, among those three-spined sticklebacks, particularly in lakes with lymnaeid snails, its first intermediate hosts. Cercariae released from host-snails, penetrate the skin of their fish hosts and within 24 h migrate to the immunologically inert eye lenses. Thus, individual D. pseudospathaceum are exposed to the fish immune system only for a short time, suggesting that only innate immunity can be active against the parasite. However, in nature sticklebacks are exposed to D. pseudospathaceum repeatedly since snails are shedding cercariae from late spring to autumn. Therefore, acquired immunity after initial infection would be advantageous against subsequent parasite encounters.

METHODS

We investigated if sticklebacks originating from a lake with high and from a river with low prevalence of D. pseudospathaceum differ in susceptibility to repeated exposure to the parasite. We compared infection success and immune functions in laboratory-bred sticklebacks from both habitats in naïve fish with fish that had been pre-exposed to eye flukes. Head kidney leukocytes (HKL) from experimental sticklebacks were investigated for respiratory burst activity and the proliferation of lymphocytes and monocytes 1.5, 5 and 15 days after infection.

RESULTS

Lake sticklebacks were less susceptible than river sticklebacks, however, in both populations pre-exposure led to a similar relative reduction in infection success. The respiratory burst activity was higher with HKL from lake sticklebacks and was up-regulated in pre-exposed fish but dropped 1.5d after an additional exposure, suggesting that activation of phagocytic cells is crucial for the defense against D. pseudospathaceum. Changes in lymphocyte proliferation were only detectable 1.5d after the last exposure in lake sticklebacks, but not 5 and 15d post exposure, indicating that a lymphocyte mediated acquired immune response was not induced. Proliferation of monocytes was significantly increased 1.5d after the last exposure with HKL from both stickleback populations.

CONCLUSIONS

Increased resistance to D. pseudospathaceum in sticklebacks from both populations upon pre-exposure cannot be explained by a prominent adaptive immune response. Monocytic leukocytes were more responsive, suggesting that rather cells of the innate than the adaptive immune system are active in the defense of D. pseudospathaceum.

Grouping facilitates avoidance of parasites by fish

BACKGROUND

Parasite distribution is often highly heterogeneous, and intensity of infection depends, among other things, on how well hosts can avoid areas with a high concentration of parasites. We studied the role of fish behaviour in avoiding microhabitats with a high infection risk using Oncorhynchus mykiss and cercariae of Diplostomum pseudospathaceum as a model. Spatial distribution of parasites in experimental tanks was highly heterogeneous. We hypothesized that fish in groups are better at recognizing a parasitized area and avoiding it than solitary fish.

METHODS

Number of fish, either solitary or in groups of 5, was recorded in different compartments of a shuttle tank where fish could make a choice between areas with different risk of being infected. Intensity of infection was assessed and compared with the number of fish recorded in the compartment with parasites and level of fish motility.

RESULTS

Both solitary fish and fish in groups avoided parasitized areas, but fish in groups avoided it more strongly and thus acquired significantly fewer parasites than solitary fish. Prevalence of infection among grouped and solitary fish was 66 and 92 %, respectively, with the mean abundance two times higher in the solitary fish. Between-individual variation in the number of parasites per fish was higher in the "groups" treatment (across all individuals) than in the "solitary" treatment. Avoidance behaviour was more efficient when fish were allowed to explore the experimental arena prior to parasite exposure. High motility of fish was shown to increase the acquisition of D. pseudospathaceum.

CONCLUSION

Fish in groups better avoided parasitized habitat, and acquired significantly fewer parasites than solitary fish. We suggest that fish in groups benefit from information about parasites gained from other members of a group. Grouping behaviour may be an efficient mechanism of parasite avoidance, together with individual behaviour and immune responses of fishes. Avoidance of habitats with a high parasite risk can be an important factor contributing to the evolution and maintenance of grouping behaviour in fish.

Genotypic and phenotypic variation in transmission traits of a complex life cycle parasite

Characterizing genetic variation in parasite transmission traits and its contribution to parasite vigor is essential for understanding the evolution of parasite life-history traits. We measured genetic variation in output, activity, survival, and infection success of clonal transmission stages (cercaria larvae) of a complex life cycle parasite (Diplostomum pseudospathaceum). We further tested if variation in host nutritional stage had an effect on these traits by keeping hosts on limited or ad libitum diet. The traits we measured were highly variable among parasite genotypes indicating significant genetic variation in these life-history traits. Traits were also phenotypically variable, for example, there was significant variation in the measured traits over time within each genotype. However, host nutritional stage had no effect on the parasite traits suggesting that a short-term reduction in host resources was not limiting the cercarial output or performance. Overall, these results suggest significant interclonal and phenotypic variation in parasite transmission traits that are not affected by host nutritional status.