Infestation of Lymnaea stagnalis by digenean flukes in the Jeziorak Lake

Thinking "outside the box": The effect of nontarget snails in the aquatic community on mollusc-borne diseases

There is a great need to understand the impact of complex communities on the free-living parasite stages that are part of them. This task becomes more complex as nonnative species emerge, changing existing relationships and shaping new interactions in the community. A relevant question would be: Can the coexistence of nontarget snails with the target hosts contribute to trematodasis control? We used field and experimental approaches to investigate nonnative competitor-induced parasite dilution. During a three-year field study, we investigated digenean infection in Lymnaea stagnalis from eight Polish lakes inhabited or uninhabited by Potamopyrgus antipodarum. Additionally, we verified the presence of digenean infections in the populations of P. antipodarum. Moreover, we conducted an experimental infection of L. stagnalis with miracidia of Trichobilharzia szidati under increasing densities of P. antipodarum and aimed to infect P. antipodarum with them separately. The prevalence of avian schistosomes in lymnaeid snails was significantly higher in uninhabited lakes than in lakes inhabited by P. antipodarum. Our study indicates that waters with a higher density of invaders have a lower prevalence of avian schistosomes in lymnaeid hosts. The results of experimental studies confirmed that the presence of high densities of P. antipodarum reduces the probability of target host infection. Both field and experimental studies rule out the role of P. antipodarum as a source of avian schistosome cercariae. Here, a nonnative species was tested as a diluter, which in practice may be harmful to the local environment. This work is not a call for the introduction of nonnative species; it is intended to be a stimulus for researchers to continue searching for natural enemies of parasites because, as our results show, they exist. Finding natural enemies to the most dangerous species of human and animal parasites that will pose no threat to the local environment could be groundbreaking.

Multigenerational responses in the Lymnaea stagnalis freshwater gastropod exposed to diclofenac at environmental concentrations

Over the last decade, there has been increased concern about the occurrence of diclofenac (DCF) in aquatic ecosystems. Living organisms could be exposed to this "pseudo-persistent" pharmaceutical for more than one generation. In this multigenerational study, we assessed the DCF impact at environmentally relevant concentrations on the life history and behavioral parameters of two offspring generations (F1 and F2) of the Lymnaea stagnalis freshwater gastropod. Snail growth was affected by DCF in the F1 generation, with increased shell sizes of juveniles exposed to 0.1 µg L ^- 1 concentration and a decreased shell size at 2 and 10 µg L ^- 1. DCF also lowered food intake, enhanced locomotion activity and reduced the number of eggs/egg mass in the F1 generation. For the F2 generation, shorter time to hatch, faster growth, increased food intake and production of more egg masses/snail were induced by DCF exposure at 10 µg L ^- 1. Over time, DCF exposure led to maximization of L. stagnalis reproductive function. These results show that multigenerational studies are crucial to reveal adaptive responses to chronic contaminant exposure, which are not observable after short-term exposure.

Characterization of the complete mitochondrial genome of the fluke of turdus, Plagiorchis elegans, and phylogenetic implications

Plagiorchis elegans (Trematoda: Digenea) is mainly parasitic in the intestines of vertebrate animals, including humans, causing irreversible pathological damage and herd-spherical influences. However, little information is available about its molecular epidemiology, population genetics, and phylogeny. In the present study, we sequenced, assembled, and annotated the complete mitochondrial (mt) genome of P. elegans. Combining with the available mitochondrial data of subclass Digenea, phylogenetic analysis was performed based on Bayesian inference (BI). The results showed that the complete length of P. elegans is 13,862 bp, including 12 PCGs, 2 ribosomal RNAs (rRNAs), 22 transfer RNAs (tRNAs), and one non-coding gene (NCR). There was an obvious A + T content from 61.0% to 71.3% and the values of the Ka/Ks ratio ranged from 0.119 (cox1) to 1.053 (nad6). In the BI analysis, different from previous studies, phylogenetic analysis showed genus Glypthelmins was paraphyletic rather than monophyletic and had a closer relationship with Plagiorchis and Orientocreadium. Additionally, the BI tree also presented that the genus Echinostoma was monophyletic. Our results provided molecular data in the family Plagiorchiidae proposing new insight within Xiphidiata and Echinostomata.

Dermatological and Molecular Evidence of Human Cercarial Dermatitis in North-Eastern Poland

Swimmer's itch or human cercarial dermatitis (HCD) appears as a skin rash caused by an allergic reaction to larval (cercariae) flatworm parasites of the family Schistosomatidae. In our study, two cases of HCD were analyzed; both of them were reported in people swimming in Lake Pluszne. In the summer of 2018, a sample of 397 snails was collected at swimming sites in that area. Five Lymnaea stagnalis (1.9%) were found to host cercariae of bird schistosomes. Positive samples were selected by amplification of the Internal Transcribed Spacers (ITS) gene region. Sequence analysis confirmed that they were homologous with European isolates of Trichobilharzia szidati. The cases reported in this article are the first confirmed cases of HCD in this lake. This study demonstrates that there is a rationale for conducting screening studies of regions with a high recreational potential.

Species-specific patterns in cercarial emergence of Diplostomum spp. from snails Radix lagotis

The cercarial emergence patterns of three species of Diplostomum (Diplostomum 'mergi', Diplostomum spathaceum and Diplostomum parviventosum) parasitizing freshwater first intermediate host Radix lagotis sampled in Most Lake, Czech Republic, were studied under various experimental conditions, i.e. field, laboratory and incubator, and seasons, i.e. spring, summer and autumn. We discovered unexpected daily periodicity-dependent species-specific emergence patterns among the three Diplostomum spp. depending on experimental conditions. At the same time, the intraspecific variation of D. spathaceum cercarial release in response to seasonal conditions was observed. We found that a complex array of mechanisms can affect Diplostomum species-specific patterns in cercarial emergence, of which behavioural characteristics of fish related to reproduction and feeding processes are considered the most important factors. This might represent a specific adaptive evolutionary mechanism to maximise transmission success while avoiding competition for host resources. Our results contribute to a better understanding of ecological and epidemiological aspects with respect to specific adaptive strategies compartmentalised among species of Diplostomum and consequences for infection risk in fish hosts.

The chemotactic swimming behavior of bird schistosome miracidia in the presence of compatible and incompatible snail hosts

No effective method has yet been developed to prevent the threat posed by the emerging disease-cercarial dermatitis (swimmer's itch), caused by infective cercariae of bird schistosomes (Digenea: Schistosomatidae). In our previous studies, the New Zealand mud snail-Potamopyrgus antipodarum (Gray, 1853; Gastropoda, Tateidae)-was used as a barrier between the miracidia of Trichobilharzia regenti and the target snails Radix balthica. Since the presence of non-indigenous snails reduced the parasite prevalence under laboratory conditions, we posed three new research questions: (1) Do bird schistosomes show totally perfect efficacy for chemotactic swimming behavior? (2) Do the larvae respond to substances emitted by incompatible snail species? (3) Do the excretory-secretory products of incompatible snail species interfere with the search for a compatible snail host? The experiments were carried out in choice-chambers for the miracidia of T. regenti and T. szidati. The arms of the chambers, depending on the variant, were filled with water conditioned by P. antipodarum, water conditioned by lymnaeid hosts, and dechlorinated tap water. Miracidia of both bird schistosome species chose more frequently the water conditioned by snails-including the water conditioned by the incompatible lymnaeid host and the alien species, P. antipodarum. However, species-specific differences were noticed in the behavior of miracidia. T. regenti remained more often inside the base arm rather than in the arm filled with water conditioned by P. antipodarum or the control arm. T. szidati, however, usually left the base arm and moved to the arm filled with water conditioned by P. antipodarum. In conclusion, the non-host snail excretory-secretory products may interfere with the snail host-finding behavior of bird schistosome miracidia and therefore they may reduce the risk of swimmer's itch.

Agents of swimmer's itch-dangerous minority in the Digenea invasion of Lymnaeidae in water bodies and the first report of Trichobilharzia regenti in Poland

Trichobilharzia spp. have been identified as a causative agent of swimmers’ itch, a skin disease provoked by contact with these digenean trematodes in water. These parasites have developed a number of strategies to invade vertebrates. Since we have little understanding of the behavior of these parasites inside the human body, the monitoring of their invasion in snail host populations is highly recommended. In our research, lymnaeid snails were collected from several Polish lakes for two vegetation seasons. The prevalence of bird schistosomes in snail host populations was significantly lower than that of other digenean species. We were the first to detect the presence of the snails emitted Trichobilharzia regenti (potentially the most dangerous nasal schistosome) in Poland. In addition, by sequencing partial rDNA genes, we confirmed the presence of the snails positive with Trichobilharzia szidati in Polish water bodies, showing that swimmer’s itch is more frequent during summer months and that large snails are more often infected with bird schistosomes than small ones.

Galba robusta sp. nov. from Yemen (Gastropoda: Lymnaeidae)

Mollusks of the genus Galba Schrank, 1803, inhabiting north-east Africa and the Arabian Peninsula, are separated into two morphologically distinct groups. The first group contains two conchologically indistinguishable species, G. truncatula (O.F. Müller, 1774) and G. schirazensis (Küster, 1862). The second group includes one species characterized by significantly larger size and different shell proportions as compared to G. truncatula and G. schirazensis. This species is new one and described here as G. robusta sp. nov. with type locality situated in Yemen. It is hypothesized that G. robusta sp. nov. has a vast distribution, ranging from Central Iran southwards to East Africa. A comparison of the new taxon with two nominal species of Galba, G. mweruensis (Connolly, 1929) and G. umlaasianus (Küster, 1862) described from East and South Africa, as well as some data on conchological variation of African representatives of this genus are given.

The Early Worm Catches the Bird? Productivity and Patterns of Trichobilharzia szidati Cercarial Emission from Lymnaea stagnalis

Digenean trematodes are common and abundant in aquatic habitats and their free-living larvae, the cercariae, have recently been recognized as important components of ecosystems in terms of comprising a significant proportion of biomass and in having a potentially strong influence on food web dynamics. One strategy to enhance their transmission success is to produce high numbers of cercariae which are available during the activity peak of the next host. In laboratory experiments with 13 Lymnaea stagnalis snails infected with Trichobilharzia szidati the average daily emergence rate per snail was determined as 2,621 cercariae, with a maximum of 29,560. During a snail’s lifetime this summed up to a mass equivalent of or even exceeding the snail’s own body mass. Extrapolated for the eutrophic pond where the snails were collected, annual T. szidati biomass may reach 4.65 tons, a value equivalent to a large Asian elephant. Emission peaks were observed after the onset of illumination, indicating emission synchronizing with the high morning activities of the definitive hosts, ducks. However, high cercarial emission is possible throughout the day under favorable lightning conditions. Therefore, although bird schistosomes, such as T. szidati constitute only a fraction of the diverse trematode communities in the studied aquatic ecosystem, their cercariae can still pose a considerable risk for humans of getting cercarial dermatitis (swimmer's itch) due to the high number of cercariae emitted from infected snails.

Parasites in a man-made landscape: contrasting patterns of trematode flow in a fishpond area in Central Europe

We have explored a large body of novel data focusing on small-scale temporal and spatial patterns in the composition and structure of larval trematode communities in Lymnaea stagnalis (L.) from a typical Central European agricultural landscape. The 5 eutrophic fishponds studied provide excellent environments for the development of species-rich and abundant trematode communities. Nine prevalent species were consistently present in component communities, but had differential contribution to the parasite flow in the 5 ponds resulting in significant contrasting patterns of community similarity and the prevalence of the 3 major transmission guilds driving this similarity. Component communities split into 2 groups: (i) those from the large pond dominated by anatid and larid generalists with active miracidial transmission; and (ii) those from the smaller ponds dominated by 2 plagiorchioideans infecting snails via egg ingestion. We put forward 3 hypotheses for the remarkable differences in larval trematode flow in the similar and closely located eutrophic ponds: (i) species-specific differences in parasite colonization potential displayed by an 'active-passive' dichotomy in miracidial transmission strategies of the species; (ii) top-down effects of pond context on transmission pathways of the trematodes; and (iii) competition as an important mechanism in eutrophic environments with a bottom-up effect on component community structure.

Species diversity of Plagiorchis Lühe, 1899 (Digenea: Plagiorchiidae) in lymnaeid snails from freshwater ecosystems in central Europe revealed by molecules and morphology

Larval stages of Plagiorchis spp. are both ubiquitous and ecologically important parasites in snail populations of freshwater ecosystems in Europe. However, difficulties in distinguishing the morphologically similar cercariae used for species identification, may lead to underestimation of species diversity. In this study, 38 isolates of Plagiorchis spp. infecting two lymnaeid snails, Lymnaea stagnalis (L.) and Radix auricularia (L.), in five central European freshwater ecosystems were subjected to morphological and molecular assessment. Five morphologically homogeneous and genetically distinct lineages of Plagiorchis spp. were identified via matching molecular data for the mitochondrial cytochrome c oxidase subunit I (cox1) gene with detailed morphological and morphometric data of the cercariae. Comparative sequence analysis using partial 28S rDNA and ITS1-5.8S-ITS2 sequences revealed that three distinct cox1 lineages are conspecific with Plagiorchis elegans (Rudolphi, 1802), P. maculosus (Rudolphi, 1802) and P. koreanus Ogata, 1938, respectively, whereas the lineage identified based on cercarial morphology as P. neomidis Brendow, 1970 plus a single isolate that could not be assigned to a described species, did not match any of the available sequences for Plagiorchis spp. A key to the cercariae of Plagiorchis spp. parasitising lymnaeid populations in central Europe is provided to facilitate identification.

The role of spatial and temporal heterogeneity and competition in structuring trematode communities in the great pond snail, Lymnaea stagnalis (L.)

We assessed how spatial and temporal heterogeneity and competition structure larval trematode communities in the pulmonate snail Lymnaea stagnalis . To postulate a dominance hierarchy, mark-release-recapture was used to monitor replacements of trematode species within snails over time. In addition, we sampled the trematode community in snails in different ponds in 3 consecutive years. A total of 7,623 snails (10,382 capture events) was sampled in 7 fishponds in the Jindřichův Hradec and Třeboň areas in South Bohemia (Czech Republic) from August 2006 to October 2008. Overall, 39% of snails were infected by a community of 14 trematode species; 7% of snails were infected with more than 1 trematode species (constituting 16 double- and 4 triple-species combinations). Results of the null-model analyses suggested that spatial heterogeneity in recruitment among ponds isolated trematode species from each other, whereas seasonal pulses in recruitment increased species interactions in some ponds. Competitive exclusion among trematodes led to a rarity of multiple infections compared to null-model expectations. Competitive relationships among trematode species were hypothesized as a dominance hierarchy based on direct evidence of replacement and invasion and on indirect evidence. Seven top dominant species with putatively similar competitive abilities (6 rediae and 1 sporocyst species) reduced the prevalence of the other trematode species developing in sporocysts only.

Small-scale to large-scale and back: larval trematodes in Lymnaea stagnalis and Planorbarius corneus in Central Europe

We examined the small-scale temporal and spatial variability in composition and structure of larval trematode communities in Lymnaea stagnalis and Planorbarius corneus in two fish ponds in the Czech Republic and compared the patterns of richness and similarity to continental and regional trematode faunas of these hosts. The levels of parasitism in the populations of both hosts were high, the former parasitized predominantly by allogenic species maturing in a wide range of birds and the latter infected by relatively more species completing their life cycles in micromammals. Communities in both hosts exhibited a congruent pattern of seasonal change in overall infection rates and community composition with lower levels of infection in spring. Both temporal and spatial variation was closely related to the structure of snail populations, and no significant differentiation of community composition with respect to pond was observed. Comparisons with large-scale inventories revealed overall congruent patterns of decreased richness and similarity and increased variability at the smaller scales in both host-parasite systems. The relative compositional homogeneity of larval communities in both snail hosts irrespective of scale suggests that historical data at small to medium regional scales may provide useful estimates of past richness and composition of larval trematode communities in these snail hosts.

Larval trematode communities in Radix auricularia and Lymnaea stagnalis in a reservoir system of the Ruhr River

Background

Analysis of the data available from traditional faunistic approaches to mollusc-trematode systems covering large spatial and/or temporal scales in Europe convinced us that a parasite community approach in well-defined aquatic ecosystems is essential for the substantial advancement of our understanding of the parasite response to anthropogenic pressures in urbanised areas which are typical on a European scale. Here we describe communities of larval trematodes in two lymnaeid species, Radix auricularia and Lymnaea stagnalis in four man-made interconnected reservoirs of the Ruhr River (Germany) focusing on among- and within-reservoir variations in parasite prevalence and component community composition and structure.

Results

The mature reservoir system on the Ruhr River provides an excellent environment for the development of species-rich and abundant trematode communities in Radix auricularia (12 species) and Lymnaea stagnalis (6 species). The lake-adapted R. auricularia dominated numerically over L. stagnalis and played a major role in the trematode transmission in the reservoir system. Both host-parasite systems were dominated by bird parasites (13 out of 15 species) characteristic for eutrophic water bodies. In addition to snail size, two environmental variables, the oxygen content and pH of the water, were identified as important determinants of the probability of infection. Between-reservoir comparisons indicated an advanced eutrophication at Baldeneysee and Hengsteysee and the small-scale within-reservoir variations of component communities provided evidence that larval trematodes may have reflected spatial bird aggregations (infection 'hot spots'). Two life history groupings of dominant species, the 'cyprinid' and 'anatid' parasites, that depict two aspects of progressive eutrophication in this mature reservoir system, were identified.

Conclusions

We conclude that trematode communities in the lake-adapted R. auricularia are better suited for monitoring the effect of environmental change on host-parasite associations in the reservoir system on the Ruhr River and other similar systems due to the important role of this host in trematode transmission in lakes. Whereas variations in trematode community diversity and abundance may indicate the degree of eutrophication on a larger scale (among reservoirs), the infection rates of the two life history groups of dominant species, the 'cyprinid' and 'anatid' assemblages, may be particularly useful in depicting environmental variability, eutrophication effects and infection 'hot spots' on smaller spatial scales.

A role for host-parasite interactions in the horizontal transfer of transposons across phyla

Horizontal transfer (HT), or the passage of genetic material between non-mating species, is increasingly recognized as an important force in the evolution of eukaryotic genomes. Transposons, with their inherent ability to mobilize and amplify within genomes, may be especially prone to HT. However, the means by which transposons can spread across widely diverged species remain elusive. Here we present evidence that host-parasite interactions have promoted the HT of four transposon families between invertebrates and vertebrates. We found that Rhodnius prolixus, a triatomine bug feeding on the blood of various tetrapods and vector of Chagas' disease in humans, carries in its genome four distinct transposon families that also invaded the genomes of a diverse, but overlapping, set of tetrapods. The bug transposons are approximately 98% identical and cluster phylogenetically with those of the opossum and squirrel monkey, two of its preferred mammalian hosts in South America. We also identified one of these transposon families in the pond snail Lymnaea stagnalis, a cosmopolitan vector of trematodes infecting diverse vertebrates, whose ancestral sequence is nearly identical and clusters with those found in Old World mammals. Together these data provide evidence for a previously hypothesized role of host-parasite interactions in facilitating HT among animals. Furthermore, the large amount of DNA generated by the amplification of the horizontally transferred transposons supports the idea that the exchange of genetic material between hosts and parasites influences their genomic evolution.

One hundred years of research on the natural infection of freshwater snails by trematode larvae in Europe

Research on the infection of snails by trematodes has been conducted in Europe for over a hundred years. The initial poor knowledge of the intra-molluscan stages of these parasites together with the difficulty of classifying them constituted a serious obstacle to the undertaking of integrated parasitological and malacological efforts to gain a better understanding of the phenomenon. The compilation of morphological and anatomical results of research on trematode larvae resulted in the publication of keys to designate species of parasites, but was not sufficient to encourage malacologists to collaborate with parasitologists. This paper undertakes to collect data published over the last hundred years on the natural infection of European populations of freshwater snails by trematode larvae. The aim of this undertaking is to make researchers of malacofauna and, above all, experts on freshwater snails aware of the scale of the problem of molluscs being exploited as intermediate hosts of trematodes and, consequently, to encourage parasitologists and malacologists to collaborate on this phenomenon that is crucial for both parasites and hosts.