Trichobilharzia mergi sp. nov. (Trematoda: Digenea: Schistosomatidae), a visceral schistosome of Mergus serrator (L.) (Aves: Anatidae)

Molecular identification of Trichobilharzia species in recreational waters in North-Eastern Poland

Background

In Europe, avian schistosomes of the genus Trichobilharzia are the most common etiological agents involved in human cercarial dermatitis (swimmer's itch). Manifested by a skin rash, the condition is caused by an allergic reaction to cercariae of nonhuman schistosomes. Humans are an accidental host in this parasite's life cycle, while water snails are the intermediate, and waterfowl are the final hosts. The study aimed to conduct a molecular and phylogenetic analysis of Trichobilharzia species occurring in recreational waters in North-Eastern Poland.

Methodology

The study area covered three water bodies (Lake Skanda, Lake Ukiel, and Lake Tyrsko) over the summer of 2021. In total, 747 pulmonate freshwater snails (Radix spp., Lymnaea stagnalis) were collected. Each snail was subjected to 1-2 h of light stimulation to induce cercarial expulsion. The phylogenetic analyses of furcocercariae were based on the partial sequence of the ITS region (ITS1, 5.8S rDNA, ITS2 and 28SrDNA). For Radix spp. phylogenetic analyses were based on the ITS-2 region.

Results

The prevalence of the Trichobilharzia species infection in snails was 0.5%. Two out of 478 (0.4%) L. stagnaliswere found to be infected with Trichobilharzia szidati. Moreover, two out of 269 (0.7%) snails of the genus Radix were positive for schistosome cercariae. Both snails were identified as Radix auricularia. One of them was infected with Trichobilharzia franki and the other with Trichobilharzia sp.

Conclusions

Molecular identification of avian schistosome species, both at the intermediate and definitive hosts level, constitutes an important source of information on a potential threat and prognosis of local swimmer's itch occurrence, and helps to determine species diversity in a particular area.

Zoonotic Threats: The (Re)emergence of Cercarial Dermatitis, Its Dynamics, and Impact in Europe

Cercarial dermatitis (CD), or "Swimmer's itch" as it is also known, is a waterborne illness caused by a blood fluke from the family Schistosomatidae. It occurs when cercariae of trematode species that do not have humans as their definitive host accidentally penetrate human skin (in an aquatic environment) and trigger allergic symptoms at the site of contact. It is an emerging zoonosis that occurs through water and is often overlooked during differential diagnosis. Some of the factors contributing to the emergence of diseases like CD are related to global warming, which brings about climate change, water eutrophication, the colonization of ponds by snails susceptible to the parasite, and sunlight exposure in the summer, associated with migratory bird routes. Therefore, with the increase in tourism, especially at fluvial beaches, it is relevant to analyze the current epidemiological scenario of CD in European countries and the potential regions at risk.

Expanding the swimmer's itch pool of the Benelux: a first record of the neurotropic Trichobilharzia regenti and potential link to human infection

BACKGROUND

Swimmer's itch, an allergic contact dermatitis caused by avian and mammalian blood flukes, is a parasitic infection affecting people worldwide. In particular, avian blood flukes of the genus Trichobilharzia are infamous for their role in swimmer's itch cases. These parasites infect waterfowl as a final host, but incidental infections by cercariae in humans are frequently reported. Upon accidental infections of humans, parasite larvae will be recognized by the immune system and destroyed, leading to painful itchy skin lesions. However, one species, Trichobilharzia regenti, can escape this response in experimental animals and reach the spinal cord, causing neuroinflammation. In the last few decades, there has been an increase in case reports across Europe, making it an emerging zoonosis.

METHODS

Following a reported case of swimmer's itch in Kampenhout in 2022 (Belgium), the transmission site consisting of a private pond and an adjacent creek was investigated through a malacological and parasitological survey.

RESULTS

Six snail species were collected, including the widespread Ampullaceana balthica, a well-known intermediate host for Trichobilharzia parasites. Shedding experiments followed by DNA barcoding revealed a single snail specimen to be infected with T. regenti, a new species record for Belgium and by extension the Benelux. Moreover, it is the most compelling case to date of the link between this neurotropic parasite and cercarial dermatitis. Additionally, an Echinostomatidae sp. and Notocotylus sp. were isolated from two other specimens of A. balthica. However, the lack of reference DNA sequences for these groups in the online repositories prevented genus- and species-level identification, respectively.

CONCLUSIONS

The presence of T. regenti in Belgium might have severe clinical implications and its finding highlights the need for increased vigilance and diagnostic awareness among medical professionals. The lack of species-level identification of the other two parasite species showcases the barcoding void for trematodes. Overall, these findings demonstrate the need for a Belgian framework to rapidly detect and monitor zoonotic outbreaks of trematode parasites within the One Health context.

First species record of Strigea falconis Szidat, 1928 (Trematoda, Strigeidae) from gyrfalcon Falco rusticolus in Iceland-pros and cons of a complex life cycle

Strigea falconis is a common parasite of birds of prey and owls widely distributed in the Holarctic. We aimed to characterise S. falconis from Iceland via integrative taxonomic approach and to contribute to the understanding of its circulation in the Holarctic. We recovered adult S. falconis from two gyrfalcons (Falco rusticolus) collected in 2011 and 2012 in Iceland (Reykjanes Peninsula, Westfjords) and characterised them by morphological and molecular genetic (D2 of rDNA, cox1, ND1 of the mDNA) methods. We provide the first species record of S. falconis in Iceland which to the best of our knowledge is its northernmost distributional range. The presence of S. falconis in Iceland is surprising, as there are no suitable intermediate hosts allowing completion of its life cycle. Gyrfalcons are fully sedentary in Iceland; thus, the only plausible explanation is that they acquired their infection by preying upon migratory birds arriving from Europe. Our data indicate that the most likely candidates are Anseriformes and Charadriiformes. Also, we corroborate the wide geographical distribution of S. falconis, as we found a high degree of similarity between our haplotypes and sequences of mesocercariae from frogs in France and of a metacercaria from Turdus naumanni in Japan, and adults from Buteo buteo and Circus aeruginosus from the Czech Republic. The case of Strigea falconis shows the advantages of a complex life cycle and also depicts its pitfalls when a parasite is introduced to a new area with no suitable intermediate hosts. In Iceland, gyrfalcons are apparently dead-end hosts for S. falconis.

DNA Barcoding of Trichobilharzia (Trematoda: Schistosomatidae) Species and Their Detection in eDNA Water Samples

We designed and tested species-specific PCR primers to detect Trichobilharzia species via environmental DNA (eDNA) barcoding in selected Austrian water bodies. Tests were performed with eDNA samples from the field as well as with artificial samples from the lab, where snails releasing cercariae were kept in aquariums. From two localities, Trichobilharzia was documented based on the release of cercariae from snails, enabling morphological species identification. In both cases, the corresponding species were detected via eDNA: Trichobilharzia szidati and Trichobilharzia physellae. Nonetheless, the stochasticity was high in the replicates. PCR tests with aquarium water into which the cercariae had been released allowed eDNA detection even after 44 days. As in the PCRs with eDNA samples from the field, positive results of these experiments were not obtained for all samples and replicates. PCR sensitivity tests with dilution series of T. szidati genomic DNA as well as of PCR amplification products yielded successful amplification down to concentrations of 0.83 pg/µL and 0.008 pg/µL, respectively. Our results indicate that the presumed species specificity of PCR primers may not be guaranteed, even if primers were designed for specific species. This entails misidentification risks, particularly in areas with incomplete species inventories.

Threat of cercarial dermatitis in Hungary: A first report of Trichobilharzia franki from the mallard (Anas platyrhynchos) and European ear snail (Radix auricularia) using molecular methods

Cercarial dermatitis in humans is a re-emerging zoonotic disease caused by infectious larvae of avian blood flukes within the Schistosomatidae family. Upon water contact, these avian schistosome larvae directly penetrate human skin and cause irritation. Between September 2018 and September 2020, carcasses of 94 mallards (Anas platyrhynchos), two green-winged teals (Anas crecca) along with one ferruginous duck (Aythya nyroca) were examined. Birds were collected within 12 regions of Hungary, representative of 9 different counties. Inspecting both morphological characters and molecular data, the cytochrome oxidase subunit I (COI) and large subunit ribosomal DNA (LSU rDNA) sequences, Bilharziella polonica and Dendritobilharzia pulverulenta were each identified. Most importantly, Trichobilharzia franki was found for the first time in Hungary and in 5 dispersed counties in conjunction with of 3 counties where other avian schistosomes were found. Of note, these avian blood flukes were predominately encountered in wild mallards (∼50%) and not in birds reared for hunting (∼1%). In total, 245 European ear snails (Radix auricularia), a known intermediate host of Trichobilharzia spp., were collected from an urban pond in Eger, Hungary. Five snails (∼2%) consistently shed numerous furcocercariae of T. franki that were confirmed by molecular methods. Our findings help to pinpoint a contemporary life cycle of this avian schistosome within an urban environment, a location regularly visited by various wild waterfowl and the possible species of avian schistosomes responsible for cercarial dermatitis in Hungary. Taken as a whole, we demonstrate the actual and potential risk zone for cercarial dermatitis, particularly with reference to R. auricularia distributions, within Hungary and across Central Europe.

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Molecular confirmed presence of three species of avian schistosomes in birds in Hungary.

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Molecularly confirmed Tricobilharzia franki invasion in Radix auricularia in Eger.

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Potential risk of swimmers itch related to the spread of R.auricularia.

Diversity of Trichobilharzia in New Zealand with a new species and a redescription, and their likely contribution to cercarial dermatitis

In response to annual outbreaks of human cercarial dermatitis (HCD) in Lake Wanaka, New Zealand, ducks and snails were collected and screened for avian schistosomes. During the survey from 2009 to 2017, four species of Trichobilharzia were recovered. Specimens were examined both morphologically and genetically. Trichobilharzia querquedulae, a species known from four continents, was found in the visceral veins of the duck Spatula rhynchotis but the snail host remains unknown. Cercaria longicauda [i.e. Trichobilharzia longicauda (Macfarlane, 1944) Davis, 2006], considered the major aetiological agent of HCD in Lake Wanaka, was discovered, and redescribed from adults in the visceral veins of the duck Aythya novaeseelandiae and cercariae from the snail Austropeplea tomentosa. Recovered from the nasal mucosa of Ay. novaeseelandiae is a new species of Trichobilharzia that was also found to cycle naturally through Au. tomentosa. Cercariae of a fourth species of Trichobilharzia were found in Au. tomentosa but the species remains unidentified.

First Record of Trichobilharzia physellae (Talbot, 1936) in Europe, a Possible Causative Agent of Cercarial Dermatitis

Several species of avian schistosomes are known to cause dermatitis in humans worldwide. In Europe, this applies above all to species of the genus Trichobilharzia. For Austria, a lot of data are available on cercarial dermatitis and on the occurrence of Trichobilharzia, yet species identification of trematodes in most cases is doubtful due to the challenging morphological determination of cercariae. During a survey of trematodes in freshwater snails, we were able to detect a species in the snail Physella acuta (Draparnaud, 1805) hitherto unknown for Austria, Trichobilharzia physellae; this is also the first time this species has been reported in Europe. Species identification was performed by integrative taxonomy combining morphological investigations with molecular genetic analyses. The results show a very close relationship between the parasite found in Austria and North American specimens (similarity found in CO1 ≥99.57%). Therefore, a recent introduction of T. physellae into Europe can be assumed.

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.

Global prevalence status of avian schistosomes: A systematic review with meta-analysis

Objectives

Human cercarial dermatitis (HCD) is a water-borne zoonotic parasitic disease. Cercariae of the avian schistosomes of several genera are frequently recognized as the causative agent of HCD. Various studies have been performed regarding prevalence of bird schistosomes in different regions of the world. So far, no study has gathered and analyzed this data systematically. The aim of this systematic review and meta-analysis study was to determine the prevalence of avian schistosomes worldwide.

Methods

Data were extracted from six available databases for studies published from 1937 to 2017. Generally, 41 studies fulfilled the inclusion criteria and were used for data extraction in this systematic review. Most of studies have been conducted on the family Anatidae.

Results

The overall prevalence of avian schistosomes was estimated to be 34.0% (95%CI, 28%-41%) around the world. Furthermore, results displayed that, Allobilharzia visceralis and Trichobilharzia spp. had the highest frequency and their prevalence in the birds was 50.0% (95% CI, 3.0%-97.0%) and 32.0% (95% CI, 21.0%-0.36%), respectively. The results showed that the prevalence of avian schistosomes was 43.0% (95% CI, 29% - 56%) in the US and 38.0% (27.0% -50.0%) in Europe, which were higher than other continents, respectively.

Conclusions

The prevalence of 34% shows that the bird schistosomes are very common zoonotic worms among aquatic birds in the world. Also, this study shows the importance of avian schistosome research when facing animal and human health of the future.

An efficient method for collecting the full-length adults, fragments, and eggs of Trichobilharzia spp. from the liver of definitive hosts

Precise identification of avian schistosomes in the genus Trichobilharzia at the species level is difficult and requires both traditional morphological and molecular techniques. To obtain satisfactory results by traditional methods, the characteristics of the intact adults or large fragments of male and females are necessary. The present study aimed to introduce a more efficient method for collecting eggs and both fragments and intact worms for morphological identification of visceral Trichobilharzia spp. Thirty-eight domestic ducks (twenty-eight fresh and ten frozen) were studied. For fresh samples, warm saline (40-45 °C) was injected into the portal vein or liver tissue, followed by slicing of the liver to small pieces in a large Petri dish. All materials were then transferred into the laboratory sieves arranged from the largest to the smallest mesh size and while crushed with the hand, washed, and filtered using a trigger water sprayer. The collected materials were studied under a stereomicroscope for parasite eggs, fragments, and full-length worms. Out of 28 freshly killed ducks, 19 (67.9%) and of 10 frozen ducks 6 (60%) were positive for visceral Trichobilharzia spp. The full-length worms and large fragments of male worms were mostly recovered with the mesh no. 150 (diameter of 106 μm) and small fragments, especially of females, and eggs with the mesh no. 270 (diameter of 53 μm). In addition to large numbers of fragments, 15 full-length adults were obtained from fresh and 2 from frozen ducks. The number of collected full-length adults was related to the worm burden. Since morphological description of different species of the genus Trichobilharzia is primarily based on the availability of adult worms, the application of methods that provide a higher number of intact males and females will result in better characterization of the species and deposition of appropriate voucher specimens. These results show the present method as a suitable tool for the collection of quality adults of visceral Trichobilharzia spp. in ducks.

Chance or choice? Understanding parasite selection and infection in multi-host communities

Ongoing debate over the relationship between biodiversity and disease risk underscores the need to develop a more mechanistic understanding of how changes in host community composition influence parasite transmission, particularly in complex communities with multiple hosts. A key challenge involves determining how motile parasites select among potential hosts and the degree to which this process shifts with community composition. Focusing on interactions between larval amphibians and the pathogenic trematode Ribeiroia ondatrae, we designed a novel, large-volume set of choice chambers to assess how the selectivity of free-swimming infectious parasites varied among five host species and in response to changes in assemblage composition (four different permutations). In a second set of trials, cercariae were allowed to contact and infect hosts, allowing comparison of host-parasite encounter rates (parasite choice) with infection outcomes (successful infections). Cercariae exhibited consistent preferences for specific host species that were independent of the community context; large-bodied amphibians, such as larval bullfrogs (Rana catesbeiana), exhibited the highest level of parasite attraction. However, because host attractiveness was decoupled from susceptibility to infection, assemblage composition sharply affected both per-host infection as well as total infection (summed among co-occurring hosts). Species such as the non-native R. catesbeiana functioned as epidemiological 'sinks' or dilution hosts, attracting a disproportionate fraction of parasites relative to the number that established successfully, whereas Taricha granulosa and especially Pseudacris regilla supported comparatively more metacercariae relative to cercariae selection. These findings provide a framework for integrating information on parasite preference in combination with more traditional factors such as host competence and density to forecast how changes within complex communities will affect parasite transmission.

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.

Putative new genera and species of avian schistosomes potentially involved in human cercarial dermatitis in the Americas, Europe and Africa

New larval avian schistosomes found in planorbid snails from Brazil and USA were used for morphological and molecular studies. Eggs with a distinctive long polar filament were found in ducks infected experimentally with Brazilian cercariae. Similar eggs were reported previously in wild or experimentally infected anatids from Brazil, South Africa, and the Czech Republic. Molecular phylogenetic analyses showed that the North American and European schistosomes are sister taxa, which are both sister to the Brazilian species. However, these clades do not group with any named genus. Molecular data plus egg morphology suggest that these are new putative genera and species of avian schistosomes that can cause human cercarial dermatitis in the Americas, Africa and Europe.

Phylogenetic analysis of nasal avian schistosomes (Trichobilharzia) from aquatic birds in Mazandaran Province, northern Iran

Nasal schistosomes are trematodes in the family Schistosomatidae, many members of which are causative agents of human cercarial dermatitis (HCD). Little is known about the species diversity and distribution of nasal dwelling schistosomes of water birds, particularly in countries outside of Europe; even less is known in countries like Iran. Nasal schistosomes are of particular interest since these species migrate via the central nervous system to the nasal cavity once they penetrate their host. Thus, there must be efforts to determine the incidence of HCD due to nasal schistosomes. HCD outbreaks are reported seasonally in Mazandaran Province, northern Iran, an area well known for rice cultivation leading to increased person contact with water and infected snails. Such places include favorable habitat for both domestic ducks year round, and wild migratory ducks in the winter through spring. Recent reports have detected the presence of both nasal and visceral schistosomes in ducks in this area but with little species characterization. In this study, we examine a diversity of aquatic birds to determine the distribution, prevalence and bird host use of nasal schistosomes. We apply for the first time a molecular identification and phylogenetic analysis of these schistosomes. From 2012 to 2014, the nasal cavity of 508 aquatic birds from Mazandaran Province were examined that included species in Anseriformes, Gruiformes, Charadriiformes and Phoenicopteriformes. Nasal schistosomes were found in 45 (8.9%) birds belonging to Anseriformes (Anas platyrhynchos and Anas clypeata). Phylogenetic analysis of the nuclear internal transcribed spacer 1 rDNA and the mitochondrial cytochrome oxidase1 gene of isolated eggs revealed that all samples grouped in a sister clade to the European Trichobilharzia regenti. However, Trichobilharzia from this study were more similar to a unique haplotype of Trichobilharzia, isolated from the nasals of an A. clypeata in France. The genetic and phenotypic differences between the species found herein and T. regenti from Europe, may prove with additional data to be a distinct species of Trichobilharzia.

Trichobilharzia anseri n. sp. (Schistosomatidae: Digenea), a new visceral species of avian schistosomes isolated from greylag goose (Anser anser L.) in Iceland and France

Parasitological investigations carried out on birds in Iceland and France highlight the presence of four species of avian schistosomes from greylag geese (Anser anser L.): the european nasal species Trichobilharzia regenti and three visceral species, among which an unknown species isolated from blood vessels of the large intestine and liver. Morphological and molecular analyzes of different parasite stages (eggs, adults) revealed new species of Trichobilharzia genus – Trichobilharzia anseri sp. nov. Studies on host-parasite relationship under natural conditions, showed that the life-cycle includes the snail Radix balthica (syn. R. peregra) as intermediate host. The cercariae, already isolated in Iceland from two ponds of the Reykjavik capital area – the Family park and Tjörnin Lake – are the same as those isolated in 1999 by Kolářová et al. during the first study on Icelandic parasitic agents of cercarial dermatitis.

Avian schistosomes and outbreaks of cercarial dermatitis

Cercarial dermatitis (swimmer's itch) is a condition caused by infective larvae (cercariae) of a species-rich group of mammalian and avian schistosomes. Over the last decade, it has been reported in areas that previously had few or no cases of dermatitis and is thus considered an emerging disease. It is obvious that avian schistosomes are responsible for the majority of reported dermatitis outbreaks around the world, and thus they are the primary focus of this review. Although they infect humans, they do not mature and usually die in the skin. Experimental infections of avian schistosomes in mice show that in previously exposed hosts, there is a strong skin immune reaction that kills the schistosome. However, penetration of larvae into naive mice can result in temporary migration from the skin. This is of particular interest because the worms are able to migrate to different organs, for example, the lungs in the case of visceral schistosomes and the central nervous system in the case of nasal schistosomes. The risk of such migration and accompanying disorders needs to be clarified for humans and animals of interest (e.g., dogs). Herein we compiled the most comprehensive review of the diversity, immunology, and epidemiology of avian schistosomes causing cercarial dermatitis.

Schistosomes in South African penguins

During the years 2009-2012, faeces of African penguins (Spheniscus demersus L.) from South African rehabilitation centres were examined for helminths. In total, 46 out 555 samples (8.29 %), mostly belonging to adult birds, were found to contain oval schistosome eggs with a spine on one pole. Their dimensions were 153.21 ± 9.07 × 87.14 ± 8.67 μm. Selected DNA fragments (18S, 28S and ITS rDNA) were sequenced and compared to other schistosome isolates deposited in GenBank. The shape of the eggs suggests that they belong to the genus Gigantobilharzia; however, due to the insufficient stage of knowledge of the genus and limited number of species available for comparison, we were not able to assign the isolate unambiguously to this genus based on either the egg morphology or the results of molecular analysis.

Diversity and ancestry of flatworms infecting blood of nontetrapod craniates "fishes"

We herein review all published molecular studies (life history, taxonomy, and phylogeny) and summarize all GenBank sequences and primer sets for the "fish blood flukes". Further, by analysing new and all available sequence data for the partial D1-D2 domains of 28S from 83 blood fluke taxa, we explore the evolutionary expansion of flatworm parasitism in the blood of craniates. Based on this analysis, the blood flukes infecting marine bony fishes (Euteleostei) are monophyletic. The clade comprising the chondrichthyan blood fluke plus the marine euteleost blood flukes is the sister group to tetrapod blood flukes (spirorchiids and schistosomes). The innominate blood fluke cercariae from freshwater gastropods were monophyletic and sister to the clade comprising spirorchiids and schistosomes, but low nodal support indicated that they may represent a distinct blood fluke lineage with phylogenetic affinities also to fish blood flukes. Blood flukes that utilize gastropod intermediate hosts were monophyletic (unidentified gastropod cercariae+tetrapod blood flukes) and those utilizing bivalves and polychaetes were monophyletic (marine fish blood flukes). Low or no taxon sampling among blood flukes of basal fish lineages and primary division freshwater fish lineages are significant data gaps needing closure. We also note that no record of an infection exists in a hagfish (Myxiniformes), lamprey (Petromyzontiformes), or nontetrapod sarcopterygiian, i.e., coelacanth (Coelacanthimorpha) or lungfish (Dipnoi). The present phylogenetic analysis reiterated support for monophyly of Schistosomatidae and paraphyly of spirorchiids, with the blood flukes of freshwater turtles basal to those of marine turtles and schistosomes.

Discovery-based studies of schistosome diversity stimulate new hypotheses about parasite biology

This review provides an update of ongoing efforts to expand our understanding of the diversity inherent within the Schistosomatidae, the parasites responsible for causing schistosomiasis and cercarial dermatitis. By revealing more of the species present, particularly among understudied avian schistosomes, we gain increased understanding of patterns of schistosome diversification, and their abilities to colonize new hosts and habitats. Schistosomes reveal a surprising ability to switch into new snail and vertebrate host species, into new intrahost habitats, and may adopt novel body forms in the process. Often these changes are not associated with deep splits or long branches in their phylogeny, suggesting some are of relatively recent origin. Several hypotheses prompted by the new observations are discussed, helping to focus thinking on processes influencing not only schistosome diversification but also their pathogenicity and abundance.