This review synthesizes recent advances in the discovery of schistosomicidal candidates, with particular emphasis on the medicinal chemistry of heterocyclic scaffolds and the identification of novel molecular targets. To this end, a contextual bibliometric assessment of the literature indexed in the Web of Science and Scopus databases was conducted, considering the chemical classes investigated, antiparasitic activity, parasite life stages evaluated, mechanistic evidence, biological targets explored, and the translational potential of the compounds. Overall, recent studies indicate that the discovery of new agents against Schistosoma mansoni still relies heavily on in vitro and in vivo phenotypic screening approaches. Nevertheless, target-based strategies have gained increasing prominence, particularly those focused on pathways essential for parasite survival, including epigenetic regulation, redox homeostasis, detoxification, nucleotide metabolism, tegument integrity, and neuromuscular function. Heterocyclic compounds, especially those containing nitrogen and sulphur atoms, have emerged as privileged scaffolds owing to their structural diversity, ease of chemical optimization, and ability to interact with multiple biological targets. The analysis also highlights the growing incorporation of computational chemistry, molecular modelling, and ADMET prediction tools in the prioritization of chemical series. Despite these advances, important gaps remain regarding the integration of structure–activity relationship (SAR) studies, target validation, elucidation of mechanisms of action, evaluation across different stages of the parasite life cycle, pharmaceutical formulation development, and pharmacokinetic and pharmacodynamic characterization. Strengthening these aspects is essential to translate promising compounds into clinically relevant candidates capable of overcoming the limitations of praziquantel-based therapy.

Anacanthorus is one of the most diverse genera of monopisthocotyl parasites associated with Neotropical Characiformes, particularly serrasalmid fishes. This study aimed to inventory the monopisthocotyl parasites of serrasalmid fish from the Lower São Francisco River, northeastern Brazil. Twenty-nine specimens of Pygocentrus nattereri were collected, and their parasites were examined using standard morphological methods. Our results allowed the description of Anacanthorus penedensis n. sp., supported by morphological and molecular data, and the assessment of its phylogenetic relationships within the Dactylogyridae using partial 28S rDNA sequences. Additionally, previously described monopisthocotyls are reported, including Anacanthorus reginae, Amphithecium falcatum, and Enallothecium aegidatum, whereas Rhinoxenus sp. was identified to the genus level and another dactylogyrid remained unidentified. Anacanthorus penedensis n. sp. can be distinguished from its congeners by having an accessory piece comprising two distinct expansions: a submedial expansion extending to the distal region and a conspicuous laminar subterminal expansion originating from one of the margins of the shaft, with a hooked acute tip, as well as by the relative lengths of the male copulatory organ and the accessory piece. Phylogenetic analyses of 28S rDNA sequences, based on maximum likelihood and Bayesian inference, placed the new species within the genus Anacanthorus and supported the monophyly of the group based on the dataset used in the present study. These findings reinforce the need for future integrative taxonomic studies that combine complementary lines of evidence for species delimitation and suggest the diversity of Anacanthorus remains underestimated.

Urinary schistosomiasis caused by Schistosoma haematobium may mimic bladder malignancy in patients evaluated outside endemic regions. We report a 20-year-old Somali university student who had been living in Türkiye for two years and had a history of freshwater exposure in Somalia. He presented with painless macroscopic haematuria and lower urinary tract symptoms. An initial 50-mL urine sample collected at 11:00 showed no parasitic structures; a repeat 20-mL terminal-stream sample collected at 13:30 on the same day revealed characteristic terminal-spined S. haematobium eggs on unstained light microscopy of centrifuged urine sediment. Cystoscopy demonstrated an approximately 2-cm sessile bladder lesion suspicious for malignancy and diffuse yellowish granular mucosal lesions (‘sandy patches’). Transurethral resection revealed eosinophil-rich chronic active cystitis and subepithelial schistosome eggs without dysplasia or malignancy. The patient received praziquantel. This case illustrates the importance of epidemiological history, repeated urine microscopy, and recognition of characteristic cystoscopic findings when evaluating bladder masses in patients from endemic regions.

The family Brachycladiidae is the most species-rich trematode taxon restricted to marine mammals, with several species being highly pathogenic. The biology of the brachycladiids remains poorly understood due to confused systematics, scarce genetic data, and unknown circulation pathways. All these challenges collide in the case of Campula oblonga Cobbold, 1858, a widespread parasite of porpoises with an unknown life cycle and signs of genetic and morphological heterogeneity across its range. To resolve this putative species complex, we sampled naticid gastropods in the Barents Sea and searched for potential brachycladiid intramolluscan stages. Several Cryptonatica affinis (Gmelin, 1791) moonsnails harboured rediae and ocellate cercariae whose nuclear (18S, 28S rDNA) and mitochondrial (nad3 mtDNA) sequences confirmed that they belong to C. oblonga. This finding represents the first unambiguous identification of an intermediate host for a trematode parasite restricted to cetaceans. Combining these life-cycle data with available genetic, morphological, and distribution records, following an integrative taxonomic approach, we hypothesised that C. oblonga may comprise three lineages: C. oblonga (to which the newly discovered intramolluscan stages belong), C. folium Ozaki, 1935, and an undescribed species, Campula sp. NWP1.

The present study aims to describe a new genus and species of a plagiorchid digenean as a parasite of the callichthyid freshwater fish Osteogaster aenea from São Paulo State, Brazil. Detailed morphological analyses using light and scanning electron microscopy, combined with molecular characterization based on partial sequences of the 28S rDNA and COI mtDNA genes, were used to characterize the new species and to describe its phylogenetic position within the Digenea. Atriumlateralis capivarensis n. gen. et n. sp. is characterized by an elongate foliaceous body with smooth tegument, a ventral sucker larger than the oral sucker, obliquely arranged testes, dextral ovary, vitelline follicles arranged in two lateral clustered fields extending intra and extracaecally, and a laterally opening genital pore positioned immediately anterior to the ventral sucker. Phylogenetic analyses consistently recovered the new taxon as a well-supported monophyletic lineage, closely related to Choledocystus hepaticus. This study represents the first record of a mature digenean infecting O. aenea and contributes novel morphological and molecular data to the understanding of plagiorchiid diversity and systematics in Neotropical freshwater fishes.

The ecosystem transformation that occurred upon the regulation of the Kureyka River to create the Kureyskoye Reservoir (66°56′47″ N, 88°20′33″ E) (Yenisey River catchment) has profoundly altered the communities and led to the formation of a seat of triaenophoriasis – a disease induced in fish by the cestode Triaenophorus crassus Forel, 1868. The least cisco, Coregonus sardinella Valenciennes, 1848, which has formed a population with unique biological characteristics in the Kureyskoye Reservoir, was used as the case for studying the prevalence and abundance distribution of T. crassus plerocercoids. The helminths were found in fish of all age groups, with prevalences varying from 27.6% in young-of-the-year (0+) to 96.6% in fish aged 4+. In fish infected at high intensities, most plerocercoids were situated freely in the musculature, not encysted in connective tissue. The distribution of plerocercoids among hosts in yearlings proved to be random (fitting the Poisson distribution), becoming aggregated from the age of two years (best approximated by the negative binomial distribution). Infection intensities grow exponentially with fish age, apparently becoming the main cause of elimination of older age groups. The hypothesis that the infection affects fish growth rates in the studied population was not supported. In this Arctic reservoir, a reduction in the trophic level and, hence, a decline in the T. crassus infection in the least cisco can be expected no earlier than 50 years after the impoundment.

As part of a broader survey on entomopathogenic nematodes (EPN), two species, Steinernema carpocapsae and S. thesami, were found for the first time in Serbia at Mt Avala. Two populations of S. carpocapsae (AV10 and AV27) were characterised using morphological and molecular methods. Sequencing of the ITS rDNA region confirmed both isolates as S. carpocapsae. Morphometric examination of infective juveniles and first- and second-generation males revealed generally typical characteristics of S. carpocapsae, although notable intraspecific variation was observed. A comparison of morphometric characters among described S. carpocapsae populations available in the literature is provided. Bacteria associated with the nematode populations were isolated from infected Galleria mellonella larvae and identified by sequencing the 16S rRNA and rpoB genes as Xenorhabdus nematophila, Serratia marcescens, Stenotrophomonas maltophilia, Enterococcus casseliflavus, Bacillus cereus, B. paranthracis, and Priestia megaterium. Pathogenicity bioassays showed that oral administration of bacterial suspensions did not cause mortality in G. mellonella larvae, whereas direct injection into the haemocoel resulted in significant mortality for most isolates. The results expand knowledge of the morphological diversity of S. carpocapsae, pointing out some possible causes that may affect intraspecific variability and providing evidence that several members of the nematode-associated microbiota possess entomopathogenic potential when intrahemocoelically administered.

Nematodes of the superfamily Thelastomatoidea are found in the digestive system of various arthropods, feeding on their host microbiome. They are sometimes considered to be ecologically intermediate forms between free-living rhabditids and parasitic Spirurina, while phylogenetically they are nested within the latter. In addition to new morphological data on the male morphology, this manuscript presents the first nuclear genome assembly of a thelastomatid species, Cranifera cranifera, using a long-read sequencing approach, making a total of three nuclear genomes available for superfamily Thelastomatoidea. The C. cranifera nuclear genome assembly presented here is 243 Mb long, consists of 7563 contigs, has an N50 of 43 kb, and includes 94% of the BUSCO nematoda_odb12 genes. The mitochondrial genome is 24644 bases long, includes a complete set of protein-coding, rRNA, and tRNA genes, and a repetitive region 9731 bases long, which includes multiple copies of tRNA-Asn(gtt) and tRNA-Lys(ttt). The nuclear assembly also contained two sequence variants of the 28S rRNA and two of the 18S rRNA genes, highlighting the presence of intragenomic variation within the rRNA operon. The newly generated assemblies (nuclear and organelle) will add to a growing body of genomic resources for underrepresented and understudied animal parasitic nematodes from Clade 3, enabling comprehensive studies in their phylogeny and trait evolution in the future.

Pseudexorchis Yamaguti, 1938 is a genus of opisthorchioid digeneans, which is currently classified as a member of the Heterophyidae. Only one species is recognised in this genus, Pseudexorchis major (Hasegawa, 1935), which parasitises freshwater silurid fishes as adults. The aim of this study is to evaluate the family affiliation of Pseudexorchis based on phylogenetic data. We obtained the sequences of several genetic markers from a single adult specimen of P. major ex Silurus asotus Linnaeus, 1758 from Japan, which allowed us to identify it as a member of a phylogenetic lineage previously named by Miura and Takisawa (2024) as Type B. The lineage Type B appeared as a sister to P. major Type A of Miura and Takisawa (2024). The degree of genetic differentiation between Type B and Type A, mainly based on the cox1 gene partial sequence, allows us to hypothesise the species status of these lineages. The trees based on 28S rRNA, 28S rRNA + ITS2, and ITS1–5.8S–ITS2 datasets were most informative for phylogenetic inferences. In our analyses, Pseudexorchis was clustered with cryptogonimids Acanthostomum yahuarcaquense Cajiao-Mora and Bullard in Cajiao-Mora, Brule, Dutton, Caicedo-Portilla, and Bullard, 2025, Neocladocystis spp., Tanganyikatrema spp., and the nonugens KC489791 and KC489792 (28S rRNA-tree), or only A. yahuarcaquense (28S rRNA + ITS2-tree), or Acanthostomum cf. americanum (Pérez-Vigueras, 1956) (ITS1–5.8S–ITS2-tree); the clade of these species was closer to the clade formed by opisthorchids and heterophyids than to other cryptogonimids. Thus, the presented data do not support the hypothesis that Pseudexorchis belongs to the Heterophyidae and demonstrate paraphyly of the Cryptogonimidae.

Cyathocotylid genera Prohemistomum Odhner, 1913, Linstowiella Szidat, 1933, and Paracoenogonimus Katsurada, 1914 have a convoluted history associated with continuing uncertainty regarding their validity and the systematic position of their constituent species. During a survey of helminths of ospreys, Pandion haliaetus (L.), we collected representatives of Prohemistomum and Neogogatea Chandler et Rausch, 1947. Morphological examination and sequencing of partial 28S rDNA and cytochrome c oxidase subunit I (cox1) mtDNA genes were conducted to assess their systematic relationships and provide sequence data for future species-level comparisons. We describe Prohemistomum ravindrai n. sp. from ospreys in Georgia and provide molecular data and a supplementary morphological description for Neogogatea kentuckiensis (Cable, 1935). Phylogenetic analysis of 28S sequences recovered a strongly supported Prohemistomum clade containing species previously assigned to Prohemistomum, Linstowiella, and Paracoenogonimus. Morphological characters traditionally used to distinguish these genera, including the presence or absence of a ventral sucker, degree of opisthosoma development, and relative sizes of the oral sucker, pharynx, and cirrus sac, were found to be inconsistent and systematically uninformative. Based on the combined molecular and morphological evidence, we consider Linstowiella and Paracoenogonimus to be junior synonyms of Prohemistomum and transfer their constituent species accordingly. Revised diagnoses for Prohemistomum and Neogogatea are provided. These findings highlight the importance of integrating morphology with molecular phylogenetic data when evaluating generic boundaries.

Ascarid infection contributes to economic losses in chicken farming, requiring targeted parasite control strategies informed by faecal egg counts (FECs). Manual FECs are time-consuming and require specialised training and so are a good target for automation. This study aimed to optimise a new automated FEC system for ascarid ova in chickens (the Parasight All-in-One [AIO]) and compared its performance to the most common manual test used in the field – the McMaster.

Automated counts using the AIO were compared to McMaster counts for correlation, agreement, sensitivity, and precision. Pooled faecal samples from 23 layer houses were collected and run in triplicate for three methods: AIO Bulk Prep (6 g subsample), AIO Single Prep (1 g subsample), and McMaster.

There was a strong correlation between AIO and McMaster counts, regardless of sample preparation method (Bulk Prep: R2 = 0.931, p < 0.001; Single Prep: R2 = 0.801, p ≤ 0.001), and AIO counted approximately 15–24× as many ova. Bland-Altman plots indicated good agreement between both AIO methods and McMaster with no obvious bias.

The larger number of ova counted by AIO was associated with elevated detection frequency. When 30 repeated counts of the same 10 eggs/gram sample were performed by McMaster and both AIO methods, positivity frequencies of 23.3% and 96.7% were obtained for McMaster and AIO, respectively, for the Single Prep method and 33.3% and 96.7%, respectively, for the Bulk Prep method. Precision was also significantly higher for both automated methods compared to McMaster (p ≤ 0.001; p ≤ 0.004).

Wildlife infection with Fasciola spp. has been documented across diverse mammalian taxa and ecological settings; however, the epidemiological significance of these infections remains difficult to determine. Serological exposure, parasite DNA, adult fluke recovery, coproantigen detection, and egg shedding represent different diagnostic states and support various levels of inference. In Fasciola transmission, a patent definitive host is only one part of an indirect cycle that also requires eggs to reach wet or aquatic habitats, compatible lymnaeid snails to become infected and amplify the parasite, and susceptible hosts to ingest the viable metacercariae. From this critical perspective, we ask when wildlife infection actually contributes to persistence or transmission to a defined target population. We used the wildlife reservoir paradox to describe situations in which measured wildlife infection is poorly aligned with the realised transmission contribution, and distinguished four context-dependent functions: incidental hosts, amplifying contributors, interface connectors, and maintenance components. We then evaluated how lymnaeid identity and ecology, environmental and hydrological connectivity, and molecular evidence could strengthen or limit reservoir attribution. Finally, we refined Fasciola Interface Management (FIM) into an operational five-step workflow that asks who contaminates, where snail amplification occurs, which target populations are connected to those sites, where parasite flow can be disrupted, and whether transmission subsequently declines. The central argument is that wildlife reservoir status should be attributed to populations within defined transmission systems, not inferred from host positivity alone.

A new entomopathogenic nematode, Steinernema chainatense n. sp., was isolated during sampling in a cassava field in Chai Nat Province, Central Thailand. A combination of morphological, morphometric, and molecular traits places the new species within the longicaudum clade of the glaseri-group. Infective juveniles (IJs) of S. chainatense n. sp. are characterised by a body length of 871 (798–939) μm, a maximum body diameter of 27 (24–30) μm, and a lateral field with eight equally developed ridges at mid-body and a formula of 2, 6, 8, 2. First-generation males are characterised by large, brown spicules, 88 (80–95) μm long, with a well-developed velum. The male genital papillae number 25 in total, comprising 12 pairs and a single midventral papilla. The internal transcribed spacer (ITS) and D2–D3 sequences differ from those of other members of the longicaudum clade by 3.6–10.4% and 1.5–3.5%, respectively. Phylogenetic analysis based on the ITS region of rDNA indicates a close relationship between S. chainatense n. sp. and S. taiwanensis, S. adamsi, S. guangdongense, and S. longicaudum, with S. taiwanensis as its sister taxon. The bacterial symbiont of S. chainatense n. sp. is the recently described Xenorhabdus thailandensis. These morphological and molecular differences support the recognition of S. chainatense n. sp. as a distinct species.

The present study describes a new species of Jainus (Dactylogyridae) parasitizing the gills of Agoniates halecinus Müller & Troschel, 1845 (Triportheidae) from Brazil. The new species differs from its congeners primarily by a unique combination of haptoral and copulatory characters. The ventral anchors possess a short superficial root and an elongate deep root, both bearing long, slender, weakly sclerotized extensions, whereas the ventral bar bears a comparatively long, weakly sclerotized median process. The new species also presents dissimilar hooks and a distinctive copulatory complex, characterized by a J-shaped male copulatory organ (MCO) forming a single counterclockwise ring and an accessory piece connected to the base of the MCO by a delicate ligament. The accessory piece is laminar, with pointed extremities, a distal opening through which the copulatory tube passes, and an elongate, strongly curved projection tapering to a pointed tip. Phylogenetic analyses based on partial large subunit (LSU) rDNA and cytochrome c oxidase subunit I (COI) mtDNA sequences recovered the two sequences of the new species together as a distinct lineage within Jainus. Molecular characterization of the new species provides additional sequence data for the genus and contributes to the current framework for investigating phylogenetic relationships within Jainus. This study represents the first record of a species of Jainus parasitizing a fish of the genus Agoniates, expanding the known host range of Jainus and increasing knowledge of its diversity within Triportheidae. These findings contribute to a broader understanding of the diversity and host associations of monopisthocotylans infecting Neotropical freshwater fishes.

Two nematode species parasitizing different developmental stages of the Indian rice frog Fejervarya kawamurai collected from Kasugai City, Japan, were examined using an integrative taxonomic approach combining morphology, scanning electron microscopy (SEM), and molecular analyses. Tadpoles of F. kawamurai were found to be infected exclusively by the oxyuroid nematode Gyrinicola japonicus, whereas adult frogs harboured a cosmocercoid nematode representing a new species of Cosmocerca, namely C. kasugaiensis n. sp. Detailed morphological observations revealed diagnostic characters of the new species, including the presence of well-developed lateral alae, equal spicules distinctly longer than the gubernaculum, five pairs of precloacal plectanes surrounded by incomplete circles of tubercles, and somatic papillae in both sexes. Molecular analyses based on nuclear 28S rDNA, internal transcribed spacer (ITS), and mitochondrial cox1 sequences showed no intraspecific variation among individuals of C. kasugaiensis n. sp., and consistently supported its distinctiveness from its congeners by Assemble Species by Automatic Partitioning (ASAP) and Bayesian inference analyses. The redescription of G. japonicus incorporates new features, including cephalic structures and egg morphological details revealed by SEM. This study also provides the ITS and cox1 sequences for this species for the first time. Morphological and molecular analyses confirmed the distinct species position within Gyrinicola.

Hookworm infection, caused by Necator americanus and Ancylostoma duodenale, is prevalent in areas with poor sanitation. Coprology is the gold standard for diagnosis, but it has low sensitivity in low parasite loads. The objective of this study was to evaluate four Polymerase Chain Reaction (PCR) targets for molecular diagnosis. Stool samples were collected from 180 residents of the rural community Tierra Blanca, Bolívar State, Venezuela, in which coproparasitological tests were performed. Four DNA extraction methods were standardized, and four PCR techniques were studied: NaITS1F/NaITS1R (ITS1), Na58/Na158R (ITS2), NA/NC2 (ITS2-28S) and NC1/NC2 (ITS2-28S) for N. americanus. Only two of these techniques could be standardized and evaluated, as amplification was not achieved with the other two. The two standardized techniques were applied to all samples, and diagnostic indices were obtained. The results showed that extraction with the Wizard kit was the most suitable due to the quality of the DNA obtained. Primer analysis revealed high identity, but with sequences mainly from Asia and Africa, since there are very few available from America. PCR-NaITS1F/NaITS1R (ITS1) demonstrated the best diagnostic indices, significantly surpassing coprology. PCR-Na58/Na158R (ITS2) showed lower sensitivity than coprology. Therefore, PCR-NaITS1F/NaITS1R (ITS1) is recommended for molecular diagnosis. The prevalence of N. americanus in the Tierra Blanca community was 20.6 % by coprology, 88.3 % by PCR-NaITS1F/NaITS1R (ITS1) and 94.4% by the combination of these techniques. The high prevalence of infections underscores the need for molecular techniques but these must be previously evaluated with samples from the regions where they will be used.

A new species of the genus Sychnotylenchus, S. golestanicus sp. n., is described and illustrated from northern Iran. The new species is characterised by a body length of 595–831 μm, a continuous lip region, a very slender stylet 7–10 μm long with small but distinct knobs, a lateral field with six to eight incisures, a heart-shaped median bulb with a refractive valve, and an excretory pore located at or anterior to the median bulb. The crustaformeria (eggshell-forming zone) consists of 16 cells arranged in 4 longitudinal rows of 4 large, rounded cells each, with the anterior part expanded and functioning as a spermatheca. Females have the vulva situated at 88.5–90.3% of body length, a post-vulval uterine sac measuring 1.4–2.2 times the vulval body diameter, and a short, broad tail with a truncate to slightly flattened terminus, usually showing a shallow terminal depression and lacking a mucro. Males possess smoothly curved spicules 16–21 μm long, a simple gubernaculum approximately one-third of the spicule length, and broad peloderan bursal alae. The new species is morphologically most similar to S. hibisci, but differs from the mentioned species by a combination of characters including the shape of the female tail, the number of lateral field incisures, the structure of the crustaformeria, the length of the post-vulval uterine sac, and the position of the excretory pore. Phylogenetic analysis based on 18S rDNA sequences placed S. golestanicus sp. n. within a well-supported clade (posterior probability = 1.00) comprising S. hibisci and other Sychnotylenchus species, supporting its placement within the genus Sychnotylenchus. The congruence between morphological and molecular evidence supports the recognition of S. golestanicus sp. n. as a distinct species. The present study also highlights the need for additional molecular markers and further integrative taxonomic studies, potentially including TEM investigations, to clarify the taxonomic significance of the crustaformeria and better resolve phylogenetic relationships within the Sychnotylenchinae.

Human angiostrongyliasis is an emerging foodborne zoonosis caused predominantly by Angiostrongylus cantonensis and Angiostrongylus costaricensis, which produce distinct neurological and abdominal forms of disease, respectively. Human infection occurs primarily through ingestion of raw or undercooked gastropod intermediate hosts, paratenic hosts such as freshwater prawns and crabs, or fresh produce contaminated with infective third-stage larvae (L3). A. cantonensis is the principal cause of neuroangiostrongyliasis and eosinophilic meningitis, whereas A. costaricensis causes abdominal angiostrongyliasis, characterized by eosinophilic inflammation involving the intestinal wall and mesenteric vasculature. Although historically concentrated in tropical and subtropical regions, A. cantonensis has expanded its geographic range, while A. costaricensis remains predominantly distributed in Latin America. This changing epidemiology is influenced by climate and environmental change, host redistribution, globalisation, urbanisation, and dietary practices. Diagnosis remains challenging and requires integration of epidemiological exposure, clinical findings, and laboratory and imaging results. For neuroangiostrongyliasis, cerebrospinal fluid (CSF) eosinophilia, serological testing, and molecular assays, including increasingly sensitive real-time PCR approaches, provide important diagnostic evidence, whereas abdominal angiostrongyliasis frequently requires imaging, histopathology, and, where available, molecular or serological confirmation. Treatment of neuroangiostrongyliasis remains centred on corticosteroids and management of intracranial pressure, while the role of albendazole has historically been controversial but is increasingly supported as adjunctive therapy when administered with corticosteroids. In abdominal angiostrongyliasis, surgical management remains an important component of diagnosis and treatment, particularly in patients with intestinal complications. Emerging molecular diagnostics, host-directed therapies, novel anthelmintic strategies, and improved surveillance approaches offer promising opportunities for future disease control. Continued progress will require an integrated One Health approach combining epidemiological surveillance, improved diagnostic capacity, evidence-based clinical management, food-safety education, and ecological monitoring of transmission.

Cestodes of the genus Fimbriarioides (Hymenolepididae, Fimbriariinae) parasitise waterfowl and shorebirds in the Holarctic. Using molecular and morphological methods, we studied two Fimbriarioides spp. from sea ducks of the European North of Russia: the type species F. intermedia from eiders Somateria mollissima and S. spectabilis 1758 and a closely related species F. lintoni from a previously unrecognised host, Velvet Scoter Melanitta fusca 1758. Our analysis also involved metacestodes Fimbriarioides sp. from barnacles Semibalanus balanoides of the Barents Sea and the Sea of Okhotsk. Metacestodes from S. balanoides matched the adults of F. intermedia by the molecular markers. In the phylogenetic tree based on 28S rRNA and cox1 genes, isolates from the two Fimbriarioides spp. formed distinct monophyletic groups in a clade sister to Fimbriaria spp. In the 16S rRNA tree, Fimbriaria sp. was positioned as a sister clade to F. intermedia, probably due to the marker’s limited resolution. We provided emended morphological descriptions of F. intermedia and F. lintoni and diagnostic characters for their discrimination. The haplotype network based on the 16S rRNA gene, which included the isolates from the Barents Sea, the Pechora Sea, the White Sea, and the Sea of Okhotsk, indicated that F. intermedia has a trans-Arctic distribution and is represented by the same population in all these regions. This is due to the high vagility of its definitive hosts, sea ducks, which make long-distance migrations along the coasts of the northern Palaearctic seas, and the almost circumpolar distribution of its intermediate hosts, barnacles.