An expanded dataset of caryophyllidean tapeworms (Cestoda) of the genus Promonobothrium Mackiewicz, 1968, parasites of suckers (Cypriniformes: Catostomidae) in North America, was used to reassess the host specificity of these parasites. Molecular data from newly collected specimens from diverse hosts revealed contrasting patterns in host range among congeneric tapeworms, ranging from strict specialists (half of all species, namely P. minytremi, P. fossae, P. mackiewiczi, P. papiliovarium and Promonobothrium sp. from Ictiobus niger) parasitizing a single host species, to family generalists (three species, namely P. hunteri, P. ingens and P. rogersi) occurring in fishes of different sucker subfamilies (Catostominae and Ictiobinae). Promonobothrium currani is considered a congeneric specialist and P. ulmeri a subfamily generalist. The high proportion of generalists (40%) is exceptional in the Caryophyllidea. Based on this study, it is recommended to verify records of tapeworms from atypical hosts, preferably using molecular data from vouchered specimens (hologenophores or paragenophores). https://zoobank.org/urn:lsid:zoobank.org:pub:EBFB81F5-6565-45D4-A9B8-48C146591248

Biological invasions are among the most disruptive forces affecting global biodiversity. The "killer shrimp" Dikerogammarus villosus, a pervasive Ponto-Caspian invader across Europe, serves as a key model for understanding the ecological consequences of such expansions. However, despite its extensive spread, the helminth fauna associated with this invader remains poorly characterized, obscuring potential spillover effects and the evolutionary history of its symbionts. To address this gap, we surveyed 54 European freshwater localities (2020-2025) and screened 1271 D. villosus specimens from 10 positive stations. These sites span the Danube (Slovakia) and Baltic (Poland) drainage basins, representing the southern and northern invasion corridors. Using an integrative taxonomic approach combining detailed morphology (12 fixed adults and 4 live metacercariae) and multi-locus phylogenetics (18S, 28S, ITS, and COI), we formally describe Maritrema dikerogammari sp. nov. infecting D. villosus. Our analyses place the new species within a distinct freshwater clade as the sister taxon to the Carpathian endemic Maritrema neomi. Historical records of Maritrema inusitatum from the Dnieper River were misidentifications, and we reassign them to the new species, confirming the parasite's longstanding presence in both the Danube and Dnieper basins. This study documents a clear case of "hidden co-invasion," where a cryptic parasite established itself in the invasion corridor undetected. Phylogenetic reconstruction reveals that the evolution of this group is driven by a fundamental marine-to-freshwater transition, shaped by ancient vicariance and recent host-facilitated dispersal. We refine the taxonomy of the genus Maritrema and underscore the critical role of invasion routes in the spread of associated fauna.

During an ichthyoparasitological survey in northeast Thailand in 2015, four species of acanthocephalans were found in four species of freshwater fishes. Adult stages of Pallisentisrexus Wongkham & Whitfield, 1999 and Raosentis sp. (Eoacanthocephala, Quadrigyridae) were collected from Channastriata (Anabantiformes, Channidae) and Mystusmysticetus (Siluriformes, Bagridae), respectively, and cystacanths of Arhythmorhynchus sp. (Palaeacanthocephala, Polymorphidae) were found on the visceral organs of M.albolineatus (Siluriformes, Bagridae). Adult acanthocephalans of the subgenus Acanthosentis Verma & Dutta, 1929 isolated from Barbonymusschwanenfeldii (Cypriniformes, Cyprinidae) were morphologically distinct from all described species of the subgenus in the arrangement of rings of tegumental spines in two fields with a more or less pronounced distance between them, and by the presence of a dome-shaped process with a ring of small spines at the base at the posterior end in females. Molecular data were generated for three species and phylogenetic analysis based on the 18S rDNA clearly supported the generic identification of Acanthogyrus (Acanthosentis) barbonymisp. nov. and P.rexus. While the phylogenetic position of the former species within the genus could not be clarified, the latter species formed a well-supported sister lineage in a clade with isolates of four congeneric species. Acanthogyrus (Acanthosentis) barbonymisp. nov. is formally described, the first genetic data for P.rexus are generated, a species of the genus Raosentis Datta, 1947 is reported for the first time outside the Indian subcontinent, and a key to the species of the latter genus is presented.

Estimating sample size is important for prevalence studies, as it directly influences the validity of the research outcomes. Our objective was to highlight constraints in the prevalence assessment and to provide guidance on the delineation of minimum and reasonable sample size. We also assess the prevalence properties as a function of sample size visualizing the median prevalence, confidence intervals, precision, and changes in precision. Constraint analysis indicates that a sample size of less than 15 host individuals will likely result in unacceptable precision in the most cases. Because the prevalence estimate accuracy depends on both sample size and the estimate itself, the minimum sample size may vary widely, from 16 to over 450 individuals, when the prevalence is between 1% and 99%. A sample size of 16-45 elements can be used as a minimum for estimating true prevalence between 10% and 90% with an acceptable precision. However, caution should be exercised with a such small sample size as the prevalence will have a high degree of uncertainty. A simple, practical suggestion for selecting a minimum sample size is to sample until at least 5 infected (cases) and 5 uninfected (non-cases) hosts are detected. This approach is effective in most situations, except in cases of extreme prevalence (1% or 99%). The design of a reasonable sample size should be based on a flexible strategy that takes into account the study objectives, available resources and desired precision. This strategy may based on finding the plateau phase within the precision or confidence intervals curves. As the uncertainty in prevalence decreases rapidly with increasing sample up to 110-135 individuals, but not much more with further increasing sample efforts, opting for a sample size exceeding this threshold, could be considered an optional choice within the prevalence range of 5-95%. We advise authors, editors and reviewers to track sample size in conjunction with the actual prevalence of the parasites and other pathogens. If the minimum sample size is unattainable, authors should acknowledge this limitation, as all data contribute to understanding parasite distribution.

The host diversity, complex life cycle and distribution of digeneans underline their value as bioindicators of ecosystem health. We examined the intestinal digenean trematode Cephalotrema elasticum of insectivorous mammals, sampled from Gammarus balcanicus (Crustacea: Amphipoda) as second intermediate hosts. The species is particularly relevant as part of the helminth species complex associated with protected mammals. Our study is based on an intensive survey of helminths in 8920 gammarids collected from 38 localities in Slovakia, Poland, and France. DNA sequences for three markers (18S rDNA, 28S rDNA, and CO1 mtDNA) were analysed using Bayesian inference. Molecular data suggest that C. elasticum represents a distinct genetic lineage closely related to Cryptotropidae. Rare anatomical traits, such as the marginal or submarginal genital pore arrangement, may influence mating success and reproductive isolation within Microphalloidea. Genetic and morphological evidence supports establishing a new family, Cephalotrematidae, within Microphalloidea, to include Cephalotrema, Pseudocephalotrema, and Opisthioparorchis. A family diagnosis and a key to the genera are provided to facilitate further taxonomic studies. The geographic distribution of C. elasticum highlights its utility as a biological indicator, reflecting the decline of water shrew populations due to climate-induced ecological changes, particularly in the French Pyrenees.

Fish-borne zoonoses are emerging worldwide, and although most human cases remain confined to tropical regions, particularly Southeast and East Asia, a few cases have been reported in Europe. This review summarizes published and new data on the life cycle of Metagonimus romanicus (misidentified as M. yokogawai, a human pathogen common in East Asia), a heterophyid trematode and one of the potentially fish-borne parasites in Europe. Metagonimus romanicus is distributed from the middle Danube in Central Europe (Slovakia) to eastern Ukraine (including the rivers of the Black Sea basin). Its distribution area coincides with that of its first intermediate hosts, the melanopsid snails Microcolpia daudebartii acicularis and Esperiana esperi. While M. romanicus most likely has a strict specificity for its first snail hosts, metacercariae are generalists and have been detected in over 50 freshwater fish species from 18 families, especially leuciscids. Despite its wide distribution and frequent occurrence in freshwater fish, the zoonotic potential of M. romanicus appears to be low. This is probably due to the exclusive localization of the metacercariae in the fish scales rather than musculature, as well as the rare consumption of raw or undercooked fish in Europe. However, some risk remains and a small number of undiagnosed human infections due to accidental ingestion of scales during the preparation and cleaning of fish cannot be ruled out.

Cercariae of the zygocercous morphotype represent a fascinating and unique morphological and ecological adaptation among trematodes (Digenea). They are characterised by an unusual behaviour in which individuals join their tails to form rosette-like clusters consisting of several to hundreds of cercariae that actively move in the water by wagging their tails, resulting in a rolling motion. This aggregation behaviour is a textbook example of prey mimetism, a strategy in which a parasite resembles the prey of a predator (i.e., the next host in the life cycle) in its appearance and movements. In this article, we review all known reports of zygocercous cercariae, whose identity is largely unknown. A total of 16 putative species forming groups of 'rat-king' type cercariae (i.e., zygocercous) have been documented in freshwater and marine snails in 12 countries worldwide. Zygocercous cercariae occur in several unrelated groups of trematodes and have evolved independently several times, apparently as an effective ecological adaptation to increase the success of transmission of the cercariae to the next host, usually a fish. In addition, we report newly discovered zygocercous cercariae found in the non-pulmonate snail Microcolpia daudebartii acicularis (Férussac) (Caenogastropoda: Melanopsidae) from the Danube River in Slovakia. This is the first record of this type of cercaria in Europe in almost 150 years. Molecular data suggest that these cercariae belong to the recently resurrected genus Mesorchis Dietz, 1909 (Echinochasmidae), which includes species with 20 or 22 collar spines. Five taxa previously assigned to the genus Echinochasmus Dietz, 1909 (whose species generally have 24 collar spines) and for which molecular data are available are hereby transferred to Mesorchis as new combinations.

The present study investigates the distribution of cadmium, chromium, and mercury levels in sediments, plants, and fish from Zemplínska Šírava, a historically contaminated reservoir in Slovakia. Using fish as bioindicators, we analyzed the impact of factors such as age, gender, ecological group, and parasite burden on heavy metal accumulation. Results revealed that mercury was highest in fish muscle, particularly predatory species, whereas chromium and cadmium were predominantly found in liver tissues. Chromium accumulation was influenced by gender, with females showing higher levels. Older fish exhibit greater cadmium accumulation in liver tissue. Parasite burden was associated with elevated mercury levels in fish muscle. Notably, the cestode Caryophyllaeus laticeps had significantly higher chromium levels than its host, freshwater bream (Abramis brama). This study highlights the important role of fish and parasites in monitoring aquatic contamination and illustrates the complex interplay of biological and environmental factors in heavy metal bioaccumulation.

A major current challenge related to invasions is the absence of a comprehensive theoretical basis for preventing the introduction of alien species and controlling their populations. By using a macroecological framework, we aim to examine patterns of symbiotic species diversity across native and invasive hosts to understand the mechanisms underlying the host–parasite system stability. The symbiotic communities of seven gammarid species, two native and five invasive, were analyzed at 16 fresh and brackish water sites along the Baltic coast of Poland. We investigated the influence of four factors—locality, habitat, host species, and host origin—on diversity patterns and assessed their relative contribution to diversity variation. Our results indicate that all factors are crucial in determining the composition and abundance of symbiotic communities in gammarids. Among these factors, locality and host species apparently have a greater influence on symbiotic communities than habitat conditions and host origin. Comparative analysis of diversity indexes of symbiotic organisms from native and invasive hosts showed that the richer communities of native gammarids were paradoxically less diverse. We assume that these rich and uneven symbiotic communities keep the host–parasite system of native gammarids in equilibrium. Highly dominant symbiotic species with a high load per host individual may stabilize the growth of the host population. Symbiotic communities of the invasive host exhibited lower species richness but displayed a relatively even distribution of species with moderate loads per individual, resulting in a scarcity of heavily infected gammarids within the population. Thus, the survival rate of invasive gammarids is expected to be relatively high, facilitating their population growth and further spread. We conclude that complex communities of native gammarids constructed from many symbiotic species may prevent host populations from undergoing explosive growth, while such mechanisms may be completely or partially reduced in invasive hosts.

The tapeworms belonging to the order Caryophyllidea (Platyhelminthes, Eucestoda) represent a widespread group of intestinal parasites found in cypriniform and siluriform fishes. This group comprises four families (Balanotaeniidae, Capingentidae, Caryophyllaeidae, Lytocestidae), 42 genera and approximately 130 described species. Additionally, over a hundred new taxa have been reported from India. While many of these taxa are endemic and have unique evolutionary importance, a considerable number of putative new taxa were also invalidated because the quality and reliability of the published data did not meet the requirements. Various researchers have synonymised 28 species into a single representative, Lytocestus indicus (Moghe, 1925) Woodland, 1926. Consequently, L. indicus was chosen as a model organism to investigate whether there is taxonomic inflation or hidden diversity. Newly collected specimens of L. indicus were subjected to various fixatives for morphological assessment, and hologenophores of the same were obtained to determine whether true variations exist in their genotype or if they are merely fixation-induced variations. Despite morphological variations associated with fixation, molecular data indicated that they were indeed the same specimens. The taxonomic status of 43 new taxa was critically evaluated, with most of them tentatively synonymised with L. indicus. Others were also found to be conspecific with L. indicus but were classified as unavailable names based on their first description in unpublished dissertations.

The black bullhead Ameiurus melas is an invasive fish species that has been introduced into Europe. In this account, proteocephalid tapeworms of the subfamily Essexiellinae, which are specific intestinal parasites of ictalurid catfishes in North America, are reported from A. melas from the Danube River in Slovakia. Molecular phylogenetic analysis based on sequences of the 28S rRNA and COI genes revealed that the tapeworms from Slovakia are almost identical to Corallotaenia sp. 1 of Scholz et al. (2020) from A. melas in Oklahoma and Wisconsin, USA and are morphologically similar. This is the second report of the occurrence of essexielline tapeworms in Europe after 35 years. The relatively common occurrence of these tapeworms in black bullheads from one of the largest European river that connects Western and Eastern Europe is alarming, and their further spread throughout Europe is likely.

In this study, we performed a cytogenetic analysis of Acanthocephalus lucii specimens from three sites with different levels of environmental pollution. Standard and fluorochrome staining (CMA3/DAPI), fluorescence in situ hybridization (FISH) with 18S rDNA and histone H3 probes, and silver impregnation were performed. Chromosome complements of 2n = 7/8 (male/female), n = 1m + 2sm + 1a (X), and CMA3-positive bands in all chromosomes were found in all three populations. FISH revealed one 18S rDNA locus on the X chromosome and one locus of H3 histone genes on the first chromosome pair. At the intraspecific level, the populations differed in the presence of supernumerary B chromosomes, which were found in all specimens from Zemplínska Šírava and in 89.4% of specimens from the Laborec River, but not at the reference site. The first two sites are considered to be water bodies with high toxin contamination. Based on this fact, we assume an increased frequency of chromosome breaks leading to the formation of DNA fragments that have the potential to form B chromosomes. The present results add to the very limited data on the organization of multigene families in the genome of Acanthocephala and suggest a possible causal link between water pollution and the occurrence of B chromosomes in fish parasites.

The heterophyid trematode Metagonimus romanicus (Ciurea, 1915) (Digenea) is redescribed on the basis of type material from domestic dogs (Canis familiaris) in Romania, vouchers from experimentally infected cats (Felis catus) and adults recovered from golden hamsters (Mesocricetus auratus) infected with metacercariae from scales of chub (Squalius cephalus) and common nase (Chondrostoma nasus) (Cypriniformes: Leuciscidae) in Hungary. This trematode, endemic to Europe and neighbouring regions (northwestern Türkiye), was previously misidentified as M. yokogawai (Katsurada, 1912), a zoonotic parasite of humans in East Asia. However, the two species differ considerably both genetically and morphologically, e.g., in the position of the ventral sucker, the presence of the prepharynx, the anterior extent of the vitelline follicles and the posterior extent of the uterus. Metagonimus ciureanus (Witenberg, 1929) (syn. Dexiogonimus ciureanus Witenberg, 1929), described from domestic cats and dogs in Israel, is a valid species distributed in the Middle East and Transcaucasia, which is also confirmed by molecular data. It differs from all Metagonimus species, including M. romanicus, in having symmetrical testes instead of the oblique testes of the other congeners. The zoonotic significance of M. romanicus and M. ciureanus is unclear, but appears to be low in Europe, mainly because raw or undercooked, whole fish with scales are generally not consumed. Accidental infection of fishermen by metacercariae in the scales when cleaning fish is more likely, but has never been reported. Remains of cyprinoids with scales infected with metacercariae of Metagonimus spp. can be an important natural source of infection for dogs, cats, and other carnivores, which can serve as a reservoir for these parasites.

The spatial distribution of polychlorinated biphenyls (PCBs), in the Zemplínska Šírava water reservoir and adjacent tributaries in the Bodrog River Basin were investigated using a fish-parasite sentinel system. PCB concentrations were detected in various fish matrices (dorsal and abdominal muscles, liver and intestine) of the Wels catfish (Silurus glanis) and its intestinal cestode Glanitaenia osculata. PCB concentrations in the fish from the water reservoir, located closest to the chemical plant, the primary source of the PCB pollution, were the highest. The analysis of these contaminants in catfish matrices showed the highest concentrations in the abdominal muscle, followed by the dorsal muscle, liver and intestine. Concentrations of ∑PCBs exceeding the limits for food set by European regulations were measured in the muscle tissue of catfish at all sites, even in the Bodrog River, 60 km away from the primary source of contamination, posing a significant risk to humans in the Zemplín region. For the first time, the ability of cestode G. osculata to accumulate higher amounts of PCBs compared to fish matrices has been demonstrated. Due to the enormous ability of the parasites to accumulate PCBs, we recommend this approach for alternative biomonitoring of PCBs in contaminated aquatic environments.

Dibothriocephalus ditremus and Dibothriocephalus latus are diphyllobothriidean tapeworms autochthonous to Europe. Their larval stages (plerocercoids) may seriously alter health of their intermediate fish hosts (D. ditremus) or cause intestinal diphyllobothriosis of the final human host (D. latus). Despite numerous data on the internal structure of broad tapeworms, many aspects of the morphology and physiology related to host–parasite co-existence remain unclear for these 2 species. The main objective of this work was to elucidate functional morphology of the frontal part (scolex) of plerocercoids, which is crucial for their establishment in fish tissues and for an early attachment in final hosts. The whole-mount specimens were labelled with different antibodies and examined by confocal microscope to capture their complex 3-dimensional microanatomy. Both species exhibited similar general pattern of immunofluorescent signal, although some differences were observed. In the nervous system, FMRF amide-like immunoreactivity (IR) occurred in the bi-lobed brain, 2 main nerve cords and surrounding nerve plexuses. Differences between the species were found in the structure of the brain commissures and the size of the sensilla. Synapsin IR examined in D. ditremus occurred mainly around FMRF amide-like IR brain lobes and main cords. The unexpected finding was an occurrence of FMRF amide-like IR in terminal reservoirs of secretory gland ducts and excretory canals, which has not been observed previously in any tapeworm species. This may indicate that secretory/excretory products, which play a key role in host–parasite relationships, are likely to contain FMRF amide-related peptide/s.

Fish products in Slovakia have been heavily infected with Anisakis spp. larvae, which causes human anisakiasis. We found larvae in all tested samples of frozen Atlantic herring. Anisakid allergen t-Ani s7 testing revealed 2 positive cases in humans, signaling need for health authorities to closely monitor zoonotic marine parasites, even in inland areas.

Diphyllobothriosis is an infectious disease caused by the consumption of raw freshwater or marine fish containing larvae of broad tapeworms (Diphyllobothriidae). In the present study, we critically reviewed all cases of human diphyllobothriosis reported from Taiwan, including unpublished reports from hospitals in Taipei. Genotyping based on mitochondrial DNA marker (cox1) confirmed that two of the recent cases were caused by Dibothriocephalus nihonkaiensis, which is not native to Taiwan and was probably imported with Pacific salmon infected with larvae of D. nihonkaiensis. The causative species previously reported in Taiwan could not be definitively confirmed. However, considering the distribution of Dibothriocephalus latus, which is not endemic in Taiwan, past cases diagnosed as D. latus are questionable.

During an ichthyoparasitological survey in 2017-2019, six species of acanthocephalans were found among Taiwan's freshwater (Cypriniformes: Xenocyprididae, Cyprinidae) and marine fishes (Scombriformes: Scombridae, Trichiuridae; Anabantiformes: Channidae; Carangaria/misc: Latidae): Micracanthorhynchina dakusuiensis (Harada, 1938), Rhadinorhynchus laterospinosus Amin, Heckmann et Ha, 2011, Pallisentis rexus Wongkham et Whitfield, 1999, Longicollum sp., Bolbosoma vasculosum (Rudolphi, 1819), and one new species, Micracanthorynchina brevelemniscus sp. n. All species are morphologically characterised and illustrated using light and scanning electron microscopy. The finding of R. laterospinosus, P. rexus and B. vasculosum is the first record for these species in Taiwan. Micracanthorhynchina brevelemniscus is similar to Micracanthorhynchina motomurai (Harada, 1935) and M. dakusuiensis in proboscis armature but differs from M. motomurai by larger eggs (53-59 × 15-16 µm vs 40 × 16 µm) and by the number of cement glands (6 vs 4) and from M. dakusuiensis by shorter body length (2.2-2.9 mm vs 4.0 mm in males and 2.9-4.1 mm vs 7.6 mm in females), by the location of the organs of the male reproductive system (from level of the posterior third of the proboscis receptacle in M. brevelemniscus vs in the posterior half of the trunk in M. dakusuiensis), and by length of lemnisci (lemnisci shorter than the proboscis receptacle vs lemnisci longer than the proboscis receptacle). Phylogenetic analyses of almost complete 18S rRNA gene revealed paraphyly of the family Rhadinorhynchidae suggested in previous studies. Micracanthorhynchina dakusuiensis and M. brevelemniscus formed a strongly supported cluster, which formed the earliest diverging branch to the rest of the rhadinorhynchids and transvenids.

The cytogenetics of Acanthocephala is a neglected area in the study of this group of endoparasites. Chromosome number and/or karyotypes are known for only 12 of the 1,270 described species, and molecular cytogenetic data are limited to rDNA mapping in two species. The standard karyological technique and mapping of 18S rRNA and H3 histone genes on the chromosomes of Acanthocephalus anguillae individuals from three populations, one of which originated from the unfavorable environmental conditions of the Zemplínska Šírava reservoir in eastern Slovakia, were applied for the first time. All specimens had 2n = 7/8 (male/female); n = 1m + 1m-sm + 1a + 1a (X). Fluorescence in situ hybridization (FISH) revealed three loci of 18S rDNA on two autosomes and dispersion of H3 histone genes on all autosomes and the X chromosome. In addition to the standard A chromosome set, 34% of specimens from Zemplínska Šírava possessed a small acrocentric B chromosome, which was always found to be univalent, with no pairing observed between the B chromosome and the A complement. The B chromosome had a small amount of heterochromatin in the centromeric and telomeric regions of the chromosomal arms and showed two clusters of H3 genes. It is well known that an environment permanently polluted with chemicals leads to an increased incidence of chromosomal rearrangements. As a possible scenario for the B chromosome origin, we propose chromosomal breaks due to the mutagenic effect of pollutants in the aquatic environment. The results are discussed in comparison with previous chromosome data from Echinorhynchida species.

The caryophyllidean genus Archigetes Leuckart, 1878 is unique among all tapeworms in that its species can mature in invertebrate hosts (Oligochaeta), i.e., have a monoxenic (direct) life cycle. All five species were described as progenetic plerocercoids in oligochaetes and two of them also as adults from cypriniform fishes. Two species, A. sieboldi Leuckart, 1878 and A. iowensis Calentine, 1962, were found in North America in non-native common carp (Cyprinus carpio). A molecular study of caryophyllideans from the southern United States has revealed the occurrence of three new species in native freshwater fishes (Catostomidae, Ictiobinae): Archigetes loculotruncatus n. sp. from Ictiobus bubalus, I. niger and Carpiodes cyprinus is the largest representative of the genus and differs by a loculotruncate scolex. Archigetes megacephalus n. sp. from Ictiobus niger, I. bubalus and I. cyprinellus is characterised by a prominent, bothrioloculodiscate scolex. Archigetes vadosus n. sp. from I. bubalus is typified by a globular scolex with very shallow loculi; it differs from the closely related A. sieboldi in the shape of the body, with a distinct neck region and a scolex wider than the remaining body. Archigetes iowensis Calentine, 1962 becomes a junior synonym of Paraglaridacris limnodrili (Yamaguti, 1934). The generic diagnosis of Archigetes is amended and a key to identification of North American taxa is provided. Species of Archigetes and Paraglaridacris differ from each other most conspicuously in the structure of the ovary, which is follicular in Archigetes versus compact in Paraglaridacris.