While most higher taxa in the Hemiuroidea exhibit remarkable diversity and thrive worldwide in marine ecosystems, a few groups have successfully radiated into freshwater environments. Among them, species of Thometrema are well known from freshwater and brackish fishes in the New World. In this study, specimens of Thometrema from a single specimen of the endangered pimelodid catfish Steindachneridion scriptum in the Upper Paraná River basin, State of São Paulo, Brazil, were subjected to morphological, ultrastructural (first scanning electron microscopy data for the genus) and molecular analyses. Newly obtained sequences of the partial large subunit nuclear ribosomal RNA (28S rDNA) and partial mitochondrial cytochrome c oxidase subunit I (COI mtDNA) provided strong evidence supporting the identification of the specimens as T. patagonicum, a trematode previously known only from endemic percichthyid fishes (Percichthyidae) in the Argentinian Patagonia. This unexpected finding, along with critical morphological evaluation of new and deposited material, led us to propose T. overstreeti and T. bonariense as junior synonyms of T. patagonicum. Moreover, we reassessed certain morphological features, revealed new potential distinguishing characters (papillae) and raised doubts about the taxonomic value of commonly used features for species differentiation (e.g., vitellarium masses). To overcome taxonomic problems in Thometrema due to the paucity of morphological features to clearly set species boundaries, new descriptions could leverage integrative approaches, as employed here, to establish robust species hypotheses and ultimately unravel the actual species composition in the Americas.

For many trematode species, individual reproductive parthenitae in first intermediate host colonies senesce, die, and are replaced by newly born parthenitae. The times involved in these processes are poorly understood. Here, we present an approach to estimate parthenita death rates and lifespans that uses readily obtainable data on senescent parthenita frequencies, brood sizes, and offspring (cercaria) release rates. The onset of parthenita senescence is often marked by the degeneration and disappearance of the germinal mass, its source of new offspring. Following germinal mass loss, the remaining viable offspring in a senescent parthenita finish development and are birthed before parthenita death. Therefore, a senescing parthenita’s remaining lifespan is the time it takes for all its viable offspring to mature and exit. We can estimate this time by measuring whole-colony (infected snail) cercaria shed rates, dissecting colonies to count reproductives, and then apply the per redia cercaria production rate to the observed brood sizes of senescent parthenitae. The per-capita parthenita death rate is then calculated as the proportion of parthenitae that are senescent divided by their average remaining lifespan. Reproductive parthenita lifespan is the inverse of this death rate. We demonstrate the approach using philophthalmid trematodes, first providing documentation of a free-floating germinal mass in 4 philophthalmids, and then, for 3 of those species, estimating parthenita senescence rates, death rates, and lifespans. This method should be broadly applicable among trematode species and help inform our understanding of trematode colony dynamics, social structure, and the evolution of parthenita senescence.

The East Asian region has seen the first discovery of trematodes of the genus Psilotrema in the muskrat Ondatra zibethicus. When studying the morphology of the trematodes, two morphotypes were identified among individuals of Psilotrema spp., the differences between which at the morphological level are equivalent to those between species. The affiliation of one of these morphotypes with the bird parasite Psilotrema limosum was established based on partial sequences of genes coding for 28S ribosomal RNA, nad1, and cox1 mitochondrial DNA. Based on the analysis of these three markers, we classified individuals with this morphotype as P. limosum, despite some morphological features of worms infecting birds. At the same time, the worms with second morphotype had no interspecies differences based on the first two markers, while the cox1 gene confirmed the separation of these samples on a species level. These trematodes were described as a new species, Psilotrema elegans n. sp., based on both morphological and molecular data.

This report provides an overview of available data on the ultrathin structure and the mechanisms of formation of cysts around acanthocephalans of the classes Palaeacanthocephala and Archiacanthocephala in intermediate hosts. In most of the described cases, the cyst is bi-layered, consisting of an outer layer that is relatively thick and vesicular, and an underlying layer that is thin and amorphous. In acanthellae of some species, a lamellar layer is observed on the cyst surface, which degrades presumably at the cystacanth stage. Three probable mechanisms of cyst formation in acanthellae of different species have been identified. The first and second mechanisms involve the border of microvilli that develops on the tegument surface. In acanthellae of the first group, the cyst’s vesicular layer is formed by detachment of fragments or small vesicles from microvilli, a process resembling the micro-apocrine secretion, while the amorphous layer is composed of electron-dense material released from invaginates of the tegument’s outer membrane and the remaining microvilli. In acanthellae of the second group, the cyst is single-layered and formed presumably of whole microvilli separated from the tegument. Since acanthellae of the third group lack microvilli on the tegument surface, the vesicular layer of the cyst is formed of a few small vesicles and amorphous material synthesized deep in the tegument and in large vesicles transported to the parasite’s surface. The amorphous, electron-dense layer of the cyst is formed in the same manner as in the first group, but without microvilli.

Gyrodactylidae Cobbold, 1864 is a monophyletic family of hyperviviparous species, with 36 genera and approximately 700 species. Here, we focused on one of these genera – namely Scleroductus Jara & Cone, 1989 – characterizing its morphological variation systematically and testing its phylogenetic position in relation to other genera in the family. We collected and describe two new species of monogenean parasites of Scleroductus infecting Neotropical freshwater catfishes in Mexico. Scleroductus veracruzano n. sp. infects Guatemalan chulín, Rhamdia guatemalensis in Veracruz, while Scleroductus oaxaqueno n. sp. infects filespine chulín, Rhamdia laticauda in Oaxaca, Mexico. Sequences of the internal transcribed spacer (ITS rDNA) and 18S rDNA were generated for both species. Phylogenetic hypotheses based on maximum likelihood and Bayesian inference analyses of 18S rDNA indicate that the two new Mexican taxa are closely related to each other, and both appear as sister species to an undescribed but molecularly characterized Scleroductus sp. infecting South American catfish, Rhamdia quelen in Brazil. Our analyses confirm the placement of Scleroductus spp. within the monophyletic family Gyrodactylidae, whose sister clade is the Oogyrodactylidae. Within Gyrodactylidae, Scleroductus spp. form a clade sister to a well-supported clade composed by members of the genera Ieredactylus, Gyrodactyloides and Laminiscus.

Steringotrema microacetabularis Suriano & Martorelli, 1983 (Fellodistomidae) was described from the flounder Paralichthys orbignyanus. Later, it was redescribed, based on new material from the same host and type locality, and reconsidered as Bacciger microacetabularis (Baccigeridae). The main difference noted in the redescription was the presence of spines on the body. However, the lack of DNA data made confirming the true affiliation of this digenean challenging. New specimens sampled from P. orbignyanus allowed us to sequence the 28S, ITS, and COI genes. Fresh specimens were stained to compare their morphology with the holotype and voucher specimens. The digeneans found correspond with those reported from Mar Chiquita, described as B. microacetabularis (=S. microacetabularis). Genetic analyses clustered the newly sequenced individuals within the Cryptogonimidae, showing relationships with Oligogonotylus manteri, Tabascotrema verai, and Caecincola parvulus (28S); T. verai, Lobosorchis spp., Euryakaina manilensis, and Metadena marina (ITS); and Siphoderina spp. (COI). After comparing the species with cryptogonimids lacking spines in the oral sucker, a new genus, Surianotrema n. gen., is described. This genus increases the number of cryptogonimid genera known in South America to seven – three in freshwater and four in marine environments – with Surianotrema n. gen. being the first to be sequenced. New sequences from other South American cryptogonimids are necessary to better understand the phylogenetic relationships between genera within this family, particularly in South America.

The techniques employed to collect and store trematodes vary between research groups, and although these differences are sometimes necessitated by distinctions in the hosts examined, they are more commonly an artefact of instruction. As a general rule, we tend to follow what we were taught rather than explore new techniques. A major reason for this is that there are few technique papers in the published literature. Inspired by a collaborative workshop at the Trematodes 2024 symposium, we outline our techniques and processes for collecting adult trematodes from fishes and discuss the improvements we have made over 40 years of dissections of 20,000+ individual marine fishes. We present these techniques for two reasons: first, to encourage unified methods across the globe, with an aim to produce optimally comparable specimens across temporal periods, across geographic localities, and between research groups; and second, as a resource for inexperienced researchers. We stress the importance of understanding differences in host biology and the expected trematode fauna, which ultimately enables organised and productive dissections. We outline our dissection method for each key organ separately, discuss handling, fixation, and storage methods to generate the most uniform and comparable samples, and explore ethical considerations, issues of accurate host identification, and the importance and potential of clear record keeping.

In this study, we investigated the diversity and survival of aestivating snails in dry-season rice paddies, focusing on their role as intermediate hosts for trematodes. A total of 1,159 snails from various families were collected and analysed, revealing nine species, primarily from the Bithyniidae family, with Bithynia siamensis siamensis being the most abundant. Of the nine species collected, the most common species, B. s. siamensis, exhibited a relatively high survival rate of 81.5% throughout the three-month dry period post-harvest, whereas the remaining eight species had survival rates below 24.0% (0–23.1%). Four snail species, B. s. siamensis, Filopaludina martensi martensi, F. sumatrensis polygramma, and F. doliaris, were found infected with larval trematodes, with an overall prevalence of 8.46% (57/674). The larval trematodes identified were categorized into four types: echinostome metacercaria, Thapariella anastomusa metacercaria, unidentified metacercaria, and unidentified rediae. These findings demonstrate that trematode infections can persist in aestivating snails despite the challenging conditions during the dry season, emphasizing the potential of aestivating snails to act as vectors for trematode transmission in agricultural settings. This underscores the need for effective management strategies to mitigate the risks associated with trematode transmission.

Stephanofilaria is a genus of nematodes that cause ulcerative dermal lesions in large mammals. However, there is a dearth of knowledge on the molecular genetics of Stephanofilaria species infecting critically endangered rhinoceros. This study employed genetic barcoding genes to identify Stephanofilaria species and to determine its genetic diversity and evolution. Phylogenetic analyses on partial genes of the second internal transcribed spacer Ribosomal DNA (ITS-2) and cytochrome c oxidase subunit 1 (Cox-1), revealed a 77% and 93% bootstrap support at the Cox-1 and ITS-2 loci respectively to a clade containing previously identified Stephanofilaria species. Morphological examination also confirmed features diagnostic of Stephanofilaria dinniki previously known to infect rhinoceros. Gene diversity of Cox-1 was 0.931 ± 0.030 and 0.579 ± 0.104 for the ITS-2, whereas nucleotide diversity was 0.008 ± 0.002 and 0.00197 ± 0.0016 for the Cox-1 and ITS-2 genes respectively. Neutrality tests (Fu and Li’s D* and Fu and Li’s F*) were significantly negative (p<0.05) at all loci, whereas Tajima D and Fu’s FS were each statistically significant (p<0.05) at the Cox-1 and ITS-2 loci respectively. The high gene diversity, low nucleotide diversity and negative neutrality tests are consistent with positive selection at the Cox-1 gene. Stephanofilaria infection among rhinoceros is currently restricted to highland sanctuaries compared to a widespread distribution in both lowlands and highlands in the 1960s suggesting an adaptation to vectors thriving in cooler highland temperatures. This is the first genetic identification of S. dinniki, in rhinoceros and will aid in diagnosis, treatment, studies, and rhinoceros conservation.

The viruses associated with bats have generated significant concern; however, there is limited knowledge regarding the endoparasites that affect these mammals. This study involved the collection of seven nematode specimens (three males and four females) from the intestines of Hipposideros armiger in Shaoguan City, Guangdong, China. Next-generation sequencing was employed to obtain the mitochondrial DNA (mtDNA) genome, which was determined to be 14,130 base pairs in length. The mitochondrial genome comprised 12 protein-coding genes, 21 tRNA genes, 2 rRNA genes, and an AT-rich non-coding region. Phylogenetic analyses based on mtDNA sequences indicated that the nematode forms a sister clade to Nematodirus, exhibiting only 74% nucleotide identity. In contrast, the nuclear ITS1 gene demonstrated a high degree of nucleotide identity (98.6%–98.8%) with Durettenema guangdongense. Consequently, the parasitic nematode identified from H. armiger is likely to belong to the genus Durettenema and has been designated as Durettenema sp. 888. Furthermore, an epidemiological investigation revealed the presence of the parasitic nematode infections in H. armiger collected from Guangdong, Guangxi, and Guizhou Provinces. Given the widespread distribution of H. armiger and their tendency to inhabit areas in close proximity to human dwellings, the influence of parasite prevalence on bat population numbers and potential for human and domestic animal transmission of this pathogen warrants further investigation.

Parkinson’s disease (PD) is a prevalent neurological disorder and the second most common neurodegenerative disease. Research has explored the impact of infectious agents, such as the parasites, on neurological conditions, including PD. Given the limited studies worldwide and in Iran, this study aims to investigate the relationship between Toxocara infection and PD. This case-control study involved 91 PD patients and 90 healthy controls. After obtaining consent, serum samples and questionnaires were collected. All sera were examined using an ELISA test for IgG antibodies against Toxocara canis. Results were analyzed with SPSS, using chi-square tests, and odds ratios (OR), and confidence intervals (CI) were calculated via univariate and multivariate analyses. The prevalence of anti-Toxocara IgG was 33% (30/91) in PD patients and 33.3% (30/90) in the control group. Both univariate analysis (OR: 0.98; 95% CI: 0.52–1.82) and multivariate analysis (OR: 0.95; 95% CI: 0.49–1.83) indicated no statistically significant association. Additionally, univariate analysis (OR: 0.49; 95% CI: 0.16–1.5) and multivariate analysis (OR: 0.37; 95% CI: 0.09–1.43) suggested non-significant association between Toxocara infection and the severity of PD. Our findings do not support a statistically significant association between Toxocara infection and the PD. While the analysis suggested that Toxocara infection might reduce the severity of PD, these results were also not statistically significant. Further research with larger sample sizes and diverse populations is needed to fully understand the potential relationship between Toxocara infection and PD.

A BioBlitz is a rapid and intensive survey of a specific geographic area that brings together experts and often lay participants to assess biodiversity, typically of macrobiota that are easily observed and identifiable on-site. This concept has become popular across taxonomic fields, attracting interest globally to increase knowledge of local biodiversity. Inspired by the success of the approach, we undertook a ‘ParasiteBlitz’ at an unexplored locality (Stono Preserve, Charleston, South Carolina, USA) to determine its feasibility for parasites, whose assessment of diversity is largely neglected worldwide. We assembled a team of parasitologists with complementary expertise. Over 12 days (3 days in each habitat) in April 2023, we intensively screened fishes and aquatic invertebrates for parasites, and sampled sediment and water for environmental DNA (eDNA) metabarcoding from four aquatic habitats: wetland, freshwater pond, brackish impoundment, and tidal creek. We incorporated assistance from non-parasitologists and students. Details on methodologies and results are provided in individual papers in this Special Collection. Traditional methods revealed the presence of ca. 100 species of seven major metazoan parasite taxa, and the eDNA survey yielded over 1,000 amplicon sequence variants identified as parasites, most with sequences unmatched in GenBank, and resulting in only a few species identified as named species in the one-year post-Blitz timeframe we imposed upon ourselves for identification. Limitations and challenges of the ParasiteBlitz are discussed, and our results support that this approach can be effective for rapid discovery of the dimensions of parasite assemblages in an understudied environment and contribute to parasitology knowledge.

This study investigated the helminths of the mixed invasive population of Darevskia armeniaca and D. dahli, collected during two field trips in Denyshy, Zhytomyr region, Ukraine, in 2023. In total, 67 adult lizards (35 D. armeniaca and 32 D. dahli) were examined. Molecular and morphological approaches were used to identify the parasites. The analyses revealed six helminth species, including four nematodes (Toxocara cati, Strongyloides darevskyi, Oswaldocruzia sp., and Spirurida gen. sp.), one trematode (Pleurogenes claviger), and one cestode (Mesocestoides litteratus). Toxocara cati had the highest prevalence, found in cysts located primarily on the liver and in the body cavity of the hosts. The qualitative and quantitative comparative assessment of the helminth community suggests that, due to the introduction of these lizards, most helminth species from their native range have been lost. Additionally, most local helminth species have not yet adapted to parasitising these lizards as normal hosts of their life cycle.

Gymnophallidae is one of the digenean families featuring bivalves as first intermediate hosts. However, the exact bivalve host species remain unknown for most members of this family. Gymnophallids have been one of the targets in our continuous efforts to reveal the diversity of digeneans in the higher north. Here, we focus on Gymnophallus minor, which we found in eiders from various locations in the Arctic and sub-Arctic. Sexual adults (maritae) of G. minor can be easily identified because they have a distinctive character: the roughly equal size of the pharynx and the ventral sucker. We also matched them, using DNA markers, with the intramolluscan stages (sporocysts, cercariae, and metacercariae) from the bivalve Liocyma fluctuosa collected on Spitsbergen. Taken together, we compile the first data on the life cycle of G. minor and discuss them in the context of other gymnophallids.

A population of Oscheius insectivorus was recovered from inside the body of a live scarab beetle collected from natural forests of Jafar Abad village, Golestan province, and was reported from Iran for the first time. The Iranian isolate of the species is characterized by 1103–1942 μm long females with their vulva at 46.9–57.4% of the body, tail 75–118 μm long, males with spicules 60.0–74.5 μm long, gubernaculum 23.7–32.2 μm long, and infective juveniles measuring 790–985 μm long. Females and males of the recovered population had shorter body when compared with the type population. Molecular identification of the recovered population was performed using the SSU and D2-D3 expansion segments of LSU rDNA, and corresponding phylogenetic trees were reconstructed and discussed. The pathogenicity of the Iranian isolate was evaluated on the larvae and adults of the poplar leaf beetle, Chrysomela populi, and the larvae of the wax moth, Galleria mellonella, under laboratory conditions. A suspension of 200 infective juveniles per ml (IJs/ml) of the nematode was most effective against third instar larvae of C. populi, causing 75% mortality 72 hours post-exposure. In adults, mortality reached 70% at the same concentration and exposure time. For G. mellonella larvae, O. insectivorus caused 50% mortality at 200 IJs/ml within 72 hours post-exposure. The lethal concentration 50 (LC50) values of the nematode were 77.71 and 94.06 IJs/ml for third instar larvae and adults of C. populi, respectively, and 217.04 IJs/ml for fifth instar larvae of G. mellonella 72 hours post-exposure at 25°C and 60% relative humidity.

Equids are infected by a diversity of gastrointestinal nematode parasites, including 64 species of equine strongyle nematodes from19 genera. Despite numerous surveys of horse strongyles worldwide, certain geographic regions and rare species remain understudied. In 1964, a new species of equine strongyle, Cylicocyclus pekingensis, was described from a donkey in China. Subsequently, this species was recorded in horses from Kazakhstan and reclassified as Hsiungia pekingensis (K’ung and Yang, 1964), the only species in this genus. Since then, H. pekingensis has not been reported elsewhere, with limited knowledge on its distribution and phylogeny.

This study documents the first record of H. pekingensis in North America. Adult specimens were recovered from fecal samples of a domestic horse in Alberta, Canada, following treatment with ivermectin. Species identification involved detailed morphological examination, complemented with sequencing of the internal transcribed spacer 1 (ITS1), 5.8S rRNA gene, and the internal transcribed spacer 2 (ITS2) regions of the nuclear genome. Phylogenetic analysis indicated a close evolutionary relationship with species from Poteriostomum and Parapoteriostomum genera. Nemabiome ITS2 sequencing of a paired pre-treatment sample also detected the presence of H. pekingensis in the studied horse. Re-analysis of public equine nemabiome datasets further detected H. pekingensis in feral horses in Alberta, but not in other regions considered. This study expands the known distribution of this rare species and enhances our knowledge of its placement in the phylogeny of equine strongyles. Furthermore, our re-analysis of public nemabiome datasets highlights the value of this approach for studying the global distribution of parasite species.

Metal pollution is a major global issue in aquatic environments, affecting environmental quality and potentially altering host–parasite dynamics. This study evaluates the buffering role of a larval trematode Himasthla sp. under experimental conditions to test the effect of copper (Cu) exposure on the survival of the marine snail Echinolittorina peruviana. Snails were collected from intertidal rocky pools over a two-month period from Coloso (23°45’S, 70°28’W), northern Chile, and identified as parasitized or unparasitized. Both groups were then exposed to Cu concentrations (3 and 6 mg/L). Kaplan–Meier curves were used to determine the percentage of survival over time and the respective confidence intervals (CI). A nested ANOVA was conducted to assess whether rediae abundance per snail varied by experiment time, snail status, and Cu concentration. Snail survival was affected by both Cu-concentrations, but the effect was greater at 6 mg/L. At 3 mg/L, 57% (CI: 49.9–66.6%) of unparasitized snails were alive at 192 h, while 56% (CI: 46.6–67.4%) of parasitized snails survived at 216 h. At 6 mg/L, 42% (CI:35-51%) of unparasitized snails survived at 192 h, while 48% of parasitized snails survived at 216 h (CI:39-59%). Regardless of Cu concentration, after 240 h, all unparasitized snails had died, while 15% of parasitized snails remained alive. Dead snails harboured 125±53 rediae, while survivors had 194±73 rediae, with no significant differences between treatments. Our results show that parasitized snails survived longer than unparasitized snails, suggesting a trade-off between parasitism and host survival in polluted environments.

Iceland is an isolated, sub-Arctic, oceanic island of volcanic origin in the northern North Atlantic. With a limited faunal diversity and being the most northern point in the distributional range for some species, it is an intriguing model region to study parasite biodiversity and biogeography. Since 2006, there has been a history of intense biodiversity discoveries of freshwater trematodes (Trematoda, Digenea), thanks to the use of integrative taxonomic methods. The majority of digeneans (28 out of 41 known) were characterised with molecular genetic methods and morphological analyses, with some of their life-cycle stages and geographical distribution assessed. A surprising diversity has been discovered, comprising species of the families Allocreadiidae, Cyclocoeliidae, Diplostomidae, Echinostomatidae, Gorgoderidae, Plagiorchiidae, Notocotylidae, Schistosomatidae, and Strigeidae. Many of the recorded species complete their life cycles within Iceland, with three snail species (Ampullaceana balthica, Gyraulus parvus, Physa acuta) known as intermediate hosts. No trematodes endemic for Iceland were found; they appear to be generalists with wide geographical ranges dispersed mainly by migratory birds. Interestingly, fish trematodes recorded in Iceland were found in mainland Europe, indicating that they might be dispersed by anadromous fishes, by human activity, or by migratory birds carrying intermediate hosts. The trematode fauna is mainly Palaearctic, with few species recorded in North America. We highlight the ongoing need for precise species identification via integrative taxonomic methods, which is a baseline for any further ecological studies and adequate epidemiological and conservation measures. Also, there is still a need of obtaining well-preserved vouchers of adults for definite species delimitation.

A new species of Tereancistrum Kritsky, Thatcher & Kayton, 1980, collected from the gills of Brycon nattereri (Bryconidae) in the Cerrado, in the state of Goiás, Brazil, is described based on morphological and molecular data, based on the mitochondrial cytochrome c oxidase subunit 1 (COI) gene region. Tereancistrum campanum n. sp. is distinguished from all its congeners by the unique ‘bell-shaped’ accessory piece in the male copulatory complex. Molecular analysis revealed that the specimens analyzed formed a distinct group from other Tereancistrum sequences and that the sequences of Tereancistrum kerri showed genetic divergence when compared to T. campanum n. sp., confirming the molecular distinction between the two species. This study also expands the knowledge of parasitic diversity in the Tocantins-Araguaia River Basin and extends the geographical distribution of the genus Tereancistrum. The combination of morphological and genetic data was essential for delimiting this new species, highlighting the importance of integrative approaches in understanding the diversity of monogeneans in freshwater fish in Brazil.