This study aimed to (1) assess the diversity of Gyrodactylus species infecting Day's mystus, Mystus bleekeri (Siluriformes: Bagridae), in northern India; (2) provide genetic resources for two new species to support taxonomy, phylogeny, and diagnostics; and (3) compile an annotated checklist of Gyrodactylus species from Indian freshwater fishes. Between 2022 and 2025, two new species, Gyrodactylus octoacanthicus n. sp. and Gyrodactylus ascia n. sp., were recovered and described from the skin and gills of M. bleekeri. Species delimitation included the integration of morphometric analysis of sclerotised haptoral and reproductive structures with sequencing of the complete ITS rDNA region (ITS1-5.8S-ITS2) and partial 28S rDNA. Molecular comparisons showed substantial divergences from previously described species in two markers, ITS rDNA (19.12% in G. octoacanthicus n. sp.; 18.07% in G. ascia n. sp.) and 28S rDNA (16.83% in G. octoacanthicus n. sp.; 8.79% in G. ascia n. sp.), exceeding standard species-delineation thresholds. The newly compiled checklist included 17 taxonomically valid Indian species (including the two new species), of which sequence data are available for only seven, and eight nominal or undescribed species that were considered unavailable names due to non-compliance with the International Code of Zoological Nomenclature. The replacement names Gyrodactylus pandeyi nom. nov. and Gyrodactylus fossilisii nom. nov. were proposed for the junior homonyms, Gyrodactylus mizellei Dubey, Gupta & Agrawal, 1990 and Gyrodactylus gussevi Dubey, Gupta & Agrawal, 1990, respectively. These findings highlighted significant gaps in Indian gyrodactylid diversity and taxonomy and underscored the need for further integrative studies on parasite diagnostics, aquaculture-health management, and freshwater biodiversity conservation in India.

We report a new commensal relationship between oerstediid nemerteans and deep-sea ascidians. A total of six nemertean individuals were collected from the peribranchial cavities of several Styelidae ascidians attached to small rocks at bathyal depths (1,400-1,409 m) off northeastern Japan in the northwestern Pacific Ocean. Phylogenetic analyses based on combined mitochondrial cytochrome c oxidase subunit I (COI), 16S rRNA, histone H3, 18S rRNA, and 28S rRNA gene sequences placed this species within the 'Clade Gononemertes' of the hoplonemertean family Oerstediidae. Previous records of commensalism between nemerteans and deep-sea ascidians are restricted to the genus Culeolus (Pyuridae). The present finding therefore suggests that ascidian-host diversity may be greater in Oerstediidae than previously recognised.

Strongyloides stercoralis is a soil-transmitted helminth capable of lifelong persistence through autoinfection, placing individuals with latent infection at risk during immunosuppression. Although corticosteroids are recognized triggers for hyperinfection and dissemination, population-level data quantifying these risks remain limited. We conducted a multicentre retrospective cohort study using the TriNetX Global Collaborative Network. Adults with laboratory-confirmed latent Strongyloides infection (positive IgG serology) were included, while patients with active disease were excluded. Corticosteroid exposure was defined as receipt within 30 days of the index test. Propensity score matching compared outcomes between patients with latent infection who did and did not receive corticosteroids. Secondary analyses compared steroid-exposed patients with and without Strongyloides and evaluated prophylactic ivermectin among steroid-exposed seropositive patients. The primary outcome was 90-day mortality; secondary outcomes included sepsis, bacteraemia, and ICU admission. Cox proportional hazards models evaluated time-to-event outcomes. Among patients with latent infection, 494 received corticosteroids, and 1,501 did not. After matching, corticosteroid exposure was associated with increased 90-day mortality (9.4% vs. 4.7%; OR 2.10, 95% CI 1.19-3.72), sepsis (OR 2.05, 95% CI 1.17-3.58), bacteraemia (OR 2.13, 95% CI 1.41-3.23), and ICU admission (OR 2.85, 95% CI 2.03-3.98). In Cox models, corticosteroid exposure remained independently associated with higher hazards of 90-day mortality (HR 3.16, 95% CI 2.01-5.02) and sepsis (HR 3.01, 95% CI 1.72-5.23). Prophylactic ivermectin was associated with a lower risk of a composite outcome (12.9% vs. 27.1%; OR 0.40, 95% CI 0.18-0.89). These findings support enhanced screening and preventive strategies in at-risk patients.

Natural selection is expected to maintain a stable 1:1 sex ratio in randomly mating populations. However, obligate parasites, especially nematodes with complex host-migratory life cycles, often show pronounced departures from this equilibrium. Ascaris suum, a globally distributed nematode with an XX/XO sex-determination system, exhibits an obvious female-biased sex ratio in adults, making it an ideal model to investigate when and how sex-ratio bias arises. Progress has been limited because, at present, no practicable method exists to identify worm sex at stages other than adulthood. Here, we aimed to establish a valid approach designed to be applicable, in principle, for precise sex identification and sex-ratio estimation across all stages of A. suum. Using a chromosome-level A. suum genome assembled in our laboratory (CNP0003380), we identified single-copy sequences on autosomes and the X chromosome and selected 3C (Chr3) as an autosomal marker and 5X (Chr5X) as an X-linked marker. We developed a qPCR assay that infers sex or sex ratio from the 3C/5X copy-number ratio, with theoretical values of 1 for XX females and 2 for XO males. Validation with 30 adult females and 30 adult males yielded 3C/5X ratios of 1.009 ± 0.042 and 1.995 ± 0.054, respectively, with no significant deviation from theoretical expectations (p > 0.05). The relative abundance of X-bearing to O-bearing sperm measured by qPCR was 0.980 ± 0.093, compared with 1.081 ± 0.163 by flow cytometry. Although the mean values differed significantly between the two methods (paired Students t-test, t = -2.886, d.f. = 29, p = 0.0073), the qPCR-based estimates showed lower variability than those obtained by flow cytometry. These results indicate that this qPCR assay provides an accurate tool for sex identification and quantification of the relative abundance of X-bearing to O-bearing sperm in A. suum.

We report new collections of trematodes currently assigned to the Faustulidae Poche, 1926, from fishes of the Pomacanthidae and Ostraciidae in the tropical Indo-West Pacific. Phylogenetic analyses demonstrate that these species, and several other faustulid taxa, form a well-supported clade within the Zoogonidae Odhner, 1902 (Microphalloidea), distant from the Faustulidae sensu stricto (Gymnophalloidea). We recognise the subfamily Antorchiinae Skrjabin & Koval, 1957 within the Zoogonidae for Antorchis Linton, 1911, Neoparantorchis Hafeezullah & Siddiqi, 1971, Parantorchis Yamaguti, 1934, Paradiscogaster Yamaguti, 1934, Discogastroides Strand, 1934, Parayamagutia Machida, 1971 and Trigonocryptus Martin, 1958, and propose four new genera. Scaticola n. g. is proposed for two species of Bacciger Nicoll, 1914 that resolve within the Antorchiinae rather than with Bacciger sensu stricto. Crassitrema n. g. and Findovitellus n. g. are proposed for two species of Paradiscogaster that resolve distant from all other sequenced species of Paradiscogaster. Nasoantorchis n. g. is proposed for Nasoantorchis nasonis (Cribb, Bray, Hall & Cutmore, 2015) n. comb., previously considered a member of Antorchis. From pomacanthids, we describe two new species of Parantorchis, Parantorchis vaili n. sp. and Parantorchis parviovatus n. sp., and two new species of Paradiscogaster, Paradiscogaster hoggettae n. sp. and Paradiscogaster ningalooensis n. sp. We also report Neoparantorchis pomacanthi (Hafeezullah & Siddiqi, 1970) Hafeezullah & Siddiqi, 1971 and Paradiscogaster machidai Cribb, Anderson & Bray, 1999. From ostraciids, we report Discogastroides ostracii (Yamaguti, 1934) Skrjabin & Koval, 1957, Discogastroides macrostomus Shimazu & Kamegai, 1990, and Parayamagutia ostracionis Machida, 1971.

The host-parasite relationship is affected by various biological, ecological, and anthropogenic factors. Some of them, such as host sex, are well studied. On the other hand, the effect of genetic differences, such as the presence of supernumerary or B chromosomes (Bs), has received little attention. We investigated the independent and interactive effects of host sex and B chromosome presence on intestinal nematode abundance in 294 yellow-necked mice (Apodemus flavicollis) from 18 localities in Serbia. Nine nematode species were identified. Because parasite numbers showed high inter-individual variability, we used a Monte Carlo randomisation method to test differences in mean abundance without assuming a specific statistical distribution. Host sex had a limited influence on infection patterns. In line with existing theoretical frameworks, a significant male-biased abundance was detected, but only for Aonchotheca annulosa, whereas no overall sex bias was found for total nematode burden. B chromosome carriers were found to have a significantly higher abundance of Syphacia frederici. A significant interaction between sex and Bs was detected for total intestinal nematodes and Syphacia stroma, with females carrying Bs harbouring the highest parasite loads. The results, indicating that Bs can shape helminth infrapopulation structure and that their effects may be sex-dependent, were discussed. The absence of consistent male-biased parasitism further suggests that sex effects are parasite-specific rather than universal. Our study highlights the importance of considering cryptic genomic variation in host-parasite research and demonstrates the value of Monte Carlo methods for analysing overdispersed helminth abundance data in wildlife hosts.

Echinostomes are a globally distributed group of digeneans with complex life cycles. In South America, many species are known only from definitive hosts, and in Argentina, most larval stages and adult records remain unlinked. Formal morphological descriptions and molecular data are essential for identifying larval stages; however, molecular reference sequences of larval digeneans remain scarce. We present an integrated morphological and molecular characterisation of echinostome cercariae from Biomphalaria aff. straminea collected between January 2017 and February 2018 in a rice field from Corrientes Province, Argentina, based on partial DNA sequences of the nuclear 28S rRNA (28S) gene and the mitochondrial marker nad1 gene. Eighty-three snails (2.12%) of the 3,913 sampled were infected with three echinostomatid morphotypes: Cercaria Echinostoma sp. 20 (1.63%); Cercaria Echinostomatidae gen. sp. 21 (0.25%); and Cercaria Drepanocephalus sp. (0.23%). Available Echinostomatidae sequences in GenBank were aligned with newly generated sequences from echinostomatid cercariae. Phylogenetic analyses showed that Echinostoma sp. 20 represents a distinct lineage that does not group with other Echinostoma sequences in GenBank; Echinostomatidae gen. sp. 21 does not cluster with Echinostoma and likely belongs to a genus related to Patagifer, although its position differs between the 28S and nad1 trees and it shows morphological differences; and Drepanocephalus sp. clusters with D. spathans from Mexico with low genetic divergence (<4.6%), although additional sequence data are needed to clarify species boundaries. This study provides the first molecular identification of Drepanocephalus and Echinostoma species in freshwater snails from Argentina, expanding the molecular dataset for Drepanocephalus in South America.

Anuran amphibians play essential ecological roles and are considered important bioindicators. This is especially true during the larval stage, when tadpoles exhibit high sensitivity to parasitic infections, including trematode metacercariae. These infections can profoundly affect the physiology and morphology of juvenile hosts, potentially compromising tissues involved in osmoregulation, locomotion, and development. We analysed naturally infected tadpoles, performing parasitological screening and histopathological evaluation. Through field collection, euthanasia, histological processing, and microscopic analysis of the tissues, our results revealed that the presence of metacercariae is directly associated with progressive tissue damage. In the kidneys, after the establishment of the parasite in a previously healthy renal tubule, we observed alterations ranging from mild changes, such as a slight reduction in glomerular density in individuals with low infection intensity, to severe damage in highly infected individuals. In the tail musculature, the metacercariae were encapsulated between muscle bundles, forming cysts delimited by fibrous capsules, accompanied by alterations in the organisation of muscle fibres, disruption of connective tissue, and the presence of inflammatory infiltrate. Capsules surrounding the parasites were composed of both host tissues and parasite-derived structures. Their formation was frequently associated with necrotic areas and functional impairment. These findings demonstrate that trematode infection exerts progressive effects on the structure and morphology of renal and muscular tissues in tadpoles, with the severity of tissue damage increasing according to parasite infection intensity. Such alterations may compromise host tissue integrity and represent an important pathological consequence of natural trematode infections during the larval stage.

Amphibian helminth diversity remains poorly documented in semiarid environments, despite its relevance for understanding host-parasite interactions and biogeographical patterns. We investigated the helminth community of the hylid frog Scinax x-signatus from a Caatinga remnant in Petrolina (Pernambuco, Brazil) and reviewed the current state of knowledge on helminth infecting South American Scinax species. Twenty-five S. x-signatus (11 adult females, 9 adult males and 5 juveniles) were examined; of these, 13 specimens (52%) harboured at least 1 helminth taxon, with infracommunity richness ranging from 1 to 7 taxa. We identified four nematodes (Oswaldocruzia sp., Oxyascaris caudacutus, Oxysomatium sp., Rhabdias sp.), two digeneans (adults of Catadiscus sp. and metacercariae of Neodiplostomum sp., and cystacanths of an acanthocephalan (Centrorhynchus sp.). Oxyascaris caudacutus was the most prevalent taxon (20%) and exhibited the highest mean intensity (4.0 ± 2.77). Four taxa (Oswaldocruzia sp., O. caudacutus, Oxysomatium sp. and Catadiscus sp.) represent new host records for S. x-signatus. Our literature review identified 45 publications from 1940 to 2026 and showed that helminth have been recorded for only 13 South American Scinax species, comprising 78 taxa in total (46 Nematoda, 20 Trematoda, 8 Acanthocephala and 4 Cestoda). Records are strongly geographically biased, being restricted to five countries (Brazil, Argentina, Peru, Paraguay, and Ecuador), and most taxa remain identified only to genus or higher taxonomic levels. Our results expand knowledge of Caatinga amphibian parasites and reinforce the need for integrative surveys to better estimate helminth diversity across the South American continent.

Preschool children in the Kingdom of Eswatini are excluded from the national school-based deworming programme and had never been surveyed for parasitic infection. During June 2019 we conducted a cross-sectional baseline survey of 261 children (mean age 4.8 ± 0.75 years) attending 16 kindergartens and community care points across 4 provinces. Enterobius vermicularis was sought on peri-anal cellophane/Scotch-tape swabs (n = 261); Schistosoma haematobium in single urine specimens by the chamber-tube microscope-slide (CTMS) sedimentation method (n = 232); and intestinal helminths and protozoa in single stool specimens by the merthiolate-iodine-formaldehyde (MIF) concentration method (n = 175). Pinworm prevalence was 3.8% (10/261) and did not differ by sex (p = 0.76) or province (p = 0.83). S. haematobium ova were found in 0.9% (2/232), both in boys from recognised endemic foci. Intestinal parasites were detected in 41.1% (72/175), with no significant sex (p = 0.64) or regional (p = 0.70) variation; protozoa predominated (97.2% of positives) over helminths (4.2%; all Ascaris lumbricoides), the commonest species being Blastocystis hominis (61.1%) and Giardia lamblia (34.7%). These first baseline data reveal a high burden of intestinal protozoa alongside a low but measurable burden of pinworm and urogenital schistosomiasis in Eswatini preschool children, and support extending screening and preventive chemotherapy to this previously unmonitored age group using the simple, field-deployable cellophane-swab, CTMS and MIF methods.

We report trematodes conforming to the generic concept of Parachrisomon Madhavi, 2008, infecting multiple species of goatfishes (Mullidae) from Moreton Bay (south-eastern Queensland) and off Lizard Island (northern Great Barrier Reef) on the eastern coast of Australia, and from Ningaloo Reef on the western coast. Initial morphological analyses suggested the possible presence of multiple species; however, no morphological distinctions proved consistent. Molecular analyses demonstrate a distinction between western and eastern Australian samples, especially in mt cox1 sequence data, but the differences are interpreted as intraspecific geographical variation. Specimens from all localities share a bipartite terminal organ with an irregularly shaped and lobed anterior part; the other eight species of Parachrisomon are reported with unspecialised, bipartite terminal organs. We recognise the specimens as representing a new species, Parachrisomon debonoae n. sp., that exhibits significant intraspecific morphological variation, especially in body size, pharynx size, and shape and size of the ovary and testis. Additionally, we report novel molecular data for Parachrisomon delicatus (Manter & Pritchard, 1961) Madhavi, 2008.

Species of Hexangium (Microscaphidiidae) were collected from 12 species of rabbitfishes (Siganidae) from multiple Indo-West Pacific locations: three on the Great Barrier Reef in Queensland, two in south-east Queensland, Ningaloo Reef in Western Australia, New Caledonia and Japan; no specimens were found from Palau or French Polynesia. Parallel molecular (cox1 mtDNA, ITS2 and 28S rDNA) and morphological analyses, together with consideration of host and geographical distribution, lead us to recognise six species. Specimens from Siganus fuscescens (Houttuyn) from Japan are identified as the type species, H. sigani Goto & Ozaki, 1929, which was originally described from S. fuscescens from off Japan. Hexangium affinum Tubangui & Masiluñgan, 1944, originally described from Siganus javus (Linnaeus) from the Philippines but previously considered a synonym of H. sigani, is reinstated, and H. secundum Annereaux, 1947 is considered its junior synonym. Four new species are described: H. urabeae n. sp., H. kuramochii n. sp., H. robustum n. sp. and H. reimeri n. sp. Based on morphology alone, individual specimens of several combinations of these species are difficult to distinguish. Although none of the combinations of species are entirely mutually cryptic, we consider taxonomy of this genus to be dependent on molecular analysis; morphological distinction of species requires substantial numbers of well-preserved specimens. Most species of Siganus examined harbour two or three species of Hexangium; sometimes one species is dominant, but in several two species were both common. All six Hexangium species recognised here have been found in multiple locations; four species were found at each of two Great Barrier Reef sites.

The Allocreadiidae comprise a diverse group of digeneans infecting freshwater fishes worldwide, with South America harbouring a particularly rich fauna. Within this family, Creptotrematina includes five valid species parasitizing primarily characid fishes, most of which have been recorded from the La Plata Basin. Despite the large geographic extent and hydrological complexity of this basin, the genetic structure and evolutionary relationships of Creptotrematina species across its sub-basins remain poorly understood. The inclusion of a new ITS sequence of Martorellia liciae recovered the same topology observed in the 28S phylogeny, supporting Martorellia as the sister genus of Creptotrematina. In the present study, specimens of Creptotrematina spp. collected in the Paraná and Iguazú rivers were analysed using partial sequences of 28S rDNA, ITS, and mitochondrial COI genes. Phylogenetic analyses based on individual markers recovered the new material within Creptotrematina batalhensis with strong support. Genetic divergence between Argentine and Brazilian populations of C. batalhensis was extremely low or absent across all markers, despite the large geographic distances and differences in hosts infected. These results indicate basin-wide genealogical cohesion and the absence of detectable phylogeographic structure. The observed pattern suggests that hydrographic subdivision within the La Plata Basin has not acted as an effective evolutionary barrier for this species. Instead, the strong association of Creptotrematina spp. with closely related characid hosts supports the hypothesis that host phylogeny and host-mediated dispersal may play a central role in shaping parasite diversification, with hydrological dynamics modulating connectivity rather than driving lineage fragmentation.

A new entomopathogenic nematode, Steinernema hattingi n. sp., was isolated during a survey in Mpumalanga Province of South Africa. Morphological and morphometric traits place the species in the glaseri-group, characterised by large infective juveniles (IJs) with average body length exceeding 1000 μm. The IJs of S. hattingi n. sp. have a mean body length of 1059 μm, lateral fields composed of eight equally spaced ridges (nine incisures) at mid-body (formula 2, 6, 8, 4, 2), a relatively short tail (c ratio 16 on average) with a hyaline region occupying approximately 45% of its length, and are relatively slender (a ratio 33 on average). Male spicules are curved, brown, with a pointed tip and an elongate manubrium (length/width ratio 1.8:1); male tails are non-mucronate or occasionally with a short papilla-like mucron in second-generation males. First-generation females have a moderately protruding, slightly asymmetrical vulva without postanal swelling, whereas second-generation females display a protruding, asymmetrical vulva and a slightly developed postanal swelling. The species is further characterised by sequences of the internal transcribed spacer (ITS) and partial 28S rDNA (D2-D3) regions. Phylogenetic analyses place S. hattingi n. sp. within the Khoisanae clade of the glaseri-group.