The freshwater fish fauna of southern Africa is highly diverse; however, the magnitude of parasitic species they host is unevenly known. The region’s documented adult trematode fish fauna is sparse, while the opposite is evident for intermediate trematode stages. Perceived difficulty in identification of underdeveloped stages lead to the exclusion of reporting metacercariae or lack either morphological or molecular data resulting in a depauperate comparative molecular data repository for species of the region and Africa as a whole. In an effort to address the morphological and molecular data void of the parasite fauna of southern African freshwater fishes, we sought to comprehensively investigate and characterise this fauna. Here we report on three metacercarial forms of Clinostomum (Clinostomidae) from three fish families (Clariidae, Mochokidae, and Mormyridae), provide the first report of a species of the Cryptogonimidae from a cyprinid host in South Africa, and include molecular data for the partial 28S rDNA, ITS1–2 and COI mtDNA regions of these metacercarial forms. Our clinostomid specimens morphologically and genetically corresponded with Clinostomum brieni (e.g., Clarias gariepinus) and Clinostomum ‘morphotype 2’ and ‘morphotype 3’ per Caffara et al. (2017) from the mormyrid Marcusenius pongolensis and the mochokid catfish Chiloglanis sp., respectively. Our cryptogonimid metacercariae did not correspond with any known species or available molecular sequence data; however, the presence of robust circumoral spines on the oral sucker indicated that they are either a species of Acanthostomum or Proctocaecum. The molecular data we provide are the first for an Acanthostomum/Proctocaecum-type cryptogonimid from Africa.

Kalicephalus (Molin, 1861) comprises 33 species of gastrointestinal snake and lizard parasites with a cosmopolitan distribution, with seven taxa occurring in the Neotropical realm. In the present study, we describe Kalicephalus atroxi n. sp., found parasitising the snake Bothrops atrox, from the Eastern Amazon in the State of Amapá, North of Brazil. We used an integrative approach that included light microscopy, scanning electron microscopy, and sequencing of the internal transcribed spacer 1 (ITS1) region to describe Kalicephalus atroxi n. sp. The new species has a buccal capsule characteristic of the genus, a slight cuticular inflation in the cephalic region. The females have an amphidelphic reproductive system, a vulva with prominent lips, and a long tail, tapering posteriorly. The males have long and alate spicules, and the copulatory bursa is lobed with dorsal rays with distinct morphology compared to their congeners. Molecular analyses and phylogenetic reconstructions cluster the new species into a well-supported clade with K. costatus costatus, from Chironius fuscus, from the same locality in northern Brazil. Kalicephalus atroxi n. sp. is the eighth species of the genus in the Neotropics, the seventh in Brazil, the second described parasitising B. atrox in Brazil, and the first species of snake nematode described in the State of Amapá.

Fasciolosis, a parasitic disease of ruminants, poses significant economic and animal-health challenges in Algeria. This study aimed to assess spatial, temporal, and species-specific patterns of fasciolosis prevalence across diverse agro-climatic zones and to estimate associated economic losses. Between 2013 and 2023, eight wilayas El-Tarf, Skikda, Jijel (Region I), Blida, Mila (Region II), and M’Sila, Medea, Laghouat (Region III) were surveyed. Systematic postmortem inspections of 1,569,392 animals (349,176 cattle; 982,669 sheep; 235,639 goats; 1,882 camels; 26 horses) were performed by qualified veterinarians, with liver and bile-duct examination for Fasciola. Data on region, species, year, and season were analyzed in R 4.4.0 using ANOVA, Kruskal–Wallis, Tukey’s post hoc tests, and principal component analysis (PCA). Economic losses were calculated from condemned liver weights in Blida, Laghouat, and Jijel, converted to USD. PCA distinguished three regional prevalence profiles, with PC1 (77.7% variance) separating overall prevalence levels. Region I exhibited the highest mean prevalence (2.47%), peaking at 3.54% in 2018 – significantly greater than Region II (1.39%) and Region III (1.96%) (p < 0.01). Cattle showed the greatest infection rate (mean 4.14%), significantly higher than sheep (1.32%; p < 0.001) and goats (0.25%; p < 0.001), while horses and camels remained uninfected. Seasonal analysis revealed highest prevalence in autumn and winter (≈2.1%) versus spring (≈1.5%). Economic losses totaled USD 10.6 million in Blida, USD 1.0 million in Laghouat, and USD 142.2 million in Jijel over the study period. Targeted control strategies, adapted to regional and seasonal risk patterns, are essential. Future work should investigate environmental and management factors driving regional differences and evaluate cost-effective interventions to mitigate fasciolosis impact in Algerian livestock.

During nematode surveys conducted to investigate the biodiversity of plant-parasitic nematodes in Mediterranean olive groves with different management strategies (organic and conventional), a nematode population of the genus Neothada was detected in southern Spain. Application of integrative taxonomical approaches clearly demonstrated that it is a new species described herein as Neothada olearum sp. nov., also representing the first report of the genus in Spain. The new species is amphimictic, characterised by a short body (563–774 μm); cuticle widely annulated (2.5–3.0 μm); total number of body annuli 214–226; 16 longitudinal ridges giving a tessellate body surface; stylet without distinct basal knobs (9.0–11.0 μm); and tail elongate-conoid, with tip bluntly rounded. The results of molecular analysis of D2-D3 28S rRNA, ITS rRNA, partial 18S rRNA, and cytochrome oxidase c subunit 1 (COI) gene sequences support for the new species status and clearly separated from N. major and other species within Neothada. Phylogenetic analyses of ribosomal and mitochondrial markers of this study suggested that Neothada is a monophyletic genus, clearly separated from Thada.

Microbiomes are communities of microorganisms that form close associations with metazoan hosts and have important roles in host biological processes. With the advent of Next Generation Sequencing, the microbiomes of myriad animals and plants have been described. However, the microbiomes of parasites have received little attention, which is surprising considering their ecological and medical importance. This study characterizes, for the first time, the microbiome of Dujardinascaris helicina, a gastrointestinal nematode parasite of the American crocodile. Dujardinascaris helicina were isolated from crocodiles residing in two geographically separated habitats across Belize. Using 16S sequencing, we compare β-diversity between sampling locations using generalized linear mixed modeling. Our results show that D. helicina microbiomes differ in composition depending on location. We also show that D. helicina microbiomes show strong shifts toward consolidation of specific taxa when proximity to human modified environments increases.

Over the years, the number of parasitic helminth species discoveries has not ceased to increase and the popularisation of the use of molecular methods has contributed greatly to sustain the growth in knowledge. However, molecular approaches evolved rapidly in the last 20 years. I argue that the research community working on parasitic helminths has lagged behind in the application of molecular methods that examine multiple loci to study species diversity. In this paper, I review the recent historical trends in the molecular markers used to study trematode diversity. Except for the emergence of pioneer mitogenome studies, the use of markers has not changed in the past 10 years. It is still restricted to single locus or a combination of two, rarely three, mitochondrial and ribosomal loci. I identify past and current molecular approaches providing data on multiple loci across the genome which have found resistance in the trematode and the helminth parasitology fields over the last four decades. I discuss how the knowledge gained from the analysis of genome-wide markers would benefit research on parasite diversity today, in particular for cases of species complexes, cryptic (or nearly cryptic) species, recently diverged species, and species with a complex taxonomic history, or a history of suspected mitonuclear discordance as well as for taxa with wide geographical distributions or species with disjoint distributions. Furthermore, I argue that both, studies with classical markers and reduced-representation genome studies providing genome-wide markers should not walk different paths but feedback on each other to advance the field forward. I examine some challenges and make recommendations for obtaining high-throughput molecular data of parasitic helminths.

Five species of monorchiids are known from fishes of the family Gerreidae, of which one is from Australian waters. Here, we report it and two new monorchiids from three species of Gerres Quoy & Gaimard, 1824 from off Lizard Island, northern Great Barrier Reef, and Moreton Bay in south-eastern Queensland: Gerres oyena (Forsskål), Gerres oblongus Cuvier and Gerres subfasciatus Cuvier. One of the new species, found only in G. oblongus at Lizard Island, conforms most closely to the concept of Proctotrema Odhner, 1911. However, it differs from species of Proctotrema in oral sucker shape and location of intestinal bifurcation and termination. It is phylogenetically distinct from two sequenced species of Proctotrema; thus, we propose Obscuromonorchis ranae n. g., n. sp. The second new species infects all three gerreids, occurs at both Lizard Island and Moreton Bay, and is morphologically most similar to the concept of Monorchicestrahelmins Yamaguti, 1971. However, the combination of the length of the caeca, size of the testis and post-testicular region, and the form of spination in the genital atrium presents a clear genus-level distinction that warrants proposal of a new genus. There are no molecular data for the three recognised species of Monorchicestrahelmins. We propose Argenticola shuyinae n. g., n. sp. for this species. New specimens of Gerricola queenslandensis Wee, Cutmore & Cribb, 2021 were collected from off Lizard Island and Moreton Bay. The three species form a well-supported clade but with internal branch lengths and topology consistent with genus-level differentiation.

A new species of Loimos MacCallum, 1917 is described more than half a century after the last species was described in 1972. The new species was collected from the gills of Rhizoprionodon taylori (Ogilby, 1915) off the Central Queensland coast, Australia, and is the first Loimos species and the first representative of the Loimoinae Price, 1936 known from Oceania. A detailed morphological description and 28S rDNA molecular sequences are provided for the new species. In the molecular phylogeny based on available 28S rDNA sequences for relevant Monocotylidae, the new species grouped together with the only other Loimos sequence available in GenBank, that of the nonugen Loimos sp. from China (OM060238), sister to Loimosina wilsoni Manter, 1944. The estimated genetic divergence between the new species and the nonugen Loimos sp. sequence is low, between 0.0452 and 0.0737, suggesting that the nonugen sequence may represent the new species, or a very closely related congener. Host identity was confirmed by comparing COI sequences with those of known sharks in GenBank. We also provide the first 12S and 16S molecular sequences for this shark species.

Trematodes of the genus Paralecithodendrium parasitize bats worldwide. Among them, the identification of Paralecithodendrium longiforme and Paralecithodendrium cryptolecithum is complicated by their high morphological similarity. We studied Paralecithodendrium trematodes from the small intestine of bats inhabiting the Middle Volga region (European Russia). The aim of our study was to analyze these two Paralecithodendrium species using both morphological and molecular phylogenetic approaches. Here, we present the first complete morphological description and molecular phylogenetic analysis of Paralecithodendrium cryptolecithum from bats in Russia and confirm the validity of this species.

Data on the infection of Middendorffʼs eelpout, Hadropareia middendorffii, by metacercariae of the trematodes Cryptocotyle lingua Creplin, 1825 and Liliatrema skrjabini Gubanov, 1953, which are causative agents of black spot disease, is provided here for the first time for Taui Bay, Sea of Okhotsk. The prevalence of infection of fish by larvae of C. lingua reached 79%, while the intensity of infection varied from one to 278 individuals (with an average of 53). The prevalence of infection by L. skrjabini was lower (40%), with an intensity from one to 22 larvae (on average, eight). Metacercariae of both trematode species were surrounded by two envelopes: an outer, connective tissue capsule formed by the host’s cells, and an inner cyst formed by the parasite. The examined metacercariae were found in all regions of the fish’s body (head, trunk, and fins), with approximately equal numbers of individuals of both species found in tissues of the head and trunk regions. Metacercariae of C. lingua were localised in the fishes’ muscles, not only immediately under the skin but also in deeper layers. The capsules formed around metacercariae of both trematode species were dominated by fibroblasts and collagen fibres and did not show pronounced signs of inflammation. Studying the distribution of black spot disease and its effects on fish is crucial for understanding disease patterns in relation to fish population dynamics. It may also inform the development of effective anthelmintic treatments for use in aquaculture farms.

Trichinellosis is a parasitic zoonosis caused by a nematode parasite of the Trichinella (T.) genus. It poses significant public health issues due to limited effective and safe treatment options, especially for the muscle-encysted larval stage. Citrus paradisi (C. paradisi), with its high content of flavonoids and polyphenols, has been recorded to possess anti-parasitic properties and numerous therapeutic applications. The present work aimed to assess the efficacy of C. paradisi extract peel extract as a therapeutic agent, either alone or combined with albendazole (ABZ), against T. spiralis in experimentally infected mice. Sixty-six lab-bred Swiss albino mice were divided into control and treatment groups, then received either ABZ, C. paradisi extract, or a combination of both during the enteral, migratory, and encapsulation phases of infection. Parasitological, histopathological, and immunohistochemical examinations were performed to evaluate the efficacy of the treatments. All treated groups displayed a highly significant difference (p < 0.001) in larval counts compared to the positive control group, with the combination therapy group having the highest efficacy and the lowest mean count value during different treatment regimens. In addition, treated groups showed improved muscle integrity compared to the positive control group. Moreover, the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) showed the highest expression reduction in the combination therapy group. These findings highlight the potential of C. paradisi as a complementary therapy to albendazole for treating trichinellosis through muscle larva reduction and mitigation of inflammation.

Cystic echinococcosis is a zoonosis caused by Echinococcus granulosus sensu lato, which leads to serious public health problems. Considerable phenotypic variation has been described between parasites depending on the intermediate host and genotype. Knowledge of species/genotypes present in an endemic region is clue for control programs. Although DNA sequencing is the most accurate tool for this purpose, its high cost makes it less affordable. A multivariate study of the morphometric parameters of rostellar hooks of E. granulosus sensu lato of human origin from Neuquén was carried out using Principal Components Analysis and classification trees. A valid classification criterion to differentiate E. granulosus species using morphometry of rostellar hooks in human hydatid cysts was determined. Seventy-six human fertile hydatid cysts (48 E. granulosus sensu stricto G1 and G3, and 28 E. canadensis G6) were included. Overall, 2,280 hooks were measured. The rostellar hooks belonging to E. granulosus sensu stricto were significantly smaller than those of E. canadensis. The Large Total Length (LTL) was sufficient to classify a new isolate into the 2 species: if LTL < 25.3 μm, then it corresponds to E. granulosus sensu stricto, and if LTL ≥ 25.3 μm to E. canadensis. This is the first multivariate study that establishes a morphometric cut-off value to discriminate between 2 species of E. granulosus sensu lato from human patients, using a significant number of cysts of both species identified by sequencing of mitochondrial genes. It constitutes a useful predictive tool in endemic areas where both human infective species overlap.

Genetic evidence indicates that Progamotaenia macropodis Beveridge, 1976 (Cestoda: Anoplocephalidae), found in various macropodid host species, is a complex of cryptic species. However, the genetic data are incomplete, and no morphological re-appraisal of the species has been undertaken since its original description. Here, additional mitochondrial cytochrome c oxidase I sequence data have been added from the type host, Macropus giganteus, as well as other host species, and a morphological study of all available material undertaken. A new species, Progamotaenia mollicula sp. nov., is erected for specimens from the tammar wallaby, Notamacropus eugenii, which are smaller, and the testes in pre-mature and mature proglottids occur invariably in two lateral groups. Specimens from the remaining host species, Macropus fuliginosus, Notamacropus parryi, N. rufogriseus, Osphranter robustus, and Wallabia bicolor are highly variable with obvious differences in length and proglottid shape, but with no reliable internal morphological characters for separating the various genotypes. The study was limited by incomplete molecular data (N. rufogriseus) and the poor quality of some of the preserved material.

Haemonchosis is a fatal disease of livestock caused by Haemonchus contortus (HC) – a blood-sucking parasite of the abomasum. This parasite is quite prevalent in sheep, causing mortality and production losses. The suppressor of cytokine signalling-2 (SOCS2) gene plays a vital role in sheep’s immune response against gastrointestinal parasites. This study aimed to estimate the parasitic load of HC in three sheep breeds (Balkhi, Ghalji, and Michni) through faecal egg count and to identify SNPs in the SOCS2 gene associated with the susceptibility of sheep against HC. The results showed that the mean number of HC eggs per gram (EPG) was higher in the faecal samples of Ghalji (4022 ± 1162 EPG), followed by Michni (1988 ± 367 EPG), while the HC EPG was the lowest in Balkhi sheep (1535 ± 552 eggs/gm). Sequencing results showed polymorphisms in the SOCS2 gene between the low-infection and high-infection categories of the three sheep breeds. A total number of six genic variants were observed, of which three were SNPs, one was insertion, and two were deletions. Polymorphisms were observed in the intronic and 3′ UTR regions of the SOCS2 gene. A deletion (c.1083delGCA) in intron 1 and an insertion (c.3304insT) in intron 2 showed positive correlations (0.833 and 0.889, respectively) with the HC infection, while one SNP in the 3′ UTR region showed negative correlation (–0.654). This study provides a basis for selecting resistant sheep against HC infection based on the SOCS2 gene molecular markers.

The cestode Gerbillitaenia psammomi (Cyclophyllidea: Catenotaeniidae: Skrjabinotaeniinae) was found parasitizing fat sand rats (Psammomys obesus) in the south of Tunisia. The species was found previously in P. obesus from Egypt and in Meriones shawi from Morocco. The present finding constitutes the first record of the species in Tunisia. In the present study, G. psammomi was morphologically characterized using light microscopy and compared to previous records of the species. The main differentiating morphological features of G. psammomi with respect to other skrjabinotaenines of North African Gerbillinae murids are, in particular, the shape of strobila, size of suckers, number of testes, absence of an antero-poral ovarian lobe, length of the central uterine stem, and number of primary uterine branches. Negative density-dependent growth (crowding effect) has also been reported. Molecular analysis based on 28S rDNA was performed for G. psammomi and for Skrjabinotaenia oranensis. Thus, both species are grouped in a clade with other species of the subfamily Skrjabinotaeniinae and are clearly separated from the clade that includes species of the subfamily Catenotaeniinae. The molecular study confirmed the previously postulated synonymy of Meggittina numida with S. oranensis.

Accacoeliid digeneans associated with fish of the family Molidae exhibit enigmatically high taxonomic diversity. However, the phylogenetic relationships between species within this digenean taxon are poorly understood. In the present study, the first nuclear 28S rRNA gene, ITS2 region of nuclear DNA, and mitochondrial cox1 gene sequence datasets were obtained for two members of the Accacoeliidae, a type and only species of the genus Odhnerium Yamaguti 1934 and an unidentified Accacladocoelium sp. collected from Mola mola (Linnaeus, 1758) off Iturup Island. Analyses of molecular differentiation and phylogenetic relationships indicate that Accacladocoelium sp. is a sister species to Accacladocoelium nigroflavum (Rudolphi, 1819). The genus Odhnerium is closely related to Tetrochetus Looss, 1912, on the 28S rRNA gene-based phylogenetic tree. Results of phylogenetic analysis based on both the mitochondrial cox1 gene and the concatenated ribosomal ITS2 region and cox1 gene of mtDNA show that the genus Odhnerium is close to the A. nigroflavum + Accacladocoelium sp. clade. In turn, the genus Accacladocoelium does not have monophyletic status in the trees reconstructed from these data.

The Neotropical region is considered a biodiversity hotspot for reptiles, with eight families of terrestrial and freshwater chelonians. Parasites are of great importance to aquatic ecosystems and are essential to host communities. They help understand the patterns and phylogenetic relationships of their hosts and act to control populations. A literature survey on helminth parasites of chelonians in the Neotropical region was conducted, examining the most commonly found groups of helminths, the most parasitized hosts, compiling their biogeographical and political distribution and recording the parasite fauna of Phrynops geoffroanus in the state of Paraíba, northeastern Brazil. Considering the literature from 1850 to 2024, 202 helminth taxa were recorded. The Mesoamerican dominion had the highest number of helminth taxa, and Brazil was the political unit that recorded the most helminth taxa, followed by Mexico. Nematoda was the group with the highest number of taxa and occurred in all biogeographical units, with the species Spiroxys contortus being the most commonly found, followed by Trematoda, represented by the most commonly found species, Nematophila grandis. Hosts from the Chelidae family had the highest number of helminth taxa, followed by the Kinosternidae family. The most parasitized host species were Chelonoidis denticulatus and Phrynops hilarii. For the host Phrynops geoffroanus, the first occurrence of the nematode helminth Spiroxys contortus was recorded. This is the first study to include all parasitic helminths of terrestrial and freshwater chelonians in the Neotropical region, including a new occurrence in a chelonian host in the hinterland of Paraíba, Brazil.

In Japan, three trematode species of the genus Azygia have been recorded: A. gotoi, A. perryii, and A. rhinogobii. Here, we report the first detection of A. hwangtsiyui in Japan, identified from the introduced snakehead Channa argus. This trematode was previously known only from the snakeheads in mainland China. Between 2015 and 2024, we sampled snakeheads and snails in Japan, collecting adult trematodes and rediae. Adult trematodes were identified morphologically as A. hwangtsiyui, a determination corroborated by molecular analyses of the COI and 28S rDNA regions. Rediae extracted from the snail Sinotaia quadrata histrica were also identified molecularly as A. hwangtsiyui, indicating the snail acts as an intermediate host. Juvenile trematodes from the short-spined Japanese trident goby Tridentiger brevispinis were also morphologically identified as A. hwangtsiyui, indicating that this goby acts as a paratenic host. Given the snakehead’s introduction to Japan between 1923 and 1924, the absence of records of later introductions, and the presence of both intermediate and paratenic hosts in Japan at that time, A. hwangtsiyui was likely introduced concurrently with the snakehead. As the detection of three COI haplotypes suggests multiple introductions, subsequent introductions via the aquarium trade or other pathways involving the transport of freshwater fish and mollusks from Asia to Japan are also plausible.

Two poorly known parasitic Platyhelminthes were collected from the gills of the European barracuda Sphyraena sphyraena in the western Mediterranean. Specimens of Chauhanea mediterranea Euzet & Trilles, 1960 and Rhinecotyle crepitacula Euzet & Trilles, 1960 (Polyopisthocotyla) are redescribed based on newly collected material. A careful examination of Ch. mediterranea revealed inaccuracies in previous interpretations of the female openings. The midventral opening, previously referred to as the ‘vaginal opening’ in Pseudochauhanea Yamaguti, 1965 and used to distinguish this genus from Chauhanea Ramalingam, 1953, is demonstrated to be the uterus, as confirmed by the presence of eggs in the thin-walled duct associated with this opening. The lateral position of the cirrus and vagina is also confirmed. Consequently, Pseudochauhanea is synonymized with Chauhanea, resulting in the following new combinations: Ch. elegans Fuentes-Zambrano, 1997 n. comb.; Ch. elongata Kritsky, Bilqees & Leiby, 1972 n. comb.; Ch. macrorchis Lin, Liu & Zhang in Zhang, Yang & Liu, 2001 n. comb.; Ch. mexicana Lamothe, 1967 n. comb.; Ch. sphyraenae Yamaguti, 1965 n. comb. A detailed investigation of the clamp sclerites of Ch. mediterranea indicates that both Chauhanea and Cotyloatlantica Bravo-Hollis, 1984 share the subdivision of the basal oblique sclerite, which is a continuation of the marginal sclerite in the anterior jaw, a key feature used to distinguish these genera. As a result, Cotyloatlantica is also synonymized with Chauhanea, and its species are transferred as follows: Ch. pretiosa (Bravo-Hollis, 1984) n. comb. The reinstatement of Ch. mediterranea Euzet & Trilles, 1960 as a valid name is supported, and an amended diagnosis of Chauhanea is provided. Additionally, R. crepitacula is accurately redescribed, with a new geographical locality record and an assessment of morphometrical and anatomical variations between Mediterranean and oceanic specimens.