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.

In this study, morphological and molecular features were used to identify a new Steinernema sp. from Kerala, India. Morphological and molecular features provide evidence for placing the new species into the longicaudum clade. The new species is characterized by the following morphological features: infective juveniles with a body length of 1067 μm (914–1268 μm); a distance from the anterior end to excretory pore of 82 μm (73–92 μm); a distance from anterior end to nerve ring of 105 μm (91–118 μm). The distinguishing feature of the infective juveniles of S. keralense n. sp. is the presence of seven ridges in the mid-body region, while all other species classified within the logicaudum clade to date are characterized by eight ridges. The first-generation males are characterised by 25 genital papillae, very short spicules, with a length of 68 μm (60–72 μm), and the SW% ratio is 136 (114–169). The new species is further characterized by sequences of the internal transcribed spacer and partial 28S regions of the ribosomal DNA. Phylogenetic analyses show that S. keralense n. sp. is closely related to species within the longicaudum clade.

Biological invasions are among the main threats to global biodiversity and present the potential to disrupt host-parasite dynamics. In Brazil, the scientific reports of the occurrence of the Amazonian fish ‘pirarucu’ (Arapaima gigas) in the upper Paraná River basin, São Paulo state, were made in 2015. However, the effects of its introduction are still unknown, including those associated to its host-parasite relationships. As part of our studies on the possible effects of A. gigas introduction into this basin, the parasites from the eyes of 60 specimens of A. gigas were evaluated. We reported the occurrence of Austrodiplostomum compactum metacercariae (Trematoda, Diplostomidae) parasitizing the eyes of A. gigas supported by morphological and molecular data (COI mtDNA). The new partial sequences had a similarity of 100% to other sequences of Au. compactum previously deposited in Genbank. Five hosts (Prevalence = 8.33%) with a standard length between 69 and 116 cm were infected, with a mean intensity of infection of 31.8 ± 17.2 (1–93). Large infected hosts are unlikely to be a significant prey item for the definitive hosts (medium-sized piscivorous birds), and could act as an ‘ecological sink’, disrupting the transmission of this metacercariae. Hosts with high infection rates by these metacercariae may present cataracts, exophthalmos, and blindness, resulting in loss of visual acuity. Considering that A. gigas is a carnivorous and visual predator, the low infection observed possibly does not act as a biological filter to hinder or contain the invasion, probably presenting a low negative effect on the visual acuity of hosts.

New, well-known and predicted life cycles for trematodes of the Haploporoidea (Haploporidae and Emprostiotrematidae) and three families of the Lepocreadioidea (Enenteridae, Gorgocephalidae, Gyliauchenidae) involve encystment of the metacercaria in the open (usually on vegetation) followed by ingestion by a range of herbivorous or detritivorous fishes. These life cycles appear among relatively highly derived plagiorchiidan trematodes in which three-host life cycles incorporating an animal second intermediate host are dominant. We hypothesise that the two-host life cycles in the Haploporoidea and Lepocreadioidea arose by secondary truncation of a three-host cycle; the second intermediate host was lost in favour of encystment in the open. Modification of a three-host life cycle effective for the infection of carnivores is consistent with the understanding that fishes arose as carnivores and that multiple lineages have secondarily become detritivores and herbivores. Four of the five trematode families involved infect fishes relating to multiple orders, suggesting a complex history of host-switching. In contrast, the Gorgocephalidae, the smallest of the families, has been found only in a single family, Kyphosidae. The timing of the evolutionary events leading to this putative life cycle truncation is yet to be deduced, but the rich developing understanding of the history of the fishes creates a strong template for future analysis.

This study describes a new species of Pharyngodon Diesing, 1961 (Nematoda: Pharyngodonidae) in teiid lizards Ameivula ocellifera (Spix, 1895) (Squamata: Teiidae) from a Caatinga morphoclimatic domain in the state of Pernambuco, Brazil. Pharyngodon ameivulum n. sp., like 11 other species of the genus, features males without spicules, females with truncated eggs, and no tail spines. However, the new species is distinguished by a unique set of morphological characteristics, such as males possessing three pairs of caudal papillae, with the first pair precloacal, the second adcloacal, and the third postcloacal (arrangement 1:1:1), the second pair (adcloacal) of papillae having a bifurcated distal end, and females with lateral body alae. Molecular analysis of the 18S rDNA, 28S rDNA, and 18S + 28S concatenated sequences genes reveals that P. ameivulum n. sp. clusters with representatives of Pharyngodonidae from the genera Skrjabinodon Inglis, 1968 and Spauligodon Skrjabin, Schikhobalova & Lagodovska, 1960, forming a basal clade to the clade composed of Spauligodon spp. and Skrjabinodon trimorphi Ainsworth, 1990. These are the first phylogenetic assays to include a species of Pharyngodon.

The growth and development of schistosomes are tightly regulated by various receptors throughout their life cycle. Each stage of the parasite inhabits a distinct habitat and responds to different factors that drive its growth and development. With two hosts involved in its life cycle (mammalian and snail), the parasite must go through additional free-living stages to transition between them. Moreover, communication between male and female worms is essential for the maturation of females. The ability of adult schistosomes to survive in human hosts for up to thirty years demonstrates their capacity to efficiently utilize host nutrients for metabolic processes and growth. In Schistosoma mansoni, receptors mediate the utilization of growth factors derived from both the parasite itself and the host. Nuclear receptors, in particular, collaborate with other proteins to regulate the expression of genes essential for various developmental functions. Receptors also play a pivotal role in RNA export, which is crucial for the parasite development. Additionally, neurotransmitter receptors are essential for the growth and development of larval stages. This review aims to elucidate the mechanisms by which these receptors regulate cell proliferation, differentiation, and maturation throughout the parasite life cycle. Understanding these processes could provide insights into the role of receptors in Schistosoma mansoni development and potentially lead to innovative therapeutic strategies to combat human schistosomiasis.

This systematic review and meta-analysis examined 27 studies published between 2003 and 2024 to assess the prevalence of Fasciola hepatica infestation in various animal species in Algeria. Diagnostic methods included liver inspection (16 studies), ELISA (7 studies), coproscopy (4 studies), bile microscopy (1 study), and abattoir data analysis (1 study). For humans, coproscopy and immunoelectrophoresis (IEP) were used in one study in Algiers. Among the 1,006,751 animals examined, 15,868 tested positive, resulting in an overall prevalence of 1.57% (CI 1.55–1.59). Prevalence was higher in the northeastern regions of Algeria (El Tarf, Annaba, and Jijel) at 15.95%, compared to other regions (0.9%–2.95%) (p<0.0001). Cattle showed the highest prevalence (3.91%; CI 3.84–3.98) (p<0.001), followed by sheep (0.42%; CI 0.40–0.44) and goats (0.12%; CI 0.10–0.14). Camels had a prevalence rate of 4%. Trend analysis over 20 years indicated a progressive decrease in prevalence, from 13.29% (2004–2009) to 1.79% (2010–2019) and 1.12% (2020–2024) (p<0.0001). The ELISA method was found to be the most sensitive, revealing a prevalence of 16.40% (CI 15.23–17.57) (true adjusted prevalence is 12.38%) (p<0.0001), significantly higher than liver inspection (1.83%), coproscopy (1.04%), and abattoir data analysis (1.10%). Prevalence increased with animal age across all species. This study clearly shows that fasciolosis in Algeria is most prevalent in the northeast region and that cattle are the high-risk group of animals. As a result, control strategies are urgently needed, targeting cattle in particular in northeast Algeria, to prevent and control this disease and thus reduce Fasciola infection.

The digenetic trematode Proctoeces maculatus is a cosmopolitan parasite that infects various invertebrates and fish hosts, including the blue mussel, Mytilus edulis, along the northeastern U.S. coast. Despite its impact on mussel fitness and the region’s aquaculture, little is known about the genetic diversity and connectivity of P. maculatus in this region. This study provides the first genetic characterization of P. maculatus populations in New England using the D1–D3 region of the 28S ribosomal RNA gene. Bayesian phylogenetic analysis and a haplotype network were used to assess genetic variation and connectivity across six localities in Maine, New York, and southern New England, and to compare these populations to global samples. Our results revealed distinct geographic structuring of P. maculatus haplotypes. The ME1 haplotype, unique to Maine, reflects either recent range expansion or isolation driven by environmental and biogeographic factors, such as Cape Cod’s role as a phylogeographic barrier. The most common haplotype, US1, was shared by populations in southern New England, New York, and a single specimen from Tunisia, indicating possible historical or anthropogenic connectivity. Two divergent haplotypes from Mississippi and Chile likely represent misidentifications or cryptic species. These findings support the hypothesis that P. maculatus is likely a cryptic species complex. Molecular evidence suggests connectivity across distant regions, emphasizing the role of host movement in parasite dispersal. Continued genetic studies, particularly from under-sampled regions, are needed to unravel the diversity and biogeography of P. maculatus and its potential impact on declining mussel populations.

The effect of reservoir construction on medically important parasites is well known worldwide but lacks information in Vietnam. With 385 active hydropower plants and numerous water reservoirs, Vietnam provides an ideal setting for studying this issue. This study investigated trematode infection in snail first intermediate hosts from three hydropower reservoirs: Hoa Binh, Son La, and Thac Ba. In total, 25,299 snails representing 16 species were examined, with 959 individuals (8 species) shedding identifiable cercariae. Infection prevalence was highest in thiarid snails (5.4%–15.4%), followed by bithynid snails (2.9%–5.8%). Other snail species showed infection prevalence ranging from 0.3% to 2.9%. Infection prevalence varied significantly across regions, with the highest prevalence in Son La, followed by Hoa Binh and Thac Ba reservoirs. However, no significant differences were observed between snails collected from reservoirs versus canals and paddy fields. Morphological identification resulted in nine cercarial morphotypes, with pleurolophocercaria, xiphidiocercariae, and echinostome being the most common types, accounting for 89.2% of all cercarial infections. Echinostome cercariae were found in seven snail species, while the other cercarial morphotypes were shed by two to five species. Gabbia fuchsiana, Parafossarulus manchouricus, and Melanoides tuberculata were the most common hosts, each harboring five cercarial morphotypes, while Radix auricularia only released echinostome type. In conclusion, our findings highlight the endemic presence of trematodes in hydropower reservoirs and emphasize the need to consider the human-environment interaction around these reservoirs for a better understanding of disease transmission risks.

We report the first record of fecampiidan platyhelminths parasitic in tanaidacean crustaceans. Two fecampiidans (0.75 mm and 1.10 mm in length) were found in a female of Pseudotanais sp. (Pseudotanaidae; 1.75 mm in length) collected at 794 m depth off the southern coast of Japan, northwestern Pacific. Fresh individuals were yellow or light yellow, but completely faded in ethanol. In a maximum likelihood tree based on 28S rRNA sequences, the parasite was placed in a moderately-supported Fecampiidae clade, suggesting it is a member of Fecampiidae. The 28S sequence from the parasite was 25.0%, 32.6%, and 35.5% divergent in Kimura 2-parameter (K2P) distance from Fecampia cf. abyssicola, Kronborgia cf. amphipodicola, and Kronborgia isopodicola sequences, respectively.

Species of the genus Corynosoma (Acanthocephala: Polymorphida) mainly parasitize marine mammals and rarely marine birds, and are of veterinary and medical importance due to causing corynosomiasis in wildlife and humans. However, the current knowledge of the mitochondrial genomes and mitogenomic phylogeny of this group remains very insufficient. In the present study, the complete mitochondrial genomes of C. bullosum (von Linstow, 1892) and C. evae Zdzitowiecki, 1984 were sequenced and annotated for the first time. Both mitogenomes comprise 12 protein-coding genes (missing atp8), 22 tRNA genes, and 2 ribosomal RNAs (rrnS and rrnL), plus 2 non-coding regions (NCR1 and NCR2). Corynosoma bullosum has the largest mitogenome (14,879 bp) of any polymorphid species reported so far, while C. evae has the smallest (13,947 bp), except for Sphaerirostris lanceoides (Petrochenko, 1949). Comparative mitogenomic analysis also revealed the presence of distinct discrepancies in A + T content and gene rearrangement across the families Polymorphidae, Centrorhynchidae, and Plagiorhynchidae. Moreover, phylogenetic analyses based on the concatenated amino acid sequences of 12 protein-coding genes strongly supported the monophyly of the order Polymorphida and a close affinity between the families Polymorphidae and Centrorhynchidae in Polymorphida. The present mitogenomic phylogeny provides additional evidence for a sister relationship between the genera Corynosoma and Bolbosoma and demonstrated that C. evae has a closer relationship with C. villosum than C. bullosum in the genus Corynosoma.

Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disease that affects the joints. Treatments are symptomatic and can induce side effects in some patients. In this sense and based on previous studies, our aim was to investigate the ex vivo immunoregulatory effect of the laminated layer (LL) during rheumatoid arthritis. LL is the outside layer of parasitic cyst of the helminth Echinococcus granulosus.

Our main objective was to study the effect of LL on nitric oxide (NO) and cytokines production, matrix metalloproteinases (MMPs) activities, inducible NO synthase (iNOS) and nuclear factor κappa B (NF-κB) expression. In this context, cultures of peripheral blood mononuclear cells (PBMC) from Algerian RA patients in active (ARA) and inactive (IRA) stage of the disease were stimulated with LL extract (50, 100, 150μg/mL). However, PBMC from ARA patients were stimulated with methotrexate (MTX; 0.5μg/mL) and biological disease modifying anti-rheumatic drugs (bDMARDs): anti-TNFα (10μg/mL), anti-IL6 (10μg/mL), anti-CD20 (10μg/mL), alone or combined with LL (50μg/mL).

Our results showed that LL reduced NO, TNF-α, and IL-17A production, MMP9/2 activities, and iNOS/NF-κB expression in PBMC from ARA patients. Concomitantly, LL increases IL-10 and TGF-β1 production in the same cultures. Interestingly, the decrease in NO production induced by bDMARDs was greater in association with LL.

Collectively, our findings indicate a strong immunoregulatory effect of LL on NO, MMPs, and cytokines. LL probably acts through the NF-κB pathway. The development of biodrugs derived from LL of E. granulosus could be a potential candidate to modulate inflammation during RA.

The synonymies of the echinochasmid genera Mesorchis Dietz, 1909 and Monilifer Dietz, 1909 with Stephanoprora Odhner, 1902 remain contentious and unresolved with morphology. To explore the matter, we herein provide a supplemental description of the type species of Stephanoprora, Stephanoprora ornata Odhner, 1902, based on specimens we collected from the intestine of a Nile crocodile Crocodylus niloticus Laurenti, 1768 (Crocodylia: Crocodylidae) captured in the Kavango River, Namibia. No nucleotide information was available previously for S. ornata. Morphology plus 28S and ITS2 phylogenetic analyses suggested Stephanoprora is a monotypic genus that can be differentiated from other genera by having 26 collar spines. Stephanoprora differs from Mesorchis by the number and distribution of collar spines (26 [2 dorsal spines, 12 lateral spines, 12 corner spines] vs. 22 [2 dorsal spines, 12 lateral spines, 8 corner spines] in Mesorchis), length of the pre-pharyngeal oesophagus, pharynx position posterior to collar (vs. pre-pharyngeal oesophagus short, pharynx anterior to or at level of corner spines), and testes shape (elongate-ovoid, irregular in outline vs. ovoid, atypically elongated). Our 28S and ITS2 phylogenies recovered the new sequence of S. ornata (having 26 collar spines) sister to all remaining echinochasmid sequences, representing species that have 20–24 collar spines. Echinochasmus Dietz, 1909 and Mesorchis were recovered as paraphyletic. We retain Monilifer as a junior subjective synonym of Echinochasmus based a suite of morphological features related to body shape and genitalia and because the designated type for Monilifer was reassigned to Echinochasmus. We accept 25 species of Mesorchis.

Human toxocariasis is a neglected parasitic disease with a global distribution, treated with current anthelmintics that have low to moderate efficacy, and requires the discovery of novel drugs. Camphor derivatives have antimicrobial properties against various pathogens such as fungi and bacteria. This study aimed to identify a camphor derivative with activity against Toxocara canis larvae and evaluate its cytotoxicity, in silico bioavailability, and in vivo activity in Swiss mice infected with this parasite. Three compounds were tested in vitro in duplicate at a concentration of 1.0 to 0.05 mg/mL in a microplate containing 100 T. canis larvae in RPMI-1640 medium incubated for 48 h at 37°C and 5% CO2. The compound (E)-2-((1,7,7-trimethylbicyclo [2.2.1] heptan-2-ylidene)amino)phenol (C2) presented a minimum larvicidal concentration (MLC) of 0.25 mg/mL and was selected for the subsequent steps. This compound showed 100% cell viability in MLC and adequate bioavailability in computational models. Two subsequent in vivo tests were performed on Swiss mice inoculated with 500 T. canis infective eggs through intragastric (IG) intubation, one at 10 days post-inoculation (n=5) and the other at 30 days post-inoculation (n=10). The selected compound (10 mg/kg, via IG) and two controls (albendazole, 40 mg/kg, IG and phosphate buffered saline 0,15M, pH 7,2, via IG) were used for this evaluation. The compound reduced the intensity of infection by 75.7% and 54.8% at 10 and 30 days post inoculation, respectively (p<0.05). The results of this study demonstrate that this compound has potential as an anthelmintic candidate for visceral toxocariasis treatment.

The parasites of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) are poorly documented in the United States despite the economic importance and global introduction of this African fish. Only one metazoan parasite (Gyrodactylus cichlidarum Paperna, 1968; Gyrodactylidae) reportedly infects Nile tilapia in the United States. Examining Nile tilapia from a flow-through aquaculture system hydrologically linked to Sougahatchee Creek (Tallapoosa River, Auburn, Alabama), we observed a gill infection by Cichlidogyrus sclerosus Paperna & Thurston, 1969 (Dactylogyridae). This monogenoid was originally described from the gill of Mozambique tilapia, Oreochromis mossambicus (Peters, 1852) from Lake Victoria, Uganda. Specimens of C. sclerosus were studied for morphology and phylogenetic analyses using the 28S and ITS1. We identified our specimens as C. sclerosus because they had the following combination of morphological features: marginal hooks shorter than dorsal anchor length; anchor roots reduced; dorsal anchor point bent; dorsal bar pyriform projections approximately half as long as dorsal bar width; penis short (<100 μm), not coiled, tubular, lacking swelling, having irregularly surfaced heel; and accessory piece straight and bifid. Our 28S and ITS1 phylogenies recovered our C. sclerosus sequences in a clade with conspecific sequences and showed no obvious biogeographic pattern. Cichlidogyrus sclerosus reportedly infects 21 fishes of 11 genera and 3 families from 36 countries in Africa, Asia, North America, South America, and Europe. The study of Nile tilapia parasites, especially those exhibiting direct life cycles and low host specificity, is important because they comprise potential invasive species.

The genus Rhinebothrium (Cestoda: Rhinebothriidea) comprises tapeworm species parasitizing elasmobranch hosts, particularly batoids. Despite numerous recent findings regarding the ecological importance of marine fish parasites throughout the world, the biodiversity of cestodes inhabiting fishes of the Persian Gulf and the Gulf of Oman remains understudied. Here, two new species of Rhinebothrium from stingrays from the Persian Gulf and Gulf of Oman are described: Rhinebothrium gossi sp. nov. and Rhinebothrium palmeri sp. nov. from Maculabatis arabica and Maculabatis randalli, respectively. However, each new cestode species is found with a lower frequency in the other host species, too. These new species were already subjected to a molecular analysis and the revealed genetic distinctiveness requires detailed morphological examinations at the species level. A combination of morphomeristic characteristics including body size, scolex features, proglottid morphology, and reproductive structures distinguish the new species from the other congeners. Although these new species are morphologically similar, however, they differ from each other in the number of testes (6–8 and 8–14), and bothridial loculi (50 and 42 in R. gossi sp. nov. and R. palmeri sp. nov., respectively). These findings contribute to our understanding of marine cestode diversity and underscore the importance of further research in this ecologically significant region.

This study investigated the co-infection dynamics of Schistosoma mansoni and Ancylostoma ceylanicum in an experimental model. Two experiments were conducted to investigate varying infection orders. In experimental group 1, subjects were initially infected with A. ceylanicum, followed by S. mansoni infection. In group 2, subjects were first infected with S. mansoni, followed by A. ceylanicum infection. The co-infected groups were compared to mono-infected groups for further analysis. Parameters assessed included weight, fecal egg elimination, blood cell counts, IgG response, histopathological analysis, and granuloma morphometry. Results indicated that the timing of infections influenced weight loss. Co-infected animals lost weight similarly to A. ceylanicum-infected animals when A. ceylanicum preceded S. mansoni. The co-infected groups did not experience worsened anemia, despite both parasites being hematophagous. No linear correlation was found between S. mansoni egg counts and anti-SEA IgG production. The response to the S. mansoni adult worm antigen was significantly higher in the S. mansoni mono-infected group compared to the group previously infected with hookworm. Co-infection with A. ceylanicum after S. mansoni infection also interfered with the host response. It caused a delay in the peak of S. mansoni egg elimination compared to the S. mansoni monoinfected group. Additionally, fewer eggs were retained in the liver compared to single infections. These findings help elucidate the interactions between different parasites and their hosts in the same endemic area, providing valuable information to enhance strategies for disease control and management.

Members of the genus Oncicola Travassos, 1916 are generalist parasites able to infect a broad spectrum of carnivorous hosts, such as marsupials, procyonids, felids, and canids, and are distributed globally. Adult specimens were collected from the intestines of three white-nosed coatis (N. narica), whereas cystacanths (larval form) were found in the body cavities of two amphibian species (paratenic hosts) in localities from northern and southeastern Mexico. Morphologically, both stages were identified as O. luehei (Travassos, 1917) on the basis of the following features: trunk cylindrical, narrow anteriorly, enlarging midbody, tapering gradually to narrow posteriorly; proboscis globular with six circular rows of hooks with six hooks each, decreasing in size posteriorly; neck short with sensory papilla; tubular lemnisci long, extending to the posterior region; protonephridia dendritic type; and eight cement glands, compact with a single giant nuclei. Sequences from cytochrome c oxidase subunit 1 from mtDNA were aligned and compared with sequences available in GenBank. Phylogenetic analyses revealed that adults and cystacanths formed a clade with two other isolates identified as Oncicola sp. and O. luehei from Mexico. The intraspecific genetic divergence among the isolates was low, ranging from 0.0% to 3.0%, indicating that the two stages of the life cycle belong to the same species. The haplotype network was inferred with 11 sequences and revealed a lack of shared haplotypes between populations, suggesting a reduced recombination rate and a high pattern of genetic variation among individuals. Finally, these new records of O. luehei increase the distribution range of O. luehei on both coasts of Mexico.

Toxocara canis is a widespread parasite of canids with a wide range of paratenic hosts, but also one of the overlooked agents causing nervous system infections of humans. Previous experimental infections of mice demonstrated the impact of high infection doses of larvae on neurobehavioral disorders and pathological changes. In contrast to previous studies, we aimed to investigate the long-term (up to 100 weeks) impact of low- to high-dose infection in mice. We focused on their physical condition, motor skills, and the accompanying pathologies in the brain. Three groups of BALB/c mice were infected with 10, 100, and 1000 T. canis larvae/mouse and specific anti-T. canis excretory-secretory antigens immunoglobulin G antibody response, general condition, and motor skills were tested in defined intervals within 100 weeks after infection. The number of larvae in selected organs was assessed and the pathological changes in the brain were studied histologically. As a result, subtle to severe impairments in general condition and motor skills were detected, with generally earlier onsets occurring the higher the infection dose was. The specific immunoglobulin G antibody levels corresponding to the infection dose were detected in all infected groups. Necrosis, cellular infiltrations, and foamy cells developed in moderate- and high-infection dose mice, in contrast with hemorrhages detected in all groups. This study demonstrated the long-term negative impact of T. canis infection on the paratenic host, particularly at moderate and high infectious doses. Although pathological changes in the brain were observed even in low-infection dose mice, their physical and motor condition was comparable to the control group.

Here, we present a comprehensive morphological and molecular phylogenetic analysis of Clinostomum sp. (Digenea: Clinostomidae) metacercariae parasitizing two freshwater fish species from Southeast Brazil: Serrasalmus spilopleura (piranha) and Callichthys callichthys (tambuatá). The morphological examination revealed distinct characteristics of metacercariae in each host. Using the cytochrome c oxidase I (COI) gene barcode region, we obtained DNA sequences that allowed for accurate phylogenetic placement. Phylogenetic analyses revealed that Clinostomum sp. HM41 (metacercariae), isolated from S. spilopleura, exhibited 86% similarity to Ithyoclinostomum yamagutii, while Clinostomum sp. HM125 (metacercariae), from C. callichthys, showed 98.7% similarity to Clinostomum sp. Cr_Ha1. The phylogenetic trees constructed through Bayesian Inference and Maximum Likelihood methods indicated high biodiversity within the Clinostomum genus and strong support for distinct lineages. These findings enhance our understanding of the diversity and ecological distribution of Clinostomum species in South American freshwater environments.