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Parasitic infections caused by Clinostomum complanatum and Euclinostomum heterostomum pose significant threats to aquaculture systems and public health. This study examined their molecular, immunological, and pathological impacts in Egyptian Nile tilapia ( Oreochromis niloticus ), with COXI gene sequencing confirming both species and revealing genetic similarities to regional isolates, suggesting broad geographical distribution. Euclinostomum heterostomum -infected fish exhibited significant upregulation of immune-related genes ( IFN-γ : 1.96-fold; IL-10 : 2.06-fold; IL-12 : 1.91-fold; IL-1β : 4.07-fold; CYP-1α : 2.07-fold) and elevated oxidative stress markers (SOD: 2.96-fold; CAT: 3.52-fold; GSH: 3.07-fold; TAC: 2.58-fold) compared to uninfected controls ( p  < 0.0001 for all comparisons). Similarly, Clinostomum complanatum infections triggered upregulation of IFN-γ (2.17-fold), IL-10 (2.09-fold), IL-12 (2.25-fold), IL-1β (2.51-fold), and CYP-1α (2.58-fold), alongside increased SOD (2.63-fold), CAT (2.70-fold), GSH (2.99-fold), and TAC (2.65-fold) compared to uninfected controls ( p  < 0.0001 for all comparisons). Histopathology revealed necrosis, fibrosis, and inflammation in gill and kidney tissues, with immunohistochemical staining confirming localized inflammatory markers. These findings demonstrate systemic immune activation and physiological stress caused by clinostomid infections, providing critical insights for aquaculture disease management strategies.

Helminth neuroinfections represent serious medical conditions, but the diversity of the host-parasite interplay within the nervous tissue often remains poorly understood, partially due to the lack of laboratory models. Here, we investigated the neuroinvasion of the mouse spinal cord by Trichobilharzia regenti (Schistosomatidae). Active migration of T . regenti schistosomula through the mouse spinal cord induced motor deficits in hindlimbs but did not affect the general locomotion or working memory. Histological examination of the infected spinal cord revealed eosinophilic meningomyelitis with eosinophil-rich infiltrates entrapping the schistosomula. Flow cytometry and transcriptomic analysis of the spinal cord confirmed massive activation of the host immune response. Of note, we recorded striking upregulation of the major histocompatibility complex II pathway and M2-associated markers, such as arginase or chitinase-like 3. Arginase also dominated the proteins found in the microdissected tissue from the close vicinity of the migrating schistosomula, which unselectively fed on the host nervous tissue. Next, we evaluated the pathological sequelae of T . regenti neuroinvasion. While no demyelination or blood-brain barrier alterations were noticed, our transcriptomic data revealed a remarkable disruption of neurophysiological functions not yet recorded in helminth neuroinfections. We also detected DNA fragmentation at the host-schistosomulum interface, but schistosomula antigens did not affect the viability of neurons and glial cells in vitro . Collectively, altered locomotion, significant disruption of neurophysiological functions, and strong M2 polarization were the most prominent features of T . regenti neuroinvasion, making it a promising candidate for further neuroinfection research. Indeed, understanding the diversity of pathogen-related neuroinflammatory processes is a prerequisite for developing better protective measures, treatment strategies, and diagnostic tools.

Yellow grub disease is a parasite infection, caused by Clinostomum spp. Metacercariae and affecting axial muscles and internal organs of freshwater fish. Killifishes live in seasonal ponds and are short-lived; in the Neotropical region they are among the most vulnerable vertebrates. The current study analyzes the possible injury and inflammatory reaction caused by parasitic infection with Clinostomum spp. metacercariae in testes, ovaries, liver, intestine and mesentery in the seasonal killifishes Trigonectes aplocheiloides and Titanolebias monstrosus . Apparently, this parasite could migrate through the circulatory system to different organs, since a cercaria was found in the lumen of a hepatic sinusoid. In all organs the inflammatory response showed dilated, congested vascular areas and infiltration of numerous inflammatory cells, mainly composed of lymphocytes, eosinophilic granular cells and macrophages. However, the most affected organs were the pancreas, ovaries and testes. In these organs, severe pancreatitis with degranulation of the pancreatic acini, scarce ovarian follicles and spermatocysts dilated with few or no seminiferous cells were observed. In seasonal killifish, yellow grub disease could cause low fertility rates or even sterility by reducing or impairing their reproductive capacity. Therefore, this type of parasitism has a considerable impact on the perpetuation of this species due to the short periods they have to breed before their habitats dry out.

Background Trematode infections in ruminants cause major economic losses through reduced productivity and liver condemnation, with their epidemiology closely linked to snail habitats, rainfall patterns, and poor grazing management. A cross‐sectional study was conducted from November 2022 to March 2023 to characterize pathological changes caused by trematode infections and identify associated risk factors in ruminants slaughtered at three selected abattoirs in central Ethiopia. Methods A total of 256 ruminants, 137 bovine, 64 ovine and 55 caprine were included in the study. The sample size was determined based on the number of animals slaughtered per week and the frequency of slaughtering. Haematological analysis, coproscopic examination, and both ante‐mortem and post‐mortem inspections were performed to detect trematode infections. Results The overall prevalence trematode was 49.22%, with the prevalence of 32.42% and 16.80% for fasciolosis and paramphistomosis, respectively. There was no statistically significant association between fasciolosis and animal species, but a significant association was found between paramphistomosis and fasciolosis. Fasciolosis prevalence was highest in caprine (41.82%), followed by ovine (31.25%) and bovine (29.50%). For both fasciolosis and paramphistomosis, prevalence was higher in animals with poor body condition (47.62% and 28.57%, respectively), moderate in those with medium body condition (32.81% and 28.97%), and lowest in animals with good body condition (28.97% and 15.89%). Based on geographic origin, the highest prevalence of fasciolosis was recorded in animals from Jimma (52.63%), while paramphistomosis was most common in animals from Dukem (22.50%). Histopathological findings in Fasciola‐infected animals revealed haemorrhages, hepatocytic wall dilatation, necrosis, hypertrophy, and portal fibrosis. In Paramphistomum‐infected animals, muscular degeneration and loss of villi and microvilli were observed. Haematological analysis showed reduced red and white blood cell counts and elevated liver enzyme levels. Conclusion The study revealed a high prevalence of fasciolosis and paramphistomosis in the study areas. Further research is recommended to explore the epidemiology of trematode infections, identify intermediate snail hosts, and develop effective strategic control measures. • Trematode infections were highly prevalent (49.22%) among ruminants in central Ethiopia, with fasciolosis (32.42%) and paramphistomosis (16.80%) strongly associated with poor body condition and specific geographic origins. • Histopathological and haematological analyses revealed severe liver and intestinal damage, anaemia, and elevated liver enzymes, highlighting the need for targeted control strategies.

Flukes can cause severe and lethal diseases in various animals, including fish. Both adult and larval stages of flukes are found in fish. Haplorchiasis is an infection of fish gills by heterophyid trematodes such as Haplorchis taichui. To detect this parasite, the gills of 30 Tigris kingfish and Tigris barb collected from the Shapour River in Kazerun, Fars province were found to be parasitized with metacercarial cysts of a heterophyid trematode identified as H. taichui. Histopathological examination of the infected fish gills revealed cartilage proliferation, severe hyperplasia, fusion, S-forming, shortening and thickening, distortion, and displacement of affected secondary gill filaments leading to deformities of the filament structure, clubbing, telangiectasis, and hyperemia. Although the gill damage was evident and potentially life-threatening for the cyprinid fish, the examined fish showed no clinical signs. This finding indicates that H. taichui is pathogenic; therefore, prevention of infection and treatment should bea priority.

Paramphistomosis, a parasitic disease of ruminants caused by flukes of the family Paramphistomidae (Platyhelminthes: Digenea) and leads to significant economic losses. Primarily, their predilection site is rumen as evident from their common name ‘ruminal flukes’ and occasionally find in other visceral organs. This study aimed to determine the prevalence of paramphistome infection in the rumens and livers of livestock followed by histopathological examination of the affected tissues. A total of 384 animals were screened for paramphistome infection at a local abattoir in the Narowal district. Liver and rumen tissue samples from infected animals were excised, fixed in buffered formalin, and processed using the paraffin‐embedding technique. Tissue‐paraffin blocks were sectioned with the thickness of 5 µm through microtome and stained using the Harri's haematoxylin and eosin methods. The prevalence of paramphistomosis in ruminants was 49.74% with sex and age of host having a statistically significant association (p < 0.05) with the magnitude of infection; however, species of host and months were not significantly associated (p > 0.05). Of the total cases having rumens positive for paramphistomes, 7.29% were also tested positive for liver infections; however, no adult or immature Fasciola spp. were detected. Infected rumens had significantly (p < 0.05) reduced epithelial thickness (30.50 ± 1.83 µm), accompanied by severe haemorrhages in hepatic tissues and blood congestion in hepatic vessels. Further, the central hepatic vein was found having a significantly larger diameter (39.56 ± 2.72 µm) as compared to that of healthy livers (29.76 ± 3.15 µm). The paper provides a maiden data on the paramphistome infection in livers along with rumens of ruminants in Pakistan. Hence, the findings suggest that paramphistome infection may pose a potential threat to nutrient absorption and overall metabolic health in ruminants. Our study was specifically focused on paramphistomes, with particular attention to their prevalence and associated pathology in the rumen and liver tissues. The elevator pitch of the manuscript to present the maiden study on extra‐ruminal infection sites of the ruminal flukes.

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.

The biliary trematode Pseudamphistomum truncatum parasitizes a wide range of fish-eating mammals, including humans. Here we report the emergence of this parasite in grey seals (Halichoerus grypus) in the Baltic Sea. One hundred eighty-three of 1 554 grey seals (11.9%) examined from 2002-2013 had detectable hepatobiliary trematode infection. Parasite identification was confirmed as P. truncatum by sequencing the ITS2 region of a pool of five to 10 trematodes from each of ten seals collected off the coast of seven different Swedish counties. The proportion of seals parasitized by P. truncatum increased significantly over time and with increasing age of seals. Males were 3.1 times more likely to be parasitized than females and animals killed in fishery interactions were less likely to be parasitized than animals found dead or hunted. There was no significant difference in parasitism of seals examined from the Gulf of Bothnia versus those examined from the Baltic Proper. Although the majority of infections were mild, P. truncatum can cause severe hepatobiliary disease and resulted in liver failure in at least one seal. Because cyprinid fish are the second intermediate host for opisthorchiid trematodes, diets of grey seals from the Baltic Sea were analysed regarding presence of cyprinids. The proportion of gastrointestinal tracts containing cyprinid remains was ten times higher in seals examined from 2008 to 2013 (12.2%) than those examined from 2002 to 2007 (1.2%) and coincided with a general increase of trematode parasitism in the host population. The emergence and relatively common occurrence of P. truncatum in grey seals signals the presence of this parasite in the Baltic Sea ecosystem and demonstrates how aquatic mammals can serve as excellent sentinels of marine ecosystem change. Investigation of drivers behind P. truncatum emergence and infection risk for other mammals, including humans, is highly warranted.

In the Laboratory of Parasites of Fishes, Crustaceans and Mollusks (CEPAVE), we undertook a parasitological study on three species of fish from the Espinal and Esteros del Iberá ecoregions of Argentina. Clinostomid metacercariae were found parasitizing Characidium rachovii , Crenicichla vittata and Gymnogeophagus balzanii . In this study, we analysed the damage that these parasites inflict on their hosts through the evaluation of histological sections. In addition, Clinostomum metacercariae were identified using morphological characters and DNA barcoding. In the pathological analysis, we observed that muscle tissue was the most affected. The inflammatory response showed vascular congestion areas and infiltration of numerous inflammatory cells, mainly lymphocytes. The molecular and morphological approach supports the presence of three new lineages of clinostomid metacercariae in Argentina. This could lead to the discovery of a high number of lineages or species of Clinostomum from South America.

This paper presents the first report of parasites from the genus Renicola sp. in the kidneys of Magellanic penguins. The histological analysis revealed inflammatory infiltrate (eonsinophils, lymphocytes, and plasmocytes), together with fibroplasia and compression of the adjacent ducts.