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Myxobolus bengalensis is a significant fish parasite responsible for disease and economic losses in aquaculture; however, information on its pathogenicity and impact on fish health remains limited. This study aimed to identify the morphological and molecular features of Myxobolus bengalensis and investigate the immunological changes in infected Labeo catla . Symptomatic fish showed signs of disease, including extreme lethargy, a slender body with a large head (growth retardation), and pale gills with the presence of whitish cysts in the gill lamellae. The Myxozoan parasite was isolated from infected L. catla tissue samples and preliminarily identified as M. bengalensis based on morphology, 18S rRNA PCR sequencing, and phylogenetic analysis. Tissue specimens were collected and underwent examination of immune-related gene expression (in the liver and kidney). The M. bengalensis spores isolated from the gills were ellipsoidal, with ovoid polar capsules. Sequencing and phylogenetic analysis of the 18S rRNA region revealed a ~ 1700 bp fragment, confirming the parasite's identity and placing it within the same clade as other M. bengalensis isolates. The transcription analysis of genes associated with inflammation ( TNF-α , IL-1β , iNOS ), immune activation ( TLR 4 , C3 , MYD88 , NOD 1 ), and both innate and adaptive immune responses ( IFN‐γ , Hsp70 , Mx , IgM ) further highlights that M. bengalensis modulates the expression of genes in the liver and kidney tissue samples of infected L. catla . There is limited data available on the host–pathogen response during M. bengalensis infection; thus, this study provides key insights into the morphology, pathogenicity, and immunological impacts of M. bengalensis in L. catla .

Four new actinospore types belonging to the sphaeractinomyxon collective group (Cnidaria, Myxosporea) are described from the coelomic cavity of a marine Baltidrilus sp. (Oligochaeta, Naididae) inhabiting a northern Portuguese estuary. Host identification supports the usage of marine oligochaetes, namely of the family Naididae Ehrenberg, 1828, as definitive hosts for myxosporeans inhabiting estuarine/marine environments. The absence of mixed infections in the host specimens analysed is suggested to reflect the influence of host-, parasite- and environmental-related factors regulating myxosporean–annelid interactions. Molecular analyses matched the SSU rDNA sequences of three of the four new types with those of mugiliform-infecting Myxobolus spp., namely Myxobolus mugiliensis and a Myxobolus sp. from flathead grey mullet Mugil cephalus, and Myxobolus labrosus from thicklip grey mullet Chelon labrosus. These results directly link, for the first time, the sphaeractinomyxon collective group to a myxospore counterpart, further confirming their previously hypothesized specific involvement in the life cycle of myxobolids that infect mullets. Acknowledging this life cycle relationship, the functionality of the sphaeractinomyxon morphotype is suggested to have been decisive for the evolutionary hyperdiversification of the genus Myxobolus in mullets. Unlike other actinospore morphotypes, sphaeractinomyxon lack valvular processes, which implies a limited capability for buoyancy. Considering the benthic-feeding nature of mullets, this feature is most likely crucial in promoting successful transmission to the vertebrate host.

Mullets inhabit a wide range of habitats from tropical to temperate regions and play a critical role in their ecosystems. This commercially important fish group constitutes a significant source of food in several geographic regions, and the production of some species for consumption is an increasing trend. About 64 myxosporean species have been reported in mullets, some of which are cryptic, as is the case of Myxobolus exiguus, and M. muelleri. This paper provides, for the first time, a detailed and critical revision of the data available for myxobolids reported in mullets, determining the species that have bona fide mugiliform fish hosts, in accordance with the original species descriptions, the available molecular data and the currently accepted taxonomic and phylogenetic criteria. Phylogenetic analyses using Bayesian inference and maximum-likelihood methodologies suggest that the evolutionary history of myxobolids with bona fide mugiliform fish hosts reflects that of its vertebrate hosts, while reinforcing known evolutionary factors and old systematic issues of the clade of myxobolids. A comprehensive morphological, ultrastructural and molecular redescription is also provided for the cryptic species M. exiguus, from infections in the visceral peritoneum of the thinlip-grey mullet Chelon ramada in the River Minho, Portugal.

Forty-two species of myxozoans, including 31 species of Myxobolus Bütschli, 1882, have been described from centrarchid fishes. One species, the green sunfish ( Lepomis cyanellus ), has been reported to host at least three species of Myxobolus . Between March 2023 and June 2024, 42 L. cyanellus were collected from watersheds in Montgomery and Polk counties, Arkansas, and their gills, gallbladders, urinary bladders, fins, integument, other major organs, and musculature were examined for myxozoans. Two (5%) L. cyanellus from Polk County were found to harbor a new species of Myxobolus infecting the gill lamellae. A qualitative and quantitative morphological description was based on formalin-fixed preserved plasmodia and myxospores. Plasmodia of Myxobolus polkensis n. sp. are 133 µm long × 123 µm wide, and myxospores are 19.8 µm long × 6.5 µm wide, with two narrowly pyriform unequal polar capsules, one usually longer (9.2 µm) than the other (8.5 µm). Molecular data consisted of a 2,025 base pair sequence of the partial small subunit rRNA gene ( SSU ). Variably sized, polysporic plasmodia were randomly distributed throughout the gill lamellae in intralamellar locations. Large plasmodia displaced adjacent lamellae inducing mild epithelial proliferation but minimal inflammatory changes. Phylogenetic analysis revealed that M. polkensis n. sp. is a member of a clade of myxozoan species that predominately infect centrarchids from North America. This is the fourth report of a Myxobolus from L . cyanellus but the first report of a species infecting the gill lamellae. This article was registered in the Official Register of Zoological Nomenclature (ZooBank) as urn:lsid:zoobank.org:pub:E9BA08E0-0A7E-487B-80AE-C5C723DBB661.

A new species, Myxobolus liuyangensissp. n., was found in the gills of the exotic mrigal carp Cirrhinus mrigala during a survey of the fauna of exotic fish myxospore in China. Plasmodia were elongated pyriform, measuring 0.42 mm long and 0.15 mm wide. The mature spores were elongated pyriform in the frontal view, tapered forward, rounded posterior end, and fusiform in the sutural view, measuring 17.3 ±0.5 (16.5–18.3) μm long, 6.2 ±0.3 (5.2–6.8) μm wide, and 4.8 ±0.2 (4.4–5.1) μm thick. The two equal polar capsules of elongated pyriform in shape measured 11.3 ±0.5 (10.6–12.3) μm long and 2.5 ±0.3 (2.0–3.1) μm wide, occupying more than half the capacity of the spores. The polar filaments were coiled with fifteen to sixteen turns. No mucous envelope and caudal appendages were found. The consensus SSU rDNA gene sequence obtained here for M. liuyangensissp. n. did not match any sequences available in GenBank, but was most closely related to M. catlae that infects the gills of C. cirrhosis (MT003664, 97.99% similarity). Phylogenetic analysis indicated that the C. mrigala-infecting Myxobolus species were not clustered together, but dispersed in different clades. The present species clustered with M. catlae and M. orissae within the clade I of elongated pyriform spore shapes, revealing spore shapes may play an important role during the evolution of Myxobolus species. This is the second myxosporean infection report in the exotic mrigal carp C. mrigala.

There is a claimed increase in the global prevalence and incidence of emerging diseases observed in many organisms. Myxozoa represents an essential group of metazoan parasites that hold both economic and ecological significance. In the current study, 1% of the fish population at two commercial goldfish ( Carassius auratus ) farms in Tehran and Ghom province, Iran, developed cavitating muscular lesions resembling humps in February 2021 and January 2022. Fish displaying pathological abnormalities were transported to the Ornamental Fish Clinic and subjected to clinical examination. Light microscopy was subsequently used to examine wet smears of skin and gills, as well as whitish exudate. In addition, tissue homogenates were collected for more precise identification and molecular confirmation. The study discovered that individuals from the goldfish farms were infected with the pathogenic myxozoan Myxobolus lentisuturalis , which caused significant damage to the epaxial muscles. The spores collected from the humps had a lack of uniformity and were primarily ellipsoidal in shape. Histopathological analysis also revealed parasites in various stages of development, such as plasmodia and spores, as well as inflammatory cell infiltration (macrophage, giant cell and lymphoplasmacytic infiltration) between skeletal muscle fibers. Phylogenetic analysis of M. lentisuturalis was performed by using MEGA 11 and the maximum likelihood method. M. lentisuturalis is a myxozoan parasite that has been sparsely recorded and lacks widespread recognition. The current study is the first clinical, histopathological, and molecular characterization of M. lentisuturalis isolated from the skeletal musculature of goldfish ( C. auratus ) in Iran.

Background Myxozoa is a well-known economically and ecologically important group of metazoan parasites, phylogenetically related to Cnidaria. High diversity of myxosporeans has been recorded in Russia and China; however, most of the species were solely morphologically characterised. Here, we identified a new gibel carp-infecting Myxobolus species and morphologically and molecularly compared the Russian and Chinese isolates of this new myxosporean. Results Myxobolus pronini n. sp. was found free in the abdominal cavity of Carassius auratus gibelio (Bloch, 1782) in Lake Baikal watershed, Russia, and embedded in the visceral serous membranes of the same fish species in Lake Taibai, Hubei province, China. The morphometric data of the plasmodia and mature spores exhibited some differences between the Russian and Chinese isolates, but SSU rDNA sequences indicated that these two geographical isolates are conspecific. The mature spores from the two locations are obovate in frontal view, with wider anterior than posterior end and lemon-shaped in sutural view. Spores of the Russian isolate were 14.3–16.2 (mean 15.1 ± 0.2) μm long, 9.6–10.8 (10.1 ± 0.1) μm wide and 6.4–7.4 (6.7 ± 0.15) μm thick; those of the Chinese isolate were 13.8–15.6 (14.7 ± 0.24) μm long, 9.6–13.3 (9.6 ± 0.65) μm wide and 6.2–7.2 (6.6 ± 0.16) μm thick. The newly-generated rDNA sequences (including SSU rDNA, ITS and LSU rDNA) from the two isolates represented some variations within the intraspecific range. Homology search by BLAST showed that the newly obtained rDNA sequences do not match any sequences available on GenBank. Phylogenetic analysis based on the aligned partial SSU rDNA sequences indicated that this novel species clustered with several gibel carp-infecting Myxobolus spp. with round anterior end of spores. Additionally, phylogenetic analysis based on all obtained ITS sequences showed that distinct genetic geographical differentiation occurred for this new parasite. Conclusions Myxobolus pronini n. sp. is described by integrating morphological, ecological and molecular evidence. Two geographical isolates of this species showed some morphological and genetic differences but within the intraspecific range of variation.

Here, we investigated the early development of two closely related myxozoan parasites, the highly pathogenic Myxobolus cerebralis, the causative agent of the whirling disease in salmonids, and Myxobolus pseudodispar, a common, non-pathogenic parasite of cyprinids. The aim of our study was to examine under in vivo laboratory conditions whether fish blood is involved in the intrapiscine development of the two parasite species and investigate if there is dissimilarity between the parasite infection intensity in blood and if it varies in terms of host susceptibility and parasite pathogenicity. Highly susceptible, less susceptible and non-susceptible hosts were involved. Blood samples were taken 1 day, 1 week and 1 month post exposure to M. cerebralis and M. pseudodispar, respectively. The prevalence and infection intensity was estimated by parasite-specific quantitative real-time PCR. Although previous findings assumed that M. cerebralis might escape from host immune system by migrating via peripheral nerves, our experimental results demonstrated that M. cerebralis is present in blood during the early stage of intrapiscine development. For the non-pathogenic M. pseudodispar, the highest infection prevalence was found in the original host, common roach Rutilus rutilus, whereas the highest infection intensity was detected in rudd Scardinius erythrophthalmus, a \"dead-end\" host of the parasite. The presence of M. pseudodispar developmental stages in the blood of both susceptible and non-susceptible cyprinids suggests that the susceptibility differences remain hidden during the early stage of infection. Our findings supply further evidence that host specificity is not determined during the early, intrapiscine development involving the vascular system. Furthermore, we found remarkable differences in the infection dynamics of the two parasite species examined, possibly due to their distinct pathogenicity or variations in adaptive capabilities to immune components in host blood.

During 9–10 February 2018 and 21–22 February 2020, 7 adult Blue Suckers, Cycleptus elongatus, were collected by hoop nets from the Red River, Little River County (n = 3), and the Black River, Lawrence County (n = 4), Arkansas, and their gills, gallbladders, fins, integument, other major organs, and musculature were examined for myxozoans. All 7 (100%) were infected with an unknown species of gill-infecting Myxobolus sp. Twenty formalin-fixed plasmodia (cysts) of Myxobolus cloutmani n. sp. were elliptoidal, 407 µm long × 270 µm wide. Formalin-fixed myxospores were orbicular to broadly elliptoidal, 8.7 µm long × 7.8 µm wide. Two polar capsules were pyriform and subequal in size, extending over halfway in the myxospore. The larger polar capsule was 5.5 µm long × 3.1 µm wide, while the shorter was 5.1 × 2.9 µm. A coiled polar filament possessed 5 or 6 coils. The myxospore was 3.7 µm thick in sutural view, with a distinct sutural ridge. Qualitative and quantitative morphological data were from formalin-fixed as well as ethanol-preserved spores, while molecular data consisted of a 2,010 base pair sequence of the partial 18S ribosomal RNA gene and a 2,502 base pair sequence of the partial 28S ribosomal RNA gene. Phylogenetic analysis grouped M. cloutmani n. sp. with the other catostomid-infecting myxobolids. This is the first myxozoan reported from C. elongatus.

Myxosporean infection in marine water fishes has drawn less attention than in freshwater fishes, which resulted in a higher taxonomic variety in freshwater in Malaysia. This study aimed to address the gap by conducting a myxosporean survey on two commercially significant marine fish species, Nemipterus furcosus (Valenciennes) (Eupercaria incertae sedis: Nemipteridae) and Selar crumenophthalmus (Bloch) (Carangiformes: Carangidae), collected from the northeastern part of peninsular Malaysia. During the examination of the organs, two distinct Myxobolus Bütschli, 1882 species were discovered in the brain tissue of these fishes, despite the absence of any observable pathological signs. The two Myxobolus species were characterized through morphometry, morphology, and analysis of partial small subunit ribosomal RNA (18S rDNA) gene. As a result, Myxobolus acanthogobii Hoshina, 1952, which infects 2.3% of N. furcosus, is synonymous with a myxobolid species commonly found in Japanese waters, based on its morphological traits, tissue tropism, and molecular diagnostics. Furthermore, a novel species, Myxobolus selarin. sp., was described, infecting the brain of one (11%) individual S. crumenophthalmus. This unique species displayed distinctive features, placing it within a well-supported subclade primarily comprising brain-infecting myxobolids. Maximum likelihood analysis further revealed the close relationships among these brain-infecting myxobolids, underscoring the significance of tissue tropism and host taxonomy for myxobolids. This study represents the initial documentation of Myxobolus species within the southern South China Sea, shedding light on the potential diversity of marine myxosporean in this region. This article was registered in the Official Register of Zoological Nomenclature (ZooBank) as urn:lsid:zoobank.org:pub:7C400E35-7CB8-4DEE-92B7-F75FF3926441.