During a 2-month survey in 2023 at Tasik Telabak, Terengganu, Malaysia three distinct actinospore types, namely raabeia, triactinomyxon and aurantiactinomyxon were identified in three invertebrate host species: Aulodrilus acutus, Branchiodrilus sp., and Bothrioneurum sp. utilizing morphometric and molecular analyses. Maximum likelihood of 18S rDNA positioned the raabeia type within the Myxobolus clade from fish of the Order Cypriniformes, suggesting a detected actinospore has a potential life cycle development in Cypriniformes and the genus Myxobolus. Both triactinomyxon and aurantiactinomyxon types were described solely based on morphology and morphometrics due to preservation error preventing the acquisition of 18S rDNA sequences. The triactinomyxon type in this study exhibited distinct morphology in spore shape and dimensions, characterized by a short style and caudal processes. Conversely, the aurantiactinomyxon type described herein possesses prominent elongated pyriform polar capsules not resembling any previously known aurantiactinomyxon types. These distinctive features, along with host species and geographical location justify their classification as novel types. Histological and microscopic analyses revealed the development of pansporocysts in the intestinal epithelium of the oligochaete host. This study marks the first descriptions of actinospore stages of myxozoans in Malaysia and the initial report of actinospores infecting host species of Aulodrilus acutus, Branchiodrilus sp. and Bothrioneurum sp.

European eel, Anguilla anguilla (Linnaeus) (Elopomorpha: Anguilliformes), is a critically endangered fish of ecological and economic importance, hosting numerous parasites, including myxozoans (Cnidaria). Since its initial discovery in the kidney of European eel, Myxidium giardi Cépède, 1906 has been reported with numerous spore sizes and shapes from various tissues of multiple anguillid species. Morphological variability, wide host and tissue spectrum, and lack of sequence data raised doubts about the conspecificity of reported isolates. Subsequent studies provided 18S rDNA sequences of several isolates from anguillids and other elopiform fish, and demonstrated a split of parasite data into two distinct phylogenetic lineages, one comprising the M. giardi sequence, and the other all species infecting elopiform fishes classified under the recently established genus Paramyxidium Freeman et Kristmundsson, 2018. Myxidium giardi was, however, transferred to this genus as Paramyxidium giardi n. comb. and designated as the type species of the genus. In line with this change, the sequence originally identified as M. giardi was considered to have been incorrectly associated with this species. To shed light on the status of M. giardi originally described by Cépède (1906), we conducted microscopic and molecular examinations of various organs of 24 individuals of European eel, originating from diverse Czech habitats. Through morphometric and molecular analyses, we demonstrated that spore and polar capsule morphology, morphometry and tissue tropism of our European eel kidney parasite isolates matched the features of the original M. giardi description. Our isolates clustered in the lineage encompassing the first published M. giardi sequence. Thus, the originally described M. giardi indeed represents an existing species within the genus Myxidium Bütschli, 1882, which we formally resurrect and redescribe. Due to the morphological and molecular differences between M. giardi and P. giardi of Freeman et Kristmundsson (2018), we additionally rename the latter species as Paramyxidium freemani nom. nov.

From genus Myxobolus, cnidarians of Myxozoa class, is well known for infecting economically important fish species and, as result, relevant losses in aquaculture production can be observed. They are present in a big range of fish in its natural habitat, including the migratory Brachyplatystoma rousseauxii catfish. This study aimed is to develop an integrative characterization of a new species of Myxobolus, located in B. rousseauxii\'s gills. To accomplish this, 30 specimens of B. rousseauxii catfish were collected from Mosqueiro Island in Pará, Brazil; necropsied and analyzed for morphology, histology and molecular characteristics. Cysts with conjunctival capsule development made up of fibroblasts were observed at the gill arches; such proliferation caused bone tissue loss and cartilage compression. The cysts contained Myxobolus myxospores measuring 9.9 μm of length and 9.6 μm of width, whereas polar capsules were 5.4 μm long and 3.4 μm wide, with 8 to 9 coils of polar tubules. Phylogenetic analyses revealed that new species were included in a subclade alongside species from the same geographic location and infection site that infect Siluriformes fish. Morphological and molecular differences revealed that Myxobolus spp. parasite-host associations through histopathology supporting the designation of a new M. rousseauxii n. sp. species in B. rousseauxii, a commercially important fish.

The mapará (Hypophthalmus marginatus) is a commercially important fish in the Brazilian Amazon and has been described as a host for numerous myxosporid species. The integrated taxonomy of a new species, Myxobolus mickeyii n. sp., discovered in the urinary bladder of H. marginatus, is undertaken in this study. In 105 specimens of H. marginatus, plasmodia and myxospores were observed in the urinary bladder fluid, the myxospores measuring 20.5 (19.6-21.3) μm in length and 14.0 (13.2-14.9) μm in width. The posterior valves of the spore body were thick, with valvulogenic nuclei, endoplasmic reticulum, and the presence of secretory vesicles. Two elliptical, rounded appendages attached to the valve, containing tubular filaments. The two polar capsules, symmetry, measuring 6.1 (5.9-6.3) μm in length and 4.4 (3.6-6.2) μm in width, with polar tubules of 3 to 5 turns. Phylogenetic analyses of the small subunit ribosomal RNA gene (SSU rDNA) sequencing revealed that M. mickeyii n. sp. is part of a Myxobolidae family clade with freshwater fish of the Siluriformes order, with a genetic distance of 19% to the nearest species. This work contributes to the wide diversity of myxozoans in this host, as other taxa have previously been reported infecting different tissues.

Infectious diseases stem from disrupted interactions among hosts, parasites, and the environment. Both abiotic and biotic factors can influence infection outcomes by shaping the abundance of a parasite\'s infective stages, as well as the host\'s ability to fight infection. However, disentangling these mechanisms within natural ecosystems remains challenging. Here, combining environmental DNA analysis and niche modelling at a regional scale, we uncovered the biotic and abiotic drivers of an infectious disease of salmonid fish, triggered by the parasite Tetracapsuloides bryosalmonae. We found that the occurrence and abundance of the parasite in the water-i.e., the propagule pressure- were mainly correlated to the abundances of its two primary hosts, the bryozoan Fredericella sultana and the fish Salmo trutta, but poorly to local abiotic environmental stressors. In contrast, the occurrence and abundance of parasites within fish hosts-i.e., proxies for disease emergence-were closely linked to environmental stressors (water temperature, agricultural activities, dams), and to a lesser extent to parasite propagule pressure. These results suggest that pathogen distribution alone cannot predict the risk of disease in wildlife, and that local anthropogenic stressors may play a pivotal role in disease emergence among wild host populations, likely by modulating the hosts\' immune response. Our study sheds light on the intricate interplay between biotic and abiotic factors in shaping pathogen distribution and raises concerns about the effects of global change on pathogen emergence.

Myxidium rhodei Léger, 1905 (Cnidaria: Myxozoa) is a kidney-infecting myxosporean that was originally described from the European bitterling Rhodeus amarus. Subsequently, it has been documented based on spore morphology in more than 40 other cypriniform species, with the roach Rutilus rutilus being the most commonly reported host. This study introduces the first comprehensive data assessment of M. rhodei, conducted through morphological, ecological and molecular methods. The morphological and phylogenetic analyses of SSU rDNA sequences of Myxidium isolates obtained from European bitterling and roach did not support parasite conspecificity from these fish. In fact, the roach-infecting isolates represent three distinct parasite species. The first two, M. rutili n. sp. and M. rutilusi n. sp., are closely related cryptic species clustering with other myxosporeans in the freshwater urinary clade, sharing the same tissue tropism. The third one, M. batuevae n. sp., previously assigned to M. cf. rhodei, clustered in the hepatic biliary clade sister to bitterling-infecting M. rhodei. Our examination of diverse cypriniform fishes, coupled with molecular and morphological analyses, allowed us to untangle the cryptic species nature of M. rhodei and discover the existence of novel species. This underscores the largely undiscovered range of myxozoan diversity and highlights the need to incorporate sequence data in diagnosing novel species.
UNASSIGNED: Résoudre le casse-tête de Myxidium rhodei (Myxozoa) : aperçu de sa phylogénie et de sa spécificité d’hôte chez les Cypriniformes.
UNASSIGNED: Myxidium rhodei Léger, 1905 (Cnidaria : Myxozoa) est un Myxosporea infectant les reins qui a été décrit à l’origine chez la bouvière, Rhodeus amarus. Par la suite, il a été documenté, sur la base de la morphologie des spores, chez plus de 40 autres espèces de cypriniformes, le gardon Rutilus rutilus étant l’hôte le plus fréquemment signalé. Cette étude présente la première évaluation complète des données sur M. rhodei, réalisée par des méthodes morphologiques, écologiques et moléculaires. Les analyse morphologiques et phylogénétiques des séquences d’ADNr SSU des isolats de Myxidium obtenus à partir de bouvières et de gardons européens n’ont pas confirmé la conspécificité du parasite de ces poissons. En fait, les isolats infectant les gardons représentent trois espèces distinctes de parasites. Les deux premières, M. rutili n. sp. et M. rutilusi n. sp., sont des espèces cryptiques étroitement apparentées, regroupées avec d’autres Myxosporea du clade urinaire d’eau douce, partageant le même tropisme tissulaire. La troisième, M. batuevae n. sp., précédemment attribuée à M. cf. rhodei, appartient au clade biliaire hépatique, groupe-frère de M. rhodei infectant la bouvière. Notre examen de divers poissons cypriniformes, couplé à des analyses moléculaires et morphologiques, nous a permis de démêler la nature cryptique des espèces de M. rhodei et de découvrir l’existence de nouvelles espèces. Cela souligne la diversité largement méconnue des Myxozoaires et souligne la nécessité d’incorporer des données de séquence dans le diagnostic de nouvelles espèces.

The coastal waters of Vietnam are home to a wide diversity of fishes, but the parasite diversity of these potential hosts is much less well characterized. To begin addressing this knowledge gap, we carried out surveys of myxozoan parasites in fishes collected from Nha Trang Bay in Vietnam\'s East Sea in 2018-2019. Mugilid fishes were collected in March-April 2018, January-February 2019, and November-December 2019, and examined for myxozoans. Myxospores consistent with those of the genus Ellipsomyxa were found in the gall bladder of four mullet species, and we thoroughly characterized those from Planiliza melinoptera. Myxospores were elliptoid and devoid of striation, with a distinct sinuous suture line. Polar capsules were pyriform and oriented toward the poles of the spore. Morphological features were compared to nominal species and this species from Vietnam was distinct. Phylogenetic analysis based on partial small subunit rDNA sequence revealed that broadly, Ellipsomyxa species split into three phylogenetic lineages, and although in some branches there are groupings by host family, habitat or locality, there are no clear phylogenetic patterns. The new species we encountered in P. melinoptera had a close sister relationship with Ellipsomyxa adlardi, with both species part of a larger subclade within the Ellipsomyxa lineage. Despite this phylogenetic similarity, these species were morphologically distinct, and partial large subunit DNA sequences were only 93% similar to each other. A combination of the morphological characteristics and molecular data suggest that this is an undescribed species and we propose the name Ellipsomyxa gordeyi n. sp.

Myxozoa, a unique group of obligate endoparasites within the phylum Cnidaria, can cause emerging diseases in wild and cultured fish populations. Recently, the myxozoan Myxobolus bejeranoi has been identified as a prevalent pathogen infecting the gills of cultured hybrid tilapia, leading to systemic immune suppression and considerable mortality. Here, we employed a proteomic approach to examine the impact of M. bejeranoi infection on fish gills, focusing on the structure of the granulomata, or cyst, formed around the proliferating parasite to prevent its spread to surrounding tissue. Enrichment analysis showed increased immune response and oxidative stress in infected gill tissue, most markedly in the cyst\'s wall. The intense immune reaction included a consortium of endopeptidase inhibitors, potentially combating the myxozoan arsenal of secreted proteases. Analysis of the cyst\'s proteome and histology staining indicated that keratin intermediate filaments contribute to its structural rigidity. Moreover, we uncovered skin-specific proteins, including a grainyhead-like transcription factor and a teleost-specific S100 calcium-binding protein that may play a role in epithelial morphogenesis and cysts formation. These findings deepen our understanding of the proteomic elements that grant the cyst its distinctive nature at the critical interface between the fish host and myxozoan parasite.

The thermotactic response of brown trout (Salmo trutta) was examined with the goal to investigate potential effects of the emerging temperature-dependent fatal trout disease PKD (proliferative kidney disease). First the differences in cold-water preferences of two forms of brown trout, lacustrine (migratory) and riverine, were determined. Second, it was studied whether this preference was changed in fish infected with PKD. The experiment involved a one-week habituation period at 14 °C in a two-chamber runway followed by a week of 3 °C temperature difference between the two runways. The fish could freely move between lanes via an opening at the end where food was provided. The temperature manipulation was repeated twice, and there were 3 trials per experimental group. All fish developed a clear spatial preference in the test. Lacustrine trout demonstrated a preference for warmer water, while riverine trout preferred cooler water. This may increase the risk to PKD in the lacustrine form. Most strikingly, riverine trout experimentally exposed to Tetracapsuloides bryosalmonae, the parasite that causes PKD, demonstrated stronger cold-seeking behaviour than control fish. Cold seeking behaviour suggests the occurrence of a disease-induced behavioural chill response, which may play an important role in disease recovery. This demonstrates the significance of protecting river connectivity and cold-water sanctuaries as management strategies for preserving salmonid populations in a warming climate.

Proliferative gill disease (PGD), caused by the myxozoan Henneguya ictaluri, has been the most notorious parasitic gill disease in the US catfish aquaculture industry. In 2019, an unusual gill disease caused by massive burdens of another myxozoan, Henneguya exilis, was described in channel (Ictalurus punctatus) × blue (Ictalurus furcatus) hybrid catfish. Targeted metagenomic sequencing and in situ hybridization (ISH) were used to differentiate these conditions by comparing myxozoan communities involved in lesion development and disease pathogenesis between massive H. exilis infections and PGD cases. Thirty ethanol-fixed gill holobranchs from 7 cases of massive H. exilis infection in hybrid catfish were subjected to targeted amplicon sequencing of the 18S rRNA gene and compared to a targeted metagenomic data set previously generated from clinical PGD case submissions. Furthermore, serial sections of 14 formalin-fixed gill holobranchs (2 per case) were analyzed by RNAscope duplex chromogenic ISH assays targeting 8 different myxozoan species. Targeted metagenomic and ISH data were concordant, indicating myxozoan community compositions significantly differ between PGD and massive branchial henneguyosis. Although PGD cases often consist of mixed species infections, massive branchial henneguyosis consisted of nearly pure H. exilis infections. Still, H. ictaluri was identified by ISH in association with infrequent PGD lesions, suggesting coinfections occur, and some cases of massive branchial henneguyosis may contain concurrent PGD lesions contributing to morbidity. These findings establish a case definition for a putative emerging, myxozoan-induced gill disease of farm-raised catfish with a proposed condition name of massive branchial henneguyosis of catfish (MBHC).