How to build single-celled cnidarians with worm-like motility: Lessons from Myxozoa

Metazoans with worm-like morphologies across diverse and disparate groups typically demonstrate motility generated by hydrostatic skeletons involving tissue layers (muscles and epithelia). Here we present representative morphological, behavioural and molecular data for parasitic cnidarians (myxozoans) that demonstrate unprecedented variation in form and function, developing as cellular hydrostats. Motile elongate plasmodia characterise a remarkable radiation of species in the genus Ceratomyxa. The vermiform plasmodia inhabit gall bladders of a range of South American freshwater fish and exhibit undulatory motility reminiscent of nematodes but achieved at the cellular level. Collective insights from ultrastructure, confocal and light microscopy along with videos depicting movements highlight key features that we propose explain the unique motility of the plasmodia. These features include cytoskeletal elements (net forming microfilaments and microtubules), a large internal vacuole, a relatively rigid outer glycocalyx and peripherally arranged mitochondria. These constituents provide collective evidence for repurposing of the cnidarian epitheliomuscular cell to support worm-like motility at the cellular level. The apparent restriction of vermiform ceratomyxids to South American freshwaters suggests an origination via Cretaceous or Miocene marine transgressions and subsequent radiation.

Infection dynamics of Enteromyxum leei (Myxozoa, Myxosporea) in culture water and its effects on cultured olive flounder, Paralichthys olivaceus (Temminck & Schlegel)

Enteromyxum leei is a causative agent of enteromyxosis, with a wide range of marine fish hosts. Recently, massive morbidity and mortality were caused by E. leei infection in cultured olive flounders in Korea. To reveal a relationship between E. leei abundance in culture water and the occurrence of parasite infection in host fish, we used a quantitative PCR assay targeting the 28S rDNA of E. leei in three fish farms (two where enteromyxosis had occurred and one where it did not) from April to November 2018. The gene of E. leei was detected at levels greater than 10 cells/L in the culture water where enteromyxosis occurred from July to September. Furthermore, 2 months after the detection in the water, the parasite gene (with more than 5,000 cells per 100 mg) was detected in fish intestine samples. However, in the fish farms where enteromyxosis had not occurred, the E. leei gene was detected at <10 cells in culture water (1 L) and fish intestine samples (100 mg). The quantification method used in this research provides a baseline of the infection timeline in olive flounder to develop effective management practices.

Effect of water temperature on the morbidity of Tetracapsuloides bryosalmonae (Myxozoa) to brown trout (Salmo trutta) under laboratory conditions

Proliferative kidney disease (PKD) is a disease found in salmonid fish that is widespread in Europe and North America. The dependency of the clinical signs on the water temperature is extensively reported in rainbow trout, but detailed information on brown trout is lacking. In this study, juvenile brown trout were exposed to the spores of Tetracapsuloides bryosalmonae and then kept at different ambient water temperatures (16°C, 19°C and 22°C) for 10 weeks along with recording of morbidity throughout the experiment. At 6, 8 and 10 weeks post-exposure, fish from each temperature group were sampled and underwent pathoanatomical examination to survey disease progression. At 16°C, brown trout showed a significantly higher survival probability compared to those kept in 19°C and 22°C water. Additionally, the parasitic burden (MSQ) was higher and the clinical signs were more pronounced in the brown trout kept at 19°C and 22°C compared with the ones kept at 16°C. This study highlights the correlation of PKD outbreaks and water temperature increases related to global climate change, which will impact the future distribution of brown trout in natural waters.

Climate change-driven disease in sympatric hosts: Temporal dynamics of parasite burden and proliferative kidney disease in wild brown trout and Atlantic salmon

Global climate change is altering the abundance and spread of various parasites, which has important consequences not only for host-parasite interactions but also for the relationships between different host species. Here, we focus on the myxozoan endoparasite Tetracapsuloides bryosalmonae that causes temperature-dependent proliferative kidney disease (PKD) in salmonids. We characterized the temporal changes in the parasite load and the severity of PKD signs (renal hyperplasia, haematocrit) in two sympatric populations of wild brown trout (Salmo trutta) and Atlantic salmon (Salmo salar). We found that both the parasite load and disease signs vary considerably between individuals, species, rivers and sampling periods. We showed that Atlantic salmon was able to slow down the initial parasite proliferation rate and subsequently tolerate high parasite burden without obvious disease signs. In contrast, the initial parasite proliferation rate was much higher in brown trout, which was followed by the development of severe PKD signs. Thus, the speed of parasite proliferation, rather than the absolute number of the parasites in the host kidney, may play an important role in interspecific variation in PKD susceptibility. To conclude, this study illustrates the usefulness of temporal perspective for understanding host defence mechanisms and climate change-mediated impacts in the wild.

Ellipsomyxa tucujuensis n. sp. (Myxozoa: Ceratomyxidae), a parasite of Satanoperca jurupari (Osteichthyes: Cichlidae) from the Brazilian Amazon

The present study describes a new coelozoic, eukaryotic microparasite of the genus Ellipsomyxa Køie, 2003 (Ceratomyxidae: Myxozoa) found parasitizing the gallbladder of Satanoperca jurupari Heckel, 1840 collected in the Curiaú River Environmental Protection Area in Macapá, Amapá state, Brazil. The fish were collected using mesh cast net. The gallbladders were examined, preserved in 80% alcohol for molecular analysis (SSU rDNA gene), and fixed in Davidson for histological slide preparation. The new parasite had a prevalence of 81% in the gallbladder, asymmetric plasmodia, irregular free spores in the bladder fluid, with no cyst formation. The spores are elliptical, with characteristics of the genus Ellipsomyxa, and they had a mean length of 10.11 (8.56-10.5) μm, mean width of 7.81 (5.96-9.56) μm, and thick walls. The polar capsules are sub-spherical in shape, slightly asymmetrical, with a mean length of 3.12 (2.31-3.99) μm and mean width of 2.5 (2.22-2.95) μm, containing polar filament with five or six coils perpendicular to the longitudinal axis of the capsule. The Bayesian Inference assigned the new species to a subclade formed by a lineage of Ellipsomyxa species from the Amazon region. Ellipsomyxa tucujuensis n. sp. is the sixth species of this genus described in fish from the Amazon region, and the first for the state of Amapá.

A portrait of the immune response to proliferative kidney disease (PKD) in rainbow trout

Proliferative kidney disease (PKD), caused by the myxozoan Tetracapsuloides bryosalmonae, is one of the most serious parasitic diseases of salmonids in which outbreaks cause severe economic constraints for the aquaculture industry and declines of wild species throughout Europe and North America. Given that rainbow trout (Oncorhynchus mykiss) is one of the most widely farmed freshwater fish and an important model species for fish immunology, most of the knowledge on how the fish immune response is affected during PKD is from this organism. Once rainbow trout are infected, PKD pathogenesis results in a chronic kidney immunopathology mediated by decreasing myeloid cells and increasing lymphocytes. Transcriptional studies have revealed the regulation of essential genes related to T-helper (Th)-like functions and a dysregulated B-cell antibody type response. Recent reports have discovered unique details of teleost B-cell differentiation and functionality and characterized the differential immunoglobulin (Ig)-mediated response. These studies have solidified the rainbow trout T. bryosalmonae system as a sophisticated disease model capable of feeding key advances into mainstream immunology and have contributed essential information to design novel parasite disease prevention strategies. In our following perspective, we summarize these efforts to evaluate the immune mechanisms of rainbow trout during PKD pathogenesis.

Insights into myxozoan composition and physiology revealed by histochemical properties of myxospores

Myxozoa (phylum Cnidaria) are a diverse group of metazoan parasites that predominately infect fish. Little is known regarding the composition and physiology of their myxospore life stage. The objective of this work was to investigate the composition of myxospores and extrasporogonic stages of nine myxozoan species infecting various teleost fish using histochemical staining techniques. Thirty histochemical stains were applied to formalin-fixed, paraffin-embedded tissues processed routinely for light microscopic evaluation. The polar capsules were the most consistent stain target across the taxa examined. Polar capsule staining with Alizarin red, von Kossa and methyl green-pyronin suggests the presence of intracapsular calcium and phosphate, which may contribute to polar filament discharge or pathogenesis of host invasion. The shell valves and suture lines of most myxozoans were stained with Luna and phosphotungstic acid haematoxylin stains, consistent with the presence of chitin and microfibrils, respectively. Vacuoles were consistently highlighted by diastase-susceptible periodic acid-Schiff and Grocott's methenamine silver staining, indicating glycogen. Other histochemical stains exhibited inconsistent staining across the taxa, suggesting differences in myxospore composition potentially reflective of physiologic variations and tissue tropisms. This work provides some information on conserved features and taxa-associated composition of myxospores and lends insight into myxozoan physiology and host-parasite interactions.

Water temperature, time of exposure and population density are key parameters in Enteromyxum leei fish-to-fish experimental transmission

Enteromyxum leei is a myxozoan histozoic parasite that infects the intestine of several teleost fish species. In gilthead sea bream (Sparus aurata), it provokes a chronic disease, entailing anorexia, delayed growth, reduced marketability and mortality. Direct fish-to-fish transmission, relevant in aquaculture conditions, has been demonstrated for E. leei via effluent, cohabitation, and oral and anal routes. However, the minimum time of exposure for infection has not been established, nor the possible effect on the fish immune response. Two effluent trials were performed at different temperatures (high: average of 25.6°C; and low: constant at 18°C), different times of exposure to the effluent (1, 3, 5 and 7 weeks) and different population densities. The results showed that 1 week was enough to infect 100% of fish at high temperature and 58.3% at low temperature. High temperature not only increased the prevalence of infection in posterior intestine, but also induced a higher production of specific antibodies, limiting the progression of the infection along the intestine. Longer time of exposure to the parasite and higher fish densities facilitated E. leei infection. These results show that effective diagnosis, lowering animal density and removal of infected fish are key aspects to manage this disease in aquaculture facilities.

Effects of Enteromyxum spp. (Myxozoa) infection in the regulation of intestinal E-cadherin: Turbot against gilthead sea bream

Enteromyxoses are relevant diseases for turbot and gilthead sea bream aquaculture. The myxozoan parasites invade the intestinal mucosa, causing a cachectic syndrome associated with intestinal barrier alteration; nonetheless, their pathological impact is different. Turbot infected by Enteromyxum scophthalmi develop more severe intestinal lesions, reaching mortality rates of 100%, whereas in E. leei-infected gilthead sea bream, the disease progresses slowly, and mortality rates are lower. The mechanisms underlying the different pathogenesis are still unclear. We studied the distribution and expression changes of E-cadherin, a highly conserved protein of the adherens junctions, in the intestine of both species by immunohistochemistry and quantitative PCR, using the same immunohistochemical protocol and common primers. The regular immunostaining pattern observed in control fish turned into markedly irregular in parasitized turbot, showing an intense immunoreaction at the host-parasite interface. Nevertheless, E-cadherin gene expression was not significantly modulated in this species. On the contrary, no evident changes in the protein distribution were noticed in gilthead sea bream, whereas a significant gene downregulation occurred in advanced infection. The results contribute to the understanding of the different host-parasite interactions in enteromyxoses. Host and parasite cells appear to establish diverse relationships in these species, which could underlie the different pathological picture.

Coelozoic parasite of the family Ceratomyxidae (Myxozoa, Bivalvulida) described from motile vermiform plasmodia found in Hemiodus unimaculatus Bloch, 1794

Myxozoans of the family Ceratomyxidae are common coelozoic parasites of marine, anadromous, and freshwater fish, and may also be found, less frequently, parasitizing the tissue of these hosts. The diversity and ecology of the freshwater species of the genus Ceratomyxa have been poorly investigated, leading to a knowledge gap that restricts the understanding of the distribution and prevalence of this group of parasites. In the present study, parasites were found inside vermiform plasmodia, characterised by oscillatory movements in the characiform species Hemiodus unimaculatus. The crescent-shaped and elongated spores, perpendicular to the suture line, have a mean length of 28.9 ± 2.7 μm and width of 2.6 ± 0.1 μm, with two symmetrical oval polar capsules, 1.9 ± 0.3 μm in length and 1.7 ± 0.2 μm in width, containing polar filaments with three or four coils, located near the central suture, with symmetrical lateral elongations 14.3 ± 1.1 μm in length and binucleate amoeboid sporoplasm. The integrated comparative analysis of the morphological characteristics and partial SSU rRNA sequences supported the identification of a new species of coelozoic Ceratomyxa, found in the gallbladder of H. unimaculatus, from the Tocantins basin, in the municipalities of Estreito and Imperatriz in eastern Brazilian Amazonia.The new species was denominated Ceratomyxa fonsecai n. sp.

Transcriptomic analysis of the impacts of ethinylestradiol (EE2) and its consequences for proliferative kidney disease outcome in rainbow trout (Oncorhynchus mykiss)

Freshwater fish are threatened by the cumulative impact of multiple stressors. The purpose of this study was to unravel the molecular and organism level reactions of rainbow trout, Oncorhynchus mykiss, to the combined impact of two such stressors that occur in the natural habitat of salmonids. Fish were infected with either the myxozoan parasite, Tetracapsuloides bryosalmonae, which causes proliferative kidney disease (PKD), or exposed to ethinylestradiol (EE2) an estrogenic endocrine disrupting compound, or to a combination of both (PKD × EE2). PKD is a slow progressive chronic disease here we focused on a later time point (130-day post-infection (d.p.i.)) when parasite intensity in the fish kidney has already started to decrease. At 130 d.p.i., RNA-seq technology was applied to the posterior kidney, the main target organ for parasite development. This resulted with 280 (PKD), 14 (EE2) and 444 (PKD × EE2) differentially expressed genes (DEGs) observed in the experimental groups. In fish exposed to the combination of stressors (PKD × EE2), a number of pathways were regulated that were neither observed in the single stressor groups. Parasite infection, alone and in combination with EE2, only resulted in a low intensity immune response that negatively correlated with an upregulation of genes involved in a variety of metabolic and inflammation resolution processes. This could indicate a trade-off whereby the host increases investment in recovery/resolution processes over immune responses at a later stage of disease. When PKD infection took place under simultaneous exposure to EE2 (PKD × EE2), parasite intensity decreased and pathological alterations in the posterior kidney were reduced in comparison to the PKD only condition. These findings suggest that EE2 modulated these response profiles in PKD infected fish, attenuating the disease impact on the fish.

Acquired protective immune response in a fish-myxozoan model encompasses specific antibodies and inflammation resolution

The myxozoan parasite Enteromyxum leei causes chronic enteritis in gilthead sea bream (GSB, Sparus aurata) leading to intestinal dysfunction. Two trials were performed in which GSB that had survived a previous infection with E. leei (SUR), and naïve GSB (NAI), were exposed to water effluent containing parasite stages. Humoral factors (total IgM and IgT, specific anti-E. leei IgM, total serum peroxidases), histopathology and gene expression were analysed. Results showed that SUR maintained high levels of specific anti-E. leei IgM (up to 16 months), expressed high levels of immunoglobulins at the intestinal mucosa, particularly the soluble forms, and were resistant to re-infection. Their acquired-type response was complemented by other immune effectors locally and systemically, like cell cytotoxicity (high granzyme A expression), complement activity (high c3 and fucolectin expression), and serum peroxidases. In contrast to NAI, SUR displayed a post-inflammatory phenotype in the intestine and head kidney, characteristic of inflammation resolution (low il1β, high il10 and low hsp90α expression).

Extensive Uncharted Biodiversity: The Parasite Dimension

Parasites are often hidden in their hosts and exhibit patchy spatial distributions. This makes them relatively difficult to detect and sample. Consequently we have poor knowledge of parasite diversities, distributions, and extinction. We evaluate our general understanding of parasite diversity and highlight the enormous bias in research on parasites such as helminths and arthropods that infect vertebrate hosts. We then focus on Myxozoa as an exemplary case for demonstrating uncharted parasite diversity. Myxozoans are a poorly recognized but speciose clade of endoparasitic cnidarians with complex life cycles that have radiated to exploit freshwater, marine, and terrestrial hosts by adopting strategies convergent to those of parasitic protists. Myxozoans are estimated to represent some 20% of described cnidarian species-greatly outnumbering the combined species richness of scyphozoans, cubozoans, and staurozoans. We summarize limited understanding of myxozoan diversification and geographical distributions, and highlight gaps in knowledge and approaches for measuring myxozoan diversity. We close by reviewing methods and problems in estimating parasite extinction and concerns about extinction risks in view of the fundamental roles parasites play in ecosystem dynamics and in driving host evolutionary trajectories.

Quantitative investigation of Enteromyxum leei (Myxozoa: Myxosporea) infection and relative condition factor in cultured olive flounder Paralichthys olivaceus (Temminck and Schlegel)

Enteromyxum leei has been reported to cause emaciation disease in various fish species. To determine the effect of parasite intensity on cultured olive flounder Paralichthys olivaceus, we investigated the relationship between the relative condition factor (rCF = CF/standard CF × 100) and parasite load with quantitative polymerase chain reaction (qPCR) and the challenge test. A total of 57 cultured olive flounders were obtained from 11 fish farms and divided into five groups based on their rCF. We investigated the parasite intensity in the posterior intestine of the fish. The parasite load was closely matched to severe loss of body weight. In addition, olive flounders were inoculated either orally or anally with intestinal scrapings of infected fish or phosphate-buffered saline. The fish were reared at natural water temperature and transferred to different tanks, and the water temperature was adjusted to 20°C after 6 weeks of inoculation. When the water temperature was increased to 20°C, the rCF decreased in the experimentally infected group. The results demonstrated that qPCR can be utilized to determine the relative abundance of E. leei in olive flounders and water temperature is an important factor to track the progress of the emaciation disease.

Exploring the immune response, tolerance and resistance in proliferative kidney disease of salmonids

Proliferative kidney disease (PKD) of salmonids is a disease of economic and environmental concern caused by the myxozoan parasite Tetracapsuloides bryosalmonae. Finer details of the immune repertoire during T. bryosalmonae infection have been elucidated in rainbow trout (Oncorhynchus mykiss). In contrast, there remain many unanswered questions regarding the immune response of the wild fish host in Europe, the brown trout (Salmo trutta) to this parasite. The first aim of this study is to examine the brown trout immune response to T. bryosalmonae and compare it with the published information on rainbow trout as two species that have undergone a different coevolution with the parasite. According to ecoimmunology terminology, infected organisms may manage infection by reducing the damage caused by parasites (tolerance) or by limiting parasite burden (resistance). The second aim of this study is to investigate tolerance/resistance patterns of these species during PKD infection. Our results suggest subtle differences in sequential aspects of the immune response and of immune genes that correlate with parasite intensity for the brown trout, in contrast to rainbow trout, in terms of the B cell response and Th-like interplay that may be linked to PKD pathogenesis. These differences in the immune response also correlate with species-specific differences in tolerance/resistance patterns, in that brown trout had increased tolerance but rainbow trout had greater resistance to infection. The variance in tolerance/resistance investment resulted in a different evolutionary outcome for each host-parasite interaction. A greater exploration of these concepts and an association of immune mechanisms could open an additional gateway for interpreting fish host-parasite interactions.

Biodiversity and host-parasite cophylogeny of Sphaerospora (sensu stricto) (Cnidaria: Myxozoa)

BACKGROUND

Myxozoa are extremely diverse microscopic parasites belonging to the Cnidaria. Their life-cycles alternate between vertebrate and invertebrate hosts, predominantly in aquatic habitats. Members of the phylogenetically well-defined Sphaerospora (sensu stricto) clade predominantly infect the urinary system of marine and freshwater fishes and amphibians. Sphaerosporids are extraordinary due to their extremely long and unique insertions in the variable regions of their 18S and 28S rDNA genes and due to the formation of motile proliferative stages in the hosts' blood. To date, DNA sequences of only 19 species have been obtained and information on the patterns responsible for their phylogenetic clustering is limited.

METHODS

We screened 549 fish kidney samples from fish of various geographical locations, mainly in central Europe, to investigate sphaerosporid biodiversity microscopically and by 18S rDNA sequences. We performed multiple phylogenetic analyses to explore phylogenetic relationships and evolutionary trends within the Sphaerospora (s.s.) clade, by matching host and habitat features to the resultant 18S rDNA trees. The apparent co-clustering of species from related fish hosts inspired us to further investigate host-parasite co-diversification, using tree-based (CoRE-PA) and distance-based (ParaFit) methods.

RESULTS

Our study considerably increased the number of 18S rDNA sequence data for Sphaerospora (s.s.) by sequencing 17 new taxa. Eight new species are described and one species (Sphaerospora diminuta Li & Desser, 1985) is redescribed, accompanied by sufficient morphological data. Phylogenetic analyses showed that sphaerosporids cluster according to their vertebrate host order and habitat, but not according to geography. Cophylogenetic analyses revealed a significant congruence between the phylogenetic trees of sphaerosporids and of their vertebrate hosts and identified Cypriniformes as a host group of multiple parasite lineages and with high parasite diversity.

CONCLUSIONS

This study significantly contributed to our knowledge of the biodiversity and evolutionary history of the members of the Sphaerospora (s.s.) clade. The presence of two separate phylogenetic lineages likely indicates independent historical host entries, and the remarkable overlap of the larger clade with vertebrate phylogeny suggests important coevolutionary adaptations. Hyperdiversification of sphaerosporids in cypriniform hosts, which have undergone considerable radiations themselves, points to host-driven diversification.

Bromodeoxyuridine DNA labelling reveals host and parasite proliferation in a fish-myxozoan model

Enteromyxum leei is a myxozoan parasite responsible for enteritis in gilthead sea bream (Sparus aurata). The parasite proliferates in the paracellular space of the intestinal epithelium and induces an inflammatory reaction. To assess intestinal cell turnover and parasite proliferation, fish were infected with the parasite by anal intubation; after 17 and 64 days, the cell proliferative marker bromodeoxyuridine (BrdU) was administered; and after 24 hr, tissue samples were taken for immunohistochemical detection. Parasite exposure induced increased epithelial and immune cell proliferation in all intestinal segments at all time points, even before parasite establishment. This increased turnover was triggered early after intubation and mainly at a local level, as shown by an increased proliferating cell nuclear antigen (pcna) gene expression only at the posterior intestine after 17 days (not found in lymphohaematopoietic organs). Incorporation of BrdU in parasite secondary and tertiary daughter cells indicated that parasite endogeny is not by schizogonial division, which uses de novo synthesis pathway of pyrimidines. Altogether, BrdU immunolabelling and pcna gene expression showed the rapid proliferative response of the fish intestines upon a myxozoan infection and how this response is effectively triggered even before the parasite reaches or establishes in the site.

Outbreak of mass mortality of yearling groupers of Epinephelus (Perciformes, Serranidae) associated with the infection of a suspected new enteric Sphaerospora (Myxozoa: Myxosporea) species in South China Sea

A suspected new enteric Sphaerospora species was believed to be directly associated with the mass mortality of yearling groupers of Epinephelus spp. in South China. The epizootic generally emerged from late September to late April of the following year. The infection prevalence and mortality rate were significantly negatively correlated with fish size. Clinical signs included anorexia, cachexia and extrusion of white pulp-like substance from anus after gentle pressure on the abdomen. Upon necropsy, severe intestinal oedema, thin and transparent intestinal wall, swollen spleen, kidney and gall bladder could be observed. Wet preparation of the infected samples showed large amount of typical disporous plasmodia of the genus Sphaerospora, but no mature spores were observed. Epidemiological investigation showed that this parasite exclusively infected Epinephelus groupers. Histopathologically, this species mainly infected the epithelium of intestine and kidney tubules and caused severe epithelia sloughing and the collapse of intestinal villus. Interestingly, this enteric myxosporidiosis did not cause severe emaciation of infected fish for mass mortality usually emerged within 2-3 days after appearance of clinical signs. The species was most genetically related to Sphaerospora fugu (89% sequence identity) and phylogenetically positioned within marine Sphaerospora lineage. This is the first report of enteric sphaerosporosis of groupers.

Are brown trout Salmo trutta fario and rainbow trout Oncorhynchus mykiss two of a kind? A comparative study of salmonids to temperature-influenced Tetracapsuloides bryosalmonae infection

Proliferative kidney disease (PKD) of salmonids caused by Tetracapsuloides bryosalmonae causes high mortalities of wild brown trout (Salmo trutta fario) and farmed rainbow trout (Oncorhynchus mykiss) at elevated water temperatures. Here the aim was to compare the temperature-dependent modulation of T. bryosalmonae in the two salmonid host species, which display different temperature optima. We used a novel experimental set-up in which we exposed brown trout and rainbow trout to an identical quantified low concentration of T. bryosalmonae for a short time period (1 hr). We followed the development of the parasite in the fish hosts for 70 days. PKD prevalence and parasite kinetics were assessed using qPCR. Exposures were performed at temperatures (12°C and 15°C) that reflect an environmental scenario that may occur in the natural habitat of salmonids. T. bryosalmonae infection was confirmed earliest in brown trout kept at 15°C (day 7 post-exposure) while, in all other groups, T. bryosalmonae was not confirmed until day 15 post-exposure. Moreover, significantly greater infection prevalence and a faster increase of parasite intensity were observed in brown trout kept at 15°C than in all other groups. These results indicate that PKD is differentially modulated by water temperature in related host species.

Tetracapsuloides bryosalmonae persists in brown trout Salmo trutta for five years post exposure

Tetracapsuloides bryosalmonae is a malacosporean parasite and the causative agent of proliferative kidney disease (PKD) that seriously impacts farmed and wild salmonids. The parasite's life cycle includes an invertebrate host, the bryozoan Fredericella sultana, and a vertebrate host, salmonid fish. The persistence of T. bryosalmonae in brown trout Salmo trutta for up to 2 yr following exposure is well documented. Results from the present study confirmed that one brown trout that had recovered from PKD did not completely clear the parasite from its tissues and that T. bryosalmonae could persist in brown trout for up to 5 yr post exposure. Furthermore, recovered infected brown trout can release viable T. bryosalmonae spores that are able to infect specific pathogen-free F. sultana colonies. T. bryosalmonae DNA was detected by PCR in every organ, and parasite stages were observed in the kidney, spleen and liver following immunohistochemistry. This finding indicates that T. bryosalmonae-infected brown trout can act as asymptomatic carriers and release the parasite for several years after the initial infection, acting as a reservoir of infection, and contributing to the dissemination of the parasite to new areas.

What goes around comes around: an investigation of resistance to proliferative kidney disease in rainbow trout Oncorhynchus mykiss (Walbaum) following experimental re-exposure

Rainbow trout Oncorhynchus mykiss surviving proliferative kidney disease (PKD) are reported not to develop the disease upon re-exposure. However, the mechanisms involved in the immune response to re-exposure are unknown. We examined disease susceptibility and the immune response of naive 1+ rainbow trout when first exposed to Tetracapsuloides bryosalmonae in comparison with that of 1+ rainbow trout re-exposed to T. bryosalmonae. PKD pathogenesis, parasite burden and transcriptional signatures of the host immune response were assessed at 10, 25 and 40 d.p.e (days post-exposure). In addition, we evaluated the presence of IgM+ B cells in the blood and the posterior kidney. The exposure of 1+ rainbow trout to T. bryosalmonae for the first time resulted in 100% infection prevalence, high parasite burdens and severe clinical PKD, while re-exposed fish were either able to avoid reinfection completely or mount an earlier and more efficient adaptive-type immune response. This response was characterized by a greater amount of IgM+ B cells in the blood and elevated mRNA levels of secretory IgM in the posterior kidney which minimized pathogen burden and kidney inflammation. Our findings suggest that rainbow trout is able to develop immune protection against T. bryosalmonae.

New host record and molecular characterization of Dicauda atherinoidi Hoffman & Walker (Bivalvulida: Myxobolidae): a parasite of the emerald shiner Notropis atherinoides Rafinesque, 1818 and mimic shiner Notropis vollucellus Cope, 1865

Updated morphological and histopathological descriptions for Dicauda atherinoidi (Bivalvulida:Myxobolidae) and an expanded host range are supplemented with the first molecular data and phylogenetic analyses of the genus. Plasmodia were located on the head, ventrum/body and fins of infected emerald shiner Notropis atherinoides Rafinesque, 1818 and mimic shiner Notropis vollucellus Cope, 1865, a new host species. Myxospores were spherical, ranging 9.3-11.4 μm (10.5 ± 0.4) in length, 9.0-11.0 μm (9.7 ± 0.4) in width and 6.6-7.0 μm (6.8 ± 0.2) thick in sutural view, and possessed 2-3 caudal processes (5.3-68.3 μm, 31.1 ± 13.6) connected to the spore body at the sutural groove, all of which are consistent with the genus Dicauda. In the absence of available Dicauda sequence data, the 18S rDNA sequences from Michigan isolates were most similar to Myxobolus spp. Phylogenetic analyses clustered these isolates with myxobolid species from cyprinid fish, suggesting these parasites may represent an underpopulated group of cyprinid-infecting myxozoans. Histopathology revealed thin-walled plasmodial pseudocysts in the dermis and associated connective tissue, where granulomatous inflammation and focal scale atrophy were also present. Further sampling/sequencing of myxobolids from Notropis spp. should expand these underrepresented myxozoans and offer further insight into Myxobolidae host family tropisms.

Immune response of olive flounder (Paralichthys olivaceus) infected with the myxosporean parasite Kudoa septempunctata

This study evaluated the pathophysiological, biochemical, and immunological status of olive flounder (Paralichthys olivaceus) infected with the myxosporean parasite Kudoa septempunctata. Flounder fish collected from Kudoa-infected and uninfected farms were confirmed by microscopic and TaqMan probe-based quantitative PCR screening. Morphological, biochemical, histological, and immune gene expression analyses were performed on uninfected and infected hosts to assess the effect of K. septempunctata. Histological studies confirmed the presence of Kudoa myxospores in the trunk muscles of infected flounder fish. Serum biochemical parameters, including the levels of myeloperoxidase activity, superoxide dismutase activity, alanine aminotransferase, alkaline phosphatase, amylase, bilirubin, total protein, cholesterol, calcium, potassium, sodium, phosphorus, glucose, and galactose, were found to exhibit no significant variations (p > 0.05) between uninfected and infected flounder fish. However, immune-related genes such as Mx, lysozyme, signal transducer and activator of transcription 1, interferon-γ, interferon regulatory factor, and tumour necrosis factor showed significantly elevated expression (p < 0.05) in the trunk muscles of infected flounder fish while no significant differences were noted in uninfected fish trunk muscle and head-kidney of infected and uninfected flounder fish.

Tetracapsuloides bryosalmonae abundance in river water

Tetracapsuloides bryosalmonae is a myxozoan parasite of freshwater bryozoans and salmonids, causing proliferative kidney disease in the latter. To date, detection of the parasite has required collection of hosts and subsequent molecular or histological examination. The release of infectious spores from both hosts offers an opportunity to detect the parasite in water samples. We developed a novel SYBR® Green quantitative real-time PCR (qPCR) assay for T. bryosalmonae in water samples which provides an estimation of bryozoan malacospore numbers and tested the assay in 3 rivers in southern England (UK) over a period of 5 wk. The assay proved to be both highly sensitive and specific to the parasite, detecting low levels of spores throughout the study period. Larger-volume samples afforded greater detection likelihood, but did not increase the number of spores detected, possibly as a result of low and patchy spore distributions and lack of within-site replication of large-volume samples. Based on point-measurements, temperature was positively associated with the likelihood of detecting spores, possibly reflecting the temperature dependence of spore shedding from bryozoan hosts. The presence of T. bryosalmonae in water samples was predominantly influenced by spatial (sites within rivers, amongst rivers) and temporal (sampling dates) factors, while the latter also influenced quantification cycle (Cq) values and spore abundance. Environmental monitoring for infectious stages can complement traditional methods, providing faster and easier detection and avoiding potentially prolonged searching, collecting and destructive sampling of invertebrate and vertebrate hosts.

Structural integrity and viability of Fredericella sultana statoblasts infected with Tetracapsuloides bryosalmonae (Myxozoa) under diverse treatment conditions

Fredericella sultana is an invertebrate host of Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease in salmonids. The bryozoan produces seed-like statoblasts to facilitate its persistence during unfavourable conditions. Statoblasts from infected bryozoans can harbor T. bryosalmonae and give rise to infected bryozoan colonies when conditions improve. We aimed in the present study to evaluate the integrity and viability of T. bryosalmonae-infected statoblasts after a range of harsh treatment conditions. We tested if statoblasts could survive ingestion by either brown trout or common carp. After ingestion, the fish faeces was collected at different time points. We also tested physical stressors: statoblasts collected from infected colonies were desiccated at room temperature, or frozen with and without Bryozoan Medium C (BMC). After treatments, statoblasts were assessed for physical integrity before being incubated on BMC to allow them to hatch. After 4 weeks, hatched and unhatched statoblasts were tested by PCR for the presence of the parasite. We found that statoblasts ingested by brown trout and those frozen in BMC were completely broken. In contrast, statoblasts ingested by common carp and those subjected to dry freezing were able to survive and hatch. T. bryosalmonae was detected by PCR in both hatched and unhatched infected statoblasts, but neither from broken nor uninfected statoblasts. Our results confirmed for the first time the ability of infected statoblasts to survive passage through a fish, and freezing. These findings suggest potential pathways for both persistence and spread of T. bryosalmonae-infected statoblasts in natural aquatic systems.

Who needs the hotspot? The effect of temperature on the fish host immune response to Tetracapsuloides bryosalmonae the causative agent of proliferative kidney disease

Proliferative kidney disease (PKD) of salmonids, caused by Tetracapsuloides bryosalmonae may lead to high mortalities at elevated water temperatures. However, it has not yet been investigated how temperature affects the fish host immune response to T. bryosalmonae. We exposed YOY (young of the year) rainbow trout (Oncorhynchus mykiss) to T. bryosalmonae at two temperatures (12 °C and 15 °C) that reflect a realistic environmental scenario and could occur in the natural habitat of salmonids. We followed the development of the parasite, host pathology and immune response over seven weeks. We evaluated the composition and kinetics of the leukocytes and their major subgroups in the anterior and posterior kidney. We measured immune gene expression profiles associated with cell lineages and functional pathways in the anterior and posterior kidney. At 12 °C, both infection prevalence and pathogen load were markedly lower. While the immune response was characterized by subtle changes, mainly an increased amount of lymphocytes present in the kidney, elevated expression of Th1-like signature cytokines and strong upregulation of the natural killer cell enhancement factor, NKEF at week 6 P.E. At 15 °C the infection prevalence and pathogen burden were ominously greater. While the immune response as the disease progressed was associated with a Th2-like switch at week 6 P.E and a prominent B cell response, evidenced at the tissue, cell and transcript level. Our results highlight how a subtle, environmentally relevant difference in temperature resulted in diverse outcomes in terms of the immune response strategy, altering the type of interaction between a host and a parasite.

[Henneguya wolinensis (Myxosporea: Myxobolidae), a new for Russian fauna parasite from the perch Perca fluviatilis L.]

The infection of the perch Perea fluviatilis L. with myxosporean Henneguya wolinensis Romuk-Wodoracki, 1990 has been detected. This is the second finding of this parasite after its original descriptin and the first for Russia. Plasmodium of this species develops in the epidermis under scales throughout the body causing the formation of white cysts up to 1 mm. Spores are fusiform, large, their average length constitutes 25.5 μm without the caudal appendages and 62 μm with them. Slight morphological differences in spore structure comparing to original description have been revealed.

Life cycle inference and phylogeny of Ortholinea labracis n. sp. (Myxosporea: Ortholineidae), a parasite of the European seabass Dicentrarchus labrax (Teleostei: Moronidae), in a Portuguese fish farm

Ortholinea labracis n. sp. is described and its life cycle is inferred from a Southern Portuguese fish farm, with basis on microscopic and molecular procedures. This myxosporean parasite infects the urinary bladder of the European seabass Dicentrarchus labrax and the intestinal epithelium of a marine oligochaete of the genus Tectidrilus. Myxospores subspherical in valvular view and ellipsoidal in sutural view measuring 7.6 ± 0.3 (6.8-8.7) μm in length, 7.2 ± 0.2 (6.7-7.7) μm in width and 6.5 ± 0.4 (5.8-7.7) μm in thickness. Two polar capsules, 3.0 ± 0.2 (2.6-3.4) μm long and 2.4 ± 0.1 (2.0-2.9) μm wide, located at the same level, but with divergent orientation and opening to opposite sides of the suture line. Sequencing of the SSU rRNA gene revealed a similarity of 100% between the analysed myxospores and triactinomyxon actinospores. The phylogenetic setting of O. labracis n. sp. shows subgrouping in correlation with tissue tropism, but identifies this parasite as another exception to the main division of Myxosporea into the main freshwater and marine lineages.

Long-term epidemiological survey of Kudoa thyrsites (Myxozoa) in Atlantic salmon (Salmo salar L.) from commercial aquaculture farms

Kudoa thyrsites (Myxozoa) encysts within myocytes of a variety of fishes. While infected fish appear unharmed, parasite-derived enzymes degrade the flesh post-mortem. In regions of British Columbia (BC), Canada, up to 4-7% of fillets can be affected, thus having economic consequences and impacting the competitiveness of BC's farms. K. thyrsites was monitored in two farms having high (HP) or low (LP) historical infection prevalence. At each farm, 30 fish were sampled monthly for blood and muscle during the first year followed by nine samplings during year two. Prevalence and intensity were measured by PCR and histology of muscle samples. In parallel, fillet tests were used to quantify myoliquefaction. Infections were detected by PCR after 355 and 509 degree days at LP and HP farms, respectively. Prevalence reached 100% at the HP farm by 2265 degree days and declined during the second year, whereas it plateaued near 50% at the LP farm. Infection intensities decreased after 1 year at both farms. Blood was PCR-positive at both farms between 778 and 1113 degree days and again after 2000 degree days. This is the first monitoring project in a production environment and compares data between farms with different prevalence.

Cutaneous myxidiosis in European eel, Anguilla anguilla (Linnaeus, 1758): histopathology, histochemistry and laminin immunohistochemistry

Histopathology, histochemistry and immunohistochemistry of the integument of European eel, Anguilla anguilla (Linnaeus, 1758), infected by Myxidium sp. are reported. Skin samples from affected and unaffected eels were dissected, formalin fixed, paraffin embedded, sectioned and stained with H&E, Periodic acid-Schiff's staining method, Alcian Blue 8 GX pH 2.5/Periodic acid-Schiff's and McCallum-Goodpasture's Gram stain. Moreover, immunohistochemistry was performed using a primary polyclonal laminin antibody. Histologically, cysts (diameter 2-3 mm) were observed mainly under the scale pockets, encircled by a thin collagen layer, lined by elongated, flattened fibroblasts and containing bipolar, PAS- and Gram-positive spores with opposite polar capsules. The epidermis stretched by the underlying cyst appeared dysplastic, thinned with a significant reduction in mucous cells number. Only inconsistent and aspecific inflammatory reaction was noted around the cysts at the dermis/epidermis interface. Intense laminin-like protein immunolabel was documented in the plasmodial ectoplasm and related to host anergia. This was the first report of laminin immunolabel in a member of the Myxozoa. Epidermal dysplasia represents likely an aspecific response against the underlying tensile force exerted by the developing parasite cyst, while fibroblast and collagen encapsulation denote a parasite-driven host response protecting, rather than harming, the encircled parasite.

Production of Ceratonova shasta Myxospores from Salmon Carcasses: Carcass Removal Is Not a Viable Management Option

Severe infection by the endemic myxozoan parasite, Ceratonova (synonym, Ceratomyxa) shasta, has been associated with declines in and impaired recovery efforts of populations of fall-run Chinook Salmon Oncorhynchus tshawytscha in the Klamath River, California. The parasite has a complex life cycle involving a polychaete worm host as well as a salmon host. Myxospore transmission of this parasite, from salmon to polychaete, is a life cycle step during which there is a potential for applied disease management. A 3-year data set on prevalence, intensity, and spore characteristics of C. shasta myxospores was obtained from adult Chinook Salmon carcasses surveyed in the main stem of the Klamath River and three of its tributaries, Bogus Creek and the Shasta and Trinity rivers. Annual prevalence of myxospore detection in salmon intestines ranged from 22% to 52%, and spore concentration values per intestinal scraping ranged from 3.94 × 10(2) to 1.47 × 10(7) spores. A prevalence of 7.3% of all carcasses examined produced >5.0 × 10(5) spores, and these carcasses with "high" spore counts accounted for 76-95% of the total spores in a given spawning season. Molecular analysis of visually negative carcasses showed that 45-87% of these samples had parasite DNA, indicating they contained either low spore numbers or presporogonic stages of the parasite. Myxospores were rarely found in carcasses of freshly spawned adults but were common in decomposed carcasses of both sexes. The date of collection or age (based indirectly on FL) did not influence detection. The longer prespawn residence time for spring-run Chinook Salmon compared with that for fall-run Chinook Salmon in the Trinity River was associated with higher spore loads. The dye exclusion method for assessing spore viability in fresh smears indicated an inverse relationship in spore integrity and initial spore concentration. A carcass-removal pilot project in Bogus Creek for 6 weeks in the fall of 2008 (907 carcasses removed) and 2009 (1,799 carcasses removed) failed to measurably influence the DNA quantity of C. shasta in targeted waters. Combined with the high numbers of carcasses that contributed myxospores, we therefore deemed that this labor-intensive approach is not a viable management option to reduce the infectivity of C. shasta in Chinook Salmon in the Klamath River. Received January 23, 2015; accepted September 28, 2015.

Co-infection of Nucleospora cyclopteri (Microsporidia) and Kudoa islandica (Myxozoa) in farmed lumpfish, Cyclopterus lumpus L., in Norway: a case report

This study describes a co-infection of Kudoa islandica (Myxozoa) and Nucleospora cyclopteri (Microsporida) in farmed lumpfish, Cyclopterus lumpus L., in Norway. Several other parasites (Cryptocotyle sp., protozoan ciliates and Gyrodactylus sp.) were also found in gills. In June 2013, the mortality in a farmed lumpfish population increased to 65%. Lumpfish showed erratic swimming behaviour and loss of weight. At necropsy, nodules in the kidney were the only visible lesions. Histologically, all fish showed severe changes with gill inflammation and necrosis in the spleen, kidney and liver. Haemorrhages and necrosis were observed in some hearts. Intracellular microsporidians associated with the lesions were detected in most organs using histological examination and Calcofluor White. Kudoa spores were diagnosed in the skeletal muscle, but no inflammatory response was associated with the presence of the plasmodia. Comparison of 18S ribosomal DNA sequences showed 100% similarity to Kudoa islandica and Nucleospora cyclopteri. Kudoa islandica and N. cyclopteri have previously been described associated with lesions in wild lumpfish in Iceland. In the present case, N. cyclopteri is believed to be the main cause of systemic pathology. This is the first description of K. islandica and N. cyclopteri causing pathology in farmed lumpfish in Norway.

Immunohistochemical detection and gene expression of TNFα in turbot (Scophthalmus maximus) enteromyxosis

Enteromyxum scophthalmi (Myxozoa) constitutes one of the most devastating pathogens for turbot (Scophthalmus maximus, L.) aquaculture. This parasite causes a severe intestinal parasitosis that leads to a cachectic syndrome with high morbidity and mortality rates for which no therapeutic options are available. Presence of inflammatory infiltrates, increased apoptotic rates and epithelial detaching have been described at intestinal level, as well as leukocyte depletion in lymphohaematopoietic organs. Previous investigations on enteromyxosis in turbot showed the high susceptibility of this species to the parasite and reported the existence of a dysregulated immune response against the parasite. The pleiotropic cytokine tumour necrosis factor alpha (TNFα) plays a major role in immune response and is involved in a wide range of biological activities. In teleost, the gene expression of this cytokine has been found regulated under several pathological conditions. Teleost TNFα shows some analogous functions with its mammalian counterparts, but the extent of its activities is still poorly understood. Cytokines are generally considered as a double-edge sword and TNFα has been implicated in the pathogenesis of different inflammatory diseases as well as in wasting syndromes described in mammals. The aim of this work was to analyse the expression of TNFα during enteromyxosis with molecular (Q-PCR) and morphological (immunohistochemistry) tools. Kidney, spleen and pyloric caeca from turbot with moderate and severe infections were analysed and compared to healthy naïve fish. TNFα expression was increased in both spleen and kidney in the earlier stages of the disease, whereas in severely infected fish, the expression decreased, especially in kidney. At the intestinal level, an increase in the number of TNFα-positive cells was noticed, which was proportional to the infiltration of inflammatory cells. The results demonstrate the involvement of TNFα in the immune response to E. scophthalmi in turbot, which could be related to the development of the clinic signs and lesions.

Effects of dietary NEXT ENHANCE®150 on growth performance and expression of immune and intestinal integrity related genes in gilthead sea bream (Sparus aurata L.)

Gilthead sea bream juveniles were fed different doses (0, 50, 100, 200, 300 ppm) of NEXT ENHANCE®150 (NE) for 9 weeks. Feed gain ratio (FGR) was improved by a 10% with all the doses, but feed intake decreased in a dose dependent manner. The optimum inclusion level to achieve maximum growth was set at 100 ppm. The hepatosomatic index did not vary and only at the highest dose, viscerosomatic and splenosomatic indexes were significantly decreased. No significant changes were found in haematological parameters, plasma biochemistry, total antioxidant capacity and respiratory burst. In a second trial, NE was given at 100 ppm alone (D1) or in combination with the prebiotic PREVIDA® (0.5%) (PRE) (D2) for 17 weeks. There were no differences in the growth rates, and FGR was equally improved for D1 and D2. No significant changes in haematology and plasma antioxidant capacity were detected. The histological examination of the liver and the intestine showed no outstanding differences in the liver, but the number of mucosal foldings appeared to be higher in D1 and D2 vs CTRL diet and the density of enterocytes and goblet cells also appeared higher, particularly in the anterior intestine. A 87-gene PCR-array was constructed based on our transcriptomic database (www.nutrigroup-iats.org/seabreamdb) and applied to samples of anterior (AI) and posterior (PI) intestine. It included 54 new gene sequences and other sequences as markers of cell differentiation and proliferation, intestinal architecture and permeability, enterocyte mass and epithelial damage, interleukins and cytokines, pattern recognition receptors (PRR), and mitochondrial function and biogenesis. More than half of the studied genes had significantly different expression between AI and PI segments. The functional significance of this differential tissue expression is discussed. The experimental diets induced significant changes in the expression of 26 genes. The intensity of these changes and the number of genes that were significantly regulated were higher at PI than at AI. At PI, both diets invoked a clear down-regulation of genes involved in cell differentiation and proliferation, some involved in cell to cell communication, cytokines and several PRR. By contrast, up-regulation was mostly found for genes related to enterocyte mass, cell epithelial damage and mitochondrial activity at AI. The changes were of the same order for D1 and D2, except for fatty acid-binding proteins 2 and 6 and the PRR fucolectin, which were higher in D2 and D1 fed fish, respectively. Thus, NE alone or in combination with PRE seems to induce an anti-inflammatory and anti-proliferative transcriptomic profile with probable improvement in the absorptive capacity of the intestine that would explain the improved FGR.

Morphology, seasonality and phylogenetic relationships of Ceratomyxa husseini n. sp. from the gall-bladder of Cephalopholis hemistiktos (Rüppell) (Perciformes: Serranidae) in the Arabian Gulf off Saudi Arabia

During a survey of myxosporean parasites of marine fishes from the Arabian Gulf in Saudi Arabia, spores of Ceratomyxa husseini n. sp. were found in the gall-bladders of 50 out of 148 specimens (33.8%) of the yellowfin hind Cephalopholis hemistiktos (Rüppell) (Perciformes: Serranidae). The rates of infection showed a seasonal fluctuation, with the highest prevalence in winter and the lowest in autumn. The mature spores appeared arched in frontal view with rounded valve ends and a slightly discriminated curved suture line and measured 8-9 × 14-18 (9 × 16) μm. The two polar capsules were spherical and equal in size, 4-5 (4.5) μm in diameter. The polar filament showed four turns obliquely to the longitudinal axis of the capsules and the sporoplasm filled half of the entire spore cavity. Partial sequences of the small subunit rRNA gene of C. husseini n. sp. showed percentage of identity with other species of Ceratomyxa ranging between 79.8 and 92.7%. The morphometric and molecular data, in association, confirmed that the present new species differs from all other Ceratomyxa spp. reported to date.

Lack of Protection following Re-Exposure of Chinook Salmon to Ceratonova shasta (Myxozoa)

The recent identification of multiple genotypes of the salmonid parasite Ceratonova shasta with different virulence levels in Chinook Salmon Oncorhynchus tshawytscha suggests that it is possible to immunize fish against subsequent infection and disease. We hypothesized that exposure of Chinook Salmon to the less-virulent parasite genotype (II) prior to the more virulent parasite genotype (I) would decrease disease and/or result in fewer mature parasites compared with fish only infected with the more virulent genotype. To test this hypothesis, fish were challenged in a combination of field and laboratory exposures, and we measured infection prevalence, percent morbidity, and mature parasite production. Neither mortality nor mature parasite production were reduced when fish were exposed to genotype II prior to genotype I compared with fish exposed only to genotype I, suggesting that protection against C. shasta using a less-virulent genotype of the parasite does not occur.

Effects of Enteromyxum scophthalmi experimental infection on the neuroendocrine system of turbot, Scophthalmus maximus (L.)

Enteromyxum scophthalmi is an intestinal myxosporean parasite responsible for serious outbreaks in turbot Scophthalmus maximus (L.) culture, in North-western Spain. The disease affects the digestive tract, provokes severe catarrhal enteritis, emaciation and high rates of mortality. The digestive parasitization triggers a response with the coordinate participation of immune and neuroendocrine systems through the action of peptides released by enteroendocrine cells and present in nervous elements, acting as neuro-immune modulators. The present study was designed to assess the response of the turbot neuroendocrine system against E. scophthalmi infection. Immunohistochemical tests were applied to sections of the gastrointestinal tract of uninfected and E. scophthalmi-infected turbot to characterize the presence of bombesin (BOM), glucagon (GLUC), somatostatin (SOM), leu-enkephalin (LEU) and met-enkephalin (MET). The occurrence of E. scophthalmi in the turbot gastrointestinal tract increased the number of enteroendocrine cells immunoreactive to SOM, LEU and MET. On the other hand, BOM and GLUC immunoreactive cells were less numerous in the gastrointestinal tract of the parasitized turbot. Scarce immunoreactivity to BOM, GLUC and SOM was observed in nerve fibres and neurons of the myenteric plexus of control and infected fish. The results indicate that E. scophthalmi infection in turbot induced changes in the neuroendocrine system, with the diminution of the anorexigenic peptides BOM and GLUC; the increase of enkephalins, related to pro-inflammatory processes; and the increase of SOM, which may cause inhibitory effects on the immune response, constituting a compensatory mechanism to the exacerbated response observed in E. scophthalmi-infected turbot.

Immunohistochemical characterization of polyclonal antibodies against Enteromyxum leei and Enteromyxum scophthalmi (Myxozoa: Myxosporea), intestinal parasites of fish

The enteric myxozoan parasites Enteromyxum leei (Diamant, Lom et Dyková) and Enteromyxum scophthalmi Palenzuela, Redondo et Álvarez-Pellitero are responsible for high weight loss in infected fish, which leads to subchronic disease and low mortality rates in gilthead sea bream (GSB), Sparus aurata L., and to high mortality rates in turbot, Psetta maxima (L.). The detection of initial parasite stages in histological sections is particularly difficult, but can be simplified by means of specific antibodies. Rabbit polyclonal antibodies (pAbs) were raised against E. scophthalmi and E. leei, and direct enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry were used to characterize their sensitivity and specificity. Both pAbs were adsorbed (apAb) with non-infected intestines to avoid non-specific labelling of fish tissues and to improve their specificity. The highest titre obtained in ELISA was 1: 32 000 for apAb-Eleei and 1:16 000 for apAb-Escoph. Working dilutions in immunohistochemistry were 1:1000 for apAb-Eleei and 1:8000 for apAb-Escoph. Both apAbs labelled proliferative and sporogonic stages with high specificity. apAb-Escoph was very specific, whereas apAb-Eleei cross-reacted with Sphaerospora dicentrarchi Sitjà-Bobadilla et Álvarez-Pellitero and Sphaerospora testicularis Sitjà-Bobadilla et Álvarez-Pellitero, suggesting the presence of shared antigens. These pAbs stand as new tools for antigenic characterization and the diagnosis of both Enteromyxum species.

Persistence of Tetracapsuloides bryosalmonae (Myxozoa) in chronically infected brown trout Salmo trutta

Proliferative kidney disease (PKD) is a widespread disease of farmed and wild salmonid populations in Europe and North America, caused by the myxozoan parasite Tetracapsuloides bryosalmonae. Limited studies have been performed on the epidemiological role in spread of the disease played by fish that survive infection with T. bryosalmonae. The aim of the present study was to evaluate the persistence of T. bryosalmonae developmental stages in chronically infected brown trout Salmo trutta up to 2 yr after initial exposure to laboratory-infected colonies of the parasite's alternate host, the bryozoan Fredericella sultana. Kidney, liver, spleen, intestine, brain, gills and blood were sampled 24, 52, 78 and 104 wk post-exposure (wpe) and tested for T. bryosalmonae by PCR and immunohistochemistry (IHC). Cohabitation trials with specific pathogen free (SPF) F. sultana colonies were conducted to test the viability of T. bryosalmonae. PCR detected T. bryosalmonae DNA in all tissue samples collected at the 4 time points. Developmental stages of T. bryosalmonae were demonstrated by IHC in most samples at the 4 time points. Cohabitation of SPF F. sultana with chronically infected brown trout resulted in successful transmission of T. bryosalmonae to the bryozoan. This study verified the persistence of T. bryosalmonae in chronically infected brown trout and their ability to infect the bryozoan F. sultana up to 104 wpe.

Interleukin gene expression is strongly modulated at the local level in a fish-parasite model

The goal of this work was to identify interleukin (IL)-related genes in the gilthead sea bream (GSB) (Sparus aurata L.) and how they are modulated by the parasite Enteromyxum leei, a myxozoan that causes severe enteritis with a strong inflammatory response. A Blast-X search of our transcriptomic GSB database (www.nutrigroup-iats.org/seabreamdb) identified 16 new sequences encompassing seven ILs (IL-7, IL-8, IL-10, IL-12β, IL-15, IL-18, and IL-34), the interleukin enhancer-binding factor 2 (ILF2), and eight IL receptors (IL-R); IL-R1, IL-6RA, IL-6RB, IL-8RA, IL-10RA, IL-10RB, IL-18R1, and IL-22R. Except for ILF2, their expression, plus that of IL-1β, IL-1R2, IL-6, and TNF-α (from public repositories), were analysed by 96-well PCR array of samples of blood, spleen, head kidney, and intestine of GSB that were anally intubated with E. leei (recipient group, RCPT). Only the expression profile of the intestine of RCPT fish showed significant difference as compared to samples from PBS-inoculated fish. At 17 days post intubation (dpi), the expression of key pro-inflammatory ILs, such as IL-8, IL-8R, IL-12β, and TNFα was significantly up-regulated, whereas at 64 dpi, anti-inflammatory IL expression (IL-6, IL-6RB, IL-7, IL-10, IL-10RA, and IL-15) was predominant. These results indicate a modification of the IL expression at late times post infection, probably to protect the fish intestine from the parasite and damage inflicted by an excessive inflammatory response. Furthermore, the response is mainly mediated at the local level as no significant changes were detected in blood, spleen and head kidney.

Seasonal monitoring of Kudoa yasunagai from sea water and aquaculture water using quantitative PCR

Kudoid myxozoans pose serious chronic problems in marine fisheries by causing pathological damage to host fish, reducing the market value of infected fish and potentially threatening public health. Kudoa yasunagai is a cosmopolitan parasite that infects the brains of various marine fishes, including important aquaculture species. We developed a quantitative PCR assay to detect K. yasunagai in sea water, and we used it to monitor abundance of the parasite in the environment and in culture through spring and winter. Quantitative PCR detected K. yasunagai DNA from sea water, with the lowest reliable threshold of 162 copies 28S rDNA l-1. Parasite DNA was detected sporadically in sea water throughout the study period of May through December 2012. The highest level of detected DNA occurred in mid-December (winter), at 117180 copies-equivalent to an estimate of over 200 myxospores l-1. Parasite DNA was generally not detected in August or September, the period with the highest water temperature. The reason for this observation is unknown, but the timing of parasite development may play a role. The amount of detected DNA was not different between unfiltered culture water and water filtered through a high-speed fiber filtration system. This result and the past incidence of high infection rate of fish reared in filtered water indicate that the mechanical removal of K. yasunagai from culture water is difficult. Detecting the precise onset and time window of infection in host fish will be an important step in the development of measures to control this economically important parasite.

Tissue preference of some myxobolids (Myxozoa: Myxosporea) from the musculature of European freshwater fishes

For several species of fish myxosporeans known at present, the musculature has been designated as the location of intrapiscine development. In the majority of these cases, plasmodia and spores are actually found in the muscle cells, but there are also myxosporeans that select intermuscular connective tissue, fish bones, nerves and capillaries of the musculature as their site of development. During the plasmodial development of Myxobolus, Henneguya and Thelohanellus species in fish inhabiting Hungarian freshwaters, 3 main locations of development inside the muscles were identified. Pseudodispar-type plasmodia, such as M. cyprini, M. musculi and M. pseudodispar, form plasmodia intracellularly in the muscle cells, while the plasmodia of M. pfeifferi, M. sandrae and T. hovorkai develop in the intermuscular connective tissue. A similar development in the connective tissue of the ventricle and the bulbus arteriosus was observed for M. dogieli, a heart parasite found in some cyprinid fishes. The third type of development is represented by M. tauricus, which prefers the collagenous elements of the fin rays, but its plasmodia are commonly found in the muscle attached to the fish bones.

Study of the distribution of active caspase-3-positive cells in turbot, Scophthalmus maximus (L.), enteromyxosis

Enteromyxosis caused by Enteromyxum scophthalmi is one of the parasitizations with a higher economic impact on turbot, Scophthalmus maximus (L.), aquaculture. This myxosporean produces severe catarrhal enteritis with abundant inflammatory infiltrates in the lamina propria-submucosa (LP), epithelial detachment and leucocyte depletion of the lymphohaematopoietic organs. Some advances made on the pathogenesis pointed to a role of apoptosis in the enteromyxosis. Therefore, the main aim of this work was to employ the TUNEL assay and the anti-(active caspase-3) immunohistochemical assay to detect apoptotic cells in both healthy and E. scophthalmi-infected turbot in order to establish the presence and distribution of apoptotic cells during development of the disease. More apoptotic cells located within the gastrointestinal epithelium were observed in the initial stages of the infection in E. scophthalmi-infected turbot compared with non-infected turbot. As the infection progressed, a higher degree of apoptosis occurred in the epithelium of folds heavily parasitized. In the severely infected turbot, apoptosis was also found among the leucocytes of the intestinal inflammatory infiltrates. Moreover, the number of active caspase-3-positive cells in the lymphohaematopoietic organs tended to increase with disease severity. In view of the results, increased apoptosis in the epithelium may favour the scaling that occurs during enteromyxosis and cell death of leucocytes in the intestinal LP, contributing to leucocyte depletion in severe cases.

Morphology, seasonality and phylogeny of Zschokkella soleae sp. n. (Myxozoa, Myxosporea) parasite of Solea solea (L.) (Pleuronectiformes, Soleidae) from Ghar El Melh Lagoon, Tunisia

A new Myxosporea, Zschokkella soleae sp. n., was found in the gall bladder and the bile of common sole, Solea solea (L.), from Ghar El Melh Lagoon in north-east Tunisia. This is the first record for the presence of Zschokkella species in Tunisian waters. The parasite's plasmodia are polysporic with variable size and shape. Some plasmodia appeared attached to the gall bladder epithelium while others were found free in bile. Mature spores are ellipsoidal in frontal view 13.8±0.38 μm long and 10.86±0.40 μm wide with two equal size spherical polar capsules 3.6±0.43 μm in size. The prevalence of infection seems to correlate with host size and changes over the year with maximum percentage in summer. Based on the 18S rDNA sequence data, Z. soleae sp. n. is readily distinguishable from other myxozoan DNA sequences in GenBank. Phylogenetically, the new species is placed in the freshwater Myxidium clade including several Zschokkella spp. infecting the gall bladder. Morphology, histology as well as DNA sequence analysis indicate that the examined species differs from all previously described Zschokkella species.

Molecular crosstalk between a chemical and a biological stressor and consequences on disease manifestation in rainbow trout

The aim of the present study was to examine the molecular and organism reaction of rainbow trout, Oncorhynchus mykiss, to the combined impact of two environmental stressors. The two stressors were the myxozoan parasite, Tetracapsuloides bryosalmonae, which is the etiological agent of proliferative kidney disease (PKD) and a natural stressor to salmonid populations, and 17β-estradiol (E2) as prototype of estrogen-active chemical stressors in the aquatic environment. Both stressors, the parasite and estrogenic contaminants, co-exist in Swiss rivers and are discussed as factors contributing to the decline of Swiss brown trout populations over the last decades. Using a microarray approach contrasting parasite-infected and non-infected rainbow trout at low or high estrogen levels, it was observed that molecular response patterns under joint exposure differed from those to the single stressors. More specifically, three major response patterns were present: (i) expression responses of gene transcripts to one stressor are weakened by the presence of the second stressor; (ii) expression responses of gene transcripts to one stressor are enhanced by the presence of the second stressor; (iii) expression responses of gene transcripts at joint treatment are dominated by one of the two stressors. Organism-level responses to concurrent E2 and parasite treatment - assessed through measuring parasite loads in the fish host and cumulative mortalities of trout - were dominated by the pathogen, with no modulating influence of E2. The findings reveal function- and level-specific responses of rainbow trout to stressor combinations, which are only partly predictable from the response to the single stressors.

Establishment of medium for laboratory cultivation and maintenance of Fredericella sultana for in vivo experiments with Tetracapsuloides bryosalmonae (Myxozoa)

The freshwater bryozoan Fredericella sultana (Blumenbach) is the most common invertebrate host of the myxozoan parasite Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease in salmonid fish. Culture media play an important role in hatching of statoblasts and maintaining clean bryozoan colonies for Malacosporea research. We developed a novel culture medium, Bryozoan Medium C (BMC), for the cultivation and maintenance of F. sultana under laboratory conditions. Statoblasts of F. sultana were successfully hatched to produce transparent-walled, specific pathogen-free (SPF) colonies that were maintained >12 months in BMC at pH 6.65. Tetracapsuloides bryosalmonae was successfully transmitted from infected brown trout, Salmo trutta L., to newly hatched F. sultana colonies in BMC, then from the infected bryozoan to SPF brown trout. This study demonstrated the utility of BMC (pH 6.65) for hatching statoblasts, long-term cultivation of clean and transparent bryozoan colonies and maintenance of the Tetracapsuloides bryosalmonae life cycle in the laboratory for molecular genetic research and other studies such as host-parasiteinteraction.

[New data on parasite fauna of the Chinese sleeper Perccottus glenii (Actinopterygii: Odontobutidae) in Primorsky Territory with the description of a new myxozoan species from the genus Myxidium (Myxozoa: Myxidiidae)]

Examination of 116 individuals of Perccottus glenii from water bodies of Primorsky Territory of Russia (the Khanka Lake basin and the Peter the Great Bay basin) revealed the presence of 35 species/taxa of parasites. We give a description of a new species, Myxidium shedkoae sp. nov. Spherical plasmodoa of M. shedkoa sp. nov. are located in the gall bladder; spores are fusiform, with protrusive, pointed, or truncated-cone-shaped poles. The length of spores constitutes 12.6-14.9 (13.4) microm; width, 7.1-8.6 (7.9) microm; shell valves possess 8-10 ridges forming "papillary" pattern. Drawings and descriptions of parasites species poorly studied and recorded from Russia for the first time are given together with drawings and descriptions of some parasites determined only as genera: Triangula perccotti, Gyrodactylus sp., Phyllodistomum sp., Paracoenogonimus ussuriensis (metacercaria), Cyathocotylidae gen. sp. (metacercaria) and Spiroxys japonicus (third-stage juvenile). Parasite fauna of P. glenii in Primorsky Territory comprises 66% of species/forms known for P. glenii in the native part of its range.