Risk factors associated with Enteromyxum scophthalmi (Myxozoa) infection in cultured turbot, Scophthalmus maximus (L.)

To React or Not to React: The Dilemma of Fish Immune Systems Facing Myxozoan Infections

Myxozoans are microscopic, metazoan, obligate parasites, belonging to the phylum Cnidaria. In contrast to the free-living lifestyle of most members of this taxon, myxozoans have complex life cycles alternating between vertebrate and invertebrate hosts. Vertebrate hosts are primarily fish, although they are also reported from amphibians, reptiles, trematodes, mollusks, birds and mammals. Invertebrate hosts include annelids and bryozoans. Most myxozoans are not overtly pathogenic to fish hosts, but some are responsible for severe economic losses in fisheries and aquaculture. In both scenarios, the interaction between the parasite and the host immune system is key to explain such different outcomes of this relationship. Innate immune responses contribute to the resistance of certain fish strains and species, and the absence or low levels of some innate and regulatory factors explain the high pathogenicity of some infections. In many cases, immune evasion explains the absence of a host response and allows the parasite to proliferate covertly during the first stages of the infection. In some infections, the lack of an appropriate regulatory response results in an excessive inflammatory response, causing immunopathological consequences that are worse than inflicted by the parasite itself. This review will update the available information about the immune responses against Myxozoa, with special focus on T and B lymphocyte and immunoglobulin responses, how these immune effectors are modulated by different biotic and abiotic factors, and on the mechanisms of immune evasion targeting specific immune effectors. The current and future design of control strategies for myxozoan diseases is based on understanding this myxozoan-fish interaction, and immune-based strategies such as improvement of innate and specific factors through diets and additives, host genetic selection, passive immunization and vaccination, are starting to be considered.

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.

Integrating Genomic and Morphological Approaches in Fish Pathology Research: The Case of Turbot (Scophthalmus maximus) Enteromyxosis

Enteromyxosis, caused by Enteromyxum scophthalmi, is one of the most devastating diseases stemming from myxozoan parasites in turbot (Scophthalmus maximus L.), being a limiting factor for its production. The disease develops as a cachectic syndrome, associated to catarrhal enteritis and leukocytic depletion, with morbidity and mortality rates usually reaching 100%. To date, no effective treatment exists and there are different unknown issues concerning its pathogenesis. The gross and microscopic lesions associated to enteromyxosis have been thoroughly described, and several morphopathological studies have been carried out to elucidate the mechanisms of this host-parasite interaction. More recently, efforts have been focused on a multidisciplinary approach, combining histopathology and transcriptome analysis, which has provided significant advances in the understanding of the pathogenesis of this parasitosis. RNA-Seq technology was applied at early and advanced stages of the disease on fishes histologically evaluated and classified based on their lesional degree. In the same way, the transcriptomic data were analyzed in relation to the morphopathological picture and the course of the disease. In this paper, a comprehensive review of turbot enteromyxosis is presented, starting from the disease description up to the most novel information extracted by an integrated approach on the infection mechanisms and host response. Further, we discuss ongoing strategies toward a full understanding of host-pathogen interaction and the identification of suitable biomarkers for early diagnosis and disease management strategies.

Diagnosis and treatment of multi-species fish mortality attributed to Enteromyxum leei while in quarantine at a US aquarium

Enteromyxum leei is an enteric myxozoan parasite of fish. This myxozoan has low host specificity and is the causative agent of myxozoan emaciation disease, known for heavy mortalities and significant financial losses within Mediterranean, Red Sea, and Asian aquaculture industries. The disease has rarely been documented within public aquaria and, to our knowledge, has never been confirmed within the USA. This case report describes an outbreak of E. leei in a population of mixed-species east African/Indo-Pacific marine fish undergoing quarantine at a public aquarium within the USA. Four of 16 different species of fish in the population, each of a different taxonomic family, were confirmed infected by the myxozoan through cloacal flush or intestinal wet mount cytology at necropsy. Clinical and histopathological findings in this case are similar to previous findings describing myxozoan emaciation disease, e.g. severe emaciation, cachexia, enteritis, and death. Sequence analysis of the 18S rDNA of intestinal samples from a powder blue tang Acanthurus leucosternon and an emperor angelfish Pomacanthus imperator confirmed the parasite to have 99-100% identity with other E. leei sequences. Spore morphology and ultrastructure were consistent with previous reports of E. leei. Treatment of clinically affected fish by oral administration of the coccidiostats amprolium and salinomycin led to reduction of mortalities and resolution of clinical signs. This case report highlights the importance of thorough examination and surveillance of fish during quarantine, particularly with respect to enteric myxozoans.

Integrated transcriptomic and proteomic analyses reveal potential mechanisms linking thermal stress and depressed disease resistance in the turbot Scophthalmus maximus

A worldwide increase in the reports of diseases affecting marine organisms has paralleled the climate warming over the past few decades. In this study, we applied omics to explore the mechanisms underlying thermo-linked epizootics, by comparing both the transcriptome- and proteome-wide response of turbots to a mimic pathogen (poly I:C) between high temperature and low temperature using a time-course approach. Our results showed that myeloperoxidase (MPO) and insulin were differentially expressed transcripts shared by all five time-points post poly I:C-injection between high and low temperature and also had a consistent expression trend as differentially expressed proteins at 24 h post injection. Combined with other data, it was suggested that the elevated temperature enhanced neutrophil-mediated immunity and the resultant MPO-mediated oxidative stress, which lasted for at least 5 days. The contents of malondialdehyde and protein carbonyls, markers of oxidative damage for lipids and proteins, respectively, were compared between different temperature groups, and the results further implied the emergence of oxidative damage under high temperature. It was also suggested that metabolism disorder likely occur considering the sustained expression changes of insulin. Hence, prolonged MPO-mediated oxidative stress and metabolic disorder might be involved in the thermo-linked epizootic.

Immunity to gastrointestinal microparasites of fish

Fish intestinal parasites cause direct mortalities and also morbidity, poor growth, higher susceptibility to opportunistic pathogens and lower resistance to stress. This review is focused on microscopic parasites (Protozoa and Metazoa) that invade the gastrointestinal tract of fish. Intracellular parasites (mainly Microsporidia and Apicomplexa) evoke almost no host immune reaction while they are concealed in the cytoplasmic and nuclear compartments, and can even use fish cells (macrophages) as Trojan horses to spread in the host. Inflammatory reaction only appears when the parasite bursts infected cells. Immunity against extracellular parasites is depicted for the myxozoans Ceratonova shasta and Enteromyxum spp. The cellular and humoral innate responses and the production of antibodies are crucial for resolving some of these myxozoonoses, but an excessive inflammatory reaction (concerted by cytokines) can become a fatal pathophysiological consequence. The local immune response plays a key role, with numerous genes more strongly regulated in the intestine than at lymphohaematopoietic organs.

RNA-seq analysis of early enteromyxosis in turbot (Scophthalmus maximus): new insights into parasite invasion and immune evasion strategies

Enteromyxum scophthalmi, an intestinal myxozoan parasite, is the causative agent of a threatening disease for turbot (Scophthalmus maximus, L.) aquaculture. The colonisation of the digestive tract by this parasite leads to a cachectic syndrome associated with high morbidity and mortality rates. This myxosporidiosis has a long pre-patent period and the first detectable clinical and histopathological changes are subtle. The pathogenic mechanisms acting in the early stages of infection are still far from being fully understood. Further information on the host-parasite interaction is needed to assist in finding efficient preventive and therapeutic measures. Here, a RNA-seq-based transcriptome analysis of head kidney, spleen and pyloric caeca from experimentally-infected and control turbot was performed. Only infected fish with early signs of infection, determined by histopathology and immunohistochemical detection of E. scophthalmi, were selected. The RNA-seq analysis revealed, as expected, less intense transcriptomic changes than those previously found during later stages of the disease. Several genes involved in IFN-related pathways were up-regulated in the three organs, suggesting that the IFN-mediated immune response plays a main role in this phase of the disease. Interestingly, an opposite expression pattern had been found in a previous study on severely infected turbot. In addition, possible strategies for immune system evasion were suggested by the down-regulation of different genes encoding complement components and acute phase proteins. At the site of infection (pyloric caeca), modulation of genes related to different structural proteins was detected and the expression profile indicated the inhibition of cell proliferation and differentiation. These transcriptomic changes provide indications regarding the mechanisms of parasite attachment to and invasion of the host. The current results contribute to a better knowledge of the events that characterise the early stages of turbot enteromyxosis and provide valuable information to identify molecular markers for early detection and control of this important parasitosis.

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.

Biology and mucosal immunity to myxozoans

Myxozoans are among the most abundant parasites in nature. Their life cycles involve two hosts: an invertebrate, usually an annelid, and a vertebrate, usually a fish. They affect fish species in their natural habitats but also constitute a menace for fish aquaculture. Using different strategies they are able to parasitize and cause damage in multiple organs, including mucosal tissues, which they use also as portals of entry. In fish, the main mucosal sites include the intestine, skin and gills. Recently the finding of a specific mucosal immunoglobulin in teleost (IgT), analogous to mammalian IgA, and the capacity of fish to develop a specific mucosal immune response against different pathogens, has highlighted the importance of studying immune responses at mucosal sites. In this review, we describe the major biological characteristics of myxozoan parasites and present the data available regarding immune responses for species that infect mucosal sites. As models for mucosal immunity we review the responses to Enteromyxum spp. and Ceratomyxa shasta, both of which parasitize the intestine. The immune response at the skin and gills is also described, as these mucosal tissues are used by myxozoans as attaching surfaces and portal of entry, and some species also parasitize these sites. Finally, the development of immunoprophylactic strategies is discussed.

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.

A combined strategy involving Sanger and 454 pyrosequencing increases genomic resources to aid in the management of reproduction, disease control and genetic selection in the turbot (Scophthalmus maximus)

Background

Genomic resources for plant and animal species that are under exploitation primarily for human consumption are increasingly important, among other things, for understanding physiological processes and for establishing adequate genetic selection programs. Current available techniques for high-throughput sequencing have been implemented in a number of species, including fish, to obtain a proper description of the transcriptome. The objective of this study was to generate a comprehensive transcriptomic database in turbot, a highly priced farmed fish species in Europe, with potential expansion to other areas of the world, for which there are unsolved production bottlenecks, to understand better reproductive- and immune-related functions. This information is essential to implement marker assisted selection programs useful for the turbot industry.

Results

Expressed sequence tags were generated by Sanger sequencing of cDNA libraries from different immune-related tissues after several parasitic challenges. The resulting database (“Turbot 2 database”) was enlarged with sequences generated from a 454 sequencing run of brain-hypophysis-gonadal axis-derived RNA obtained from turbot at different development stages. The assembly of Sanger and 454 sequences generated 52,427 consensus sequences (“Turbot 3 database”), of which 23,661 were successfully annotated. A total of 1,410 sequences were confirmed to be related to reproduction and key genes involved in sex differentiation and maturation were identified for the first time in turbot (AR, AMH, SRY-related genes, CYP19A, ZPGs, STAR FSHR, etc.). Similarly, 2,241 sequences were related to the immune system and several novel key immune genes were identified (BCL, TRAF, NCK, CD28 and TOLLIP, among others). The number of genes of many relevant reproduction- and immune-related pathways present in the database was 50–90% of the total gene count of each pathway. In addition, 1,237 microsatellites and 7,362 single nucleotide polymorphisms (SNPs) were also compiled. Further, 2,976 putative natural antisense transcripts (NATs) including microRNAs were also identified.

Conclusions

The combined sequencing strategies employed here significantly increased the turbot genomic resources available, including 34,400 novel sequences. The generated database contains a larger number of genes relevant for reproduction- and immune-associated studies, with an excellent coverage of most genes present in many relevant physiological pathways. This database also allowed the identification of many microsatellites and SNP markers that will be very useful for population and genome screening and a valuable aid in marker assisted selection programs.

Carbohydrate patterns in the digestive tract of Sparus aurata L. and Psetta maxima (L.) (Teleostei) parasitized by Enteromyxum leei and E. scophthalmi (Myxozoa)

The influence of Enteromyxum spp. infections on the carbohydrate patterns of the digestive tract of gilthead sea bream (GSB) Sparus aurata L. and turbot (TB) Psetta maxima (L.) has been studied. Histochemical stainings to differentiate the types of mucins and lectin-binding assays to detect terminal carbohydrate residues were applied to histological sections of GSB and TB uninfected or infected by Enteromyxum leei and E. scophthalmi, respectively. The number of intestinal GC decreased in severely infected fish in both parasitoses, though changes in mucin patterns were limited to the decrease in the staining intensity for acidic mucins in infected GSB. The TB stomach and intestine lacked histochemically detectable acidic mucins, or sialic acid detectable by SNA, in contrast with their abundance in GSB. Glucose/mannose, fucose and GlcNAc residues were less abundant in both infected hosts with respect to uninfected fish. In contrast, D-Gal and D-GalNAc moieties (detectable by BSL I) increased in most parts of E. scophthalmi-infected TB while decreasing (oesophagus) or remaining unchanged (intestine) in E. leei-infected GSB. The decreasing in the expression of acidic mucins and of sialic acid detectable by SNA in E. leei-infected GSB is remarkable. Differences in the carbohydrate patterns between both hosts could aid to explain the differences in the severity of both enteromyxoses. In addition, the changes induced by Enteromyxum spp. infections in the digestive tract of GSB and TB suggest a role of terminal carbohydrate residues in the parasite-host interaction.

The effect of lectins on the attachment and invasion of Enteromyxum scophthalmi (Myxozoa) in turbot (Psetta maxima L.) intestinal epithelium in vitro

The involvement of the lectin/carbohydrate interaction in the invasion of the turbot intestinal epithelium by Enteromyxum scophthalmi was studied in vitro using explants of turbot intestine and pre-treatment of parasite stages with the plant lectins of Canavalia ensiformis (Con A) and Glycine max (SBA). Both lectins inhibited the attachment and invasion of E. scophthalmi stages to the intestinal epithelium, though the inhibitory effect was higher for SBA than for Con A. Such results point to the involvement of N-acetyl-galactosamine (GalNAc) and galactose (Gal) residues and also of mannose/glucose residues in the E. scophthalmi-intestinal epithelium interaction. The inhibitory effect of both lectins on the parasite adhesion and penetration points to the interest of further studies to confirm the presence of putative lectins recognising GalNAc-Gal and mannose/glucose residues in turbot intestine. The obtained results demonstrated also the adequacy of turbot intestinal explants as an in vitro model to study the interaction with E. scophthalmi.

Host and environmental risk factors associated with Cryptosporidium scophthalmi (Apicomplexa) infection in cultured turbot, Psetta maxima (L.) (Pisces, Teleostei)

An epidemiological cohort study of Cryptosporidium scophthalmi in cultured turbot Psetta maxima L. of Northwestern Spain was conducted along a four-year period. Four different ongrowing cohorts were monitored monthly from introduction into the ongrowing tanks (10-50 g) until reaching market size (400-1400 g). The association of host and environmental factors with five categories of parasite abundance was assessed using a multivariable regression framework. Epidemiological factors assessed here were water temperature, weight, length, month of collection, season, age, origin, condition factor, water filtration, and status to the myxozoan Enteromyxum scophthalmi infection. E. scophthalmi was included into the analysis because it targets the same organ than C. scophthalmi and it was prevalent in the studied population. The multivariable analysis demonstrated the statistically significant association between several factors and parasite abundance. C. scophthalmi abundance was associated (P<0.05) with age, condition factor, season, and status to E. scophthalmi infection. Young animals, with poor condition factor, during spring or summer, and not infected with the myxozoan were most likely to be highly infected by C. scophthalmi. Inclusion of these four variables significantly (P<0.05) improved the model, compared to the model that did not include any of these epidemiological factors. Increasing levels of C. scophthalmi abundance were associated (P<0.01) with higher severity of C. scophthalmi-compatible lesions. The frequency of distribution of C. scophthalmi abundance was clearly right-skewed and fitted a negative binomial distribution, whereas the intensity of infection fitted a Poisson distribution. The quantification of the variance-to-mean ratio stratified by age demonstrated overdispersion for 8-16 months old fish, although this bivariate association is likely affected by several other factors, as suggested by the results of the multivariable analysis. The negative relation between C. scophthalmi abundance and status to E. scophthalmi infection suggests differences in the transmission, onset, and course of both infections. The coarse filtration used in some cohorts did not significantly affect the levels of infection. C. scophthalmi was probably introduced into the ongrowing tanks mainly with carrier fish, though the involvement of infective oocysts from the water supply cannot be disregarded. Infection prevalence and mean intensity decreased with fish age and a seasonal distribution was found. Results presented here will help to understand the epidemiology of C. scophthalmi in turbot, to estimate the expected levels of infection associated with presence or absence of epidemiological factors, and to quantify the impact that the disease may have on susceptible turbot populations. The multivariable model used here is more powerful than the visual inspection of graphics for exploring associations in cooperative processes and can be easily extended to the assessment of epidemiological associations in other population and parasitic diseases.

Detection of carbohydrate terminals in the enteric parasite Enteromyxum scophthalmi (Myxozoa) and possible interactions with its fish host Psetta maxima

The existence and localisation of carbohydrate terminals in Enteromyxum scophthalmi stages was investigated at light (LM) and transmission electron microscopes (TEM) using lectin histochemistry techniques, with the aim of contributing to elucidate the participation of carbohydrate-lectin interactions in the parasite invasion and relationships with the fish host. The presence of abundant mannose and/or glucose residues was demonstrated by the intense staining by concanavalin A at both LM and TEM. The staining pattern obtained with soybean agglutinin and Bandeiraea simplicifolia I (BSI) indicated the abundance of N-acetyl-galactosamine and D-galactose at a lesser extent. The lectins wheat germ agglutinin, BSI and Ulex europaeus agglutinin produced weaker marks. Most lectins recognised structures present in both pre-sporogonic and sporogonic stages, though the glycosidic pattern and/or staining intensity varied between developmental stages. No staining was obtained with Sambucus nigra agglutinin. The TEM studies demonstrated glucose-mannose, N-acetyl-glucosamine, N-acetyl-galactosamine and alpha-D-galactose as dominant structures at the parasite membrane and host-parasite interface, suggesting a role in host-parasite interactions. All these terminals were also detected in the mitochondria of P cells and were scant in the S cells and nuclei. In turbot intestine, mannose-glucose terminals and N-acetyl-glucosamine were labelled on the epithelial brush border and in the mucous cells and rodlet cells. The relevance of these findings in relation to the host-parasite interaction is discussed.

Protective acquired immunity to Enteromyxum scophthalmi (Myxozoa) is related to specific antibodies in Psetta maxima (L.) (Teleostei)

The acquired protection of three groups of turbot that had survived enteromyxosis outbreaks was tested by challenging with E. scophthalmi in three different experiments. The relation of such a response with the kinetics and duration of antibody production (determined by an ELISA) was studied. The progression of the infection was evaluated by PCR. In experiments 1 and 2, in which turbot had cohabited with highly infected fish during outbreaks, parasite prevalence and mortality were very low or null, and there was a progressive and statistically significant increase in the mean antibody production up to 350 and 152 days post-exposure respectively. By contrast, in experiment 3, fish (coming from non-infected tanks during the initial outbreak), both infection prevalence and cumulative mortality reached 92.8%, and specific antibodies were detected only in two fish. The observed differences in mortality after challenge appear to be related to the production of specific antibodies and it is probably accompanied by a repertoire of mechanisms of innate immunity. The exploitation of the immune system through breeding selection programmes as a possible strategy to control the disease is discussed.