Comparison of Atlantic salmon individuals with different outcomes of cardiomyopathy syndrome (CMS)

Persistent immune responses in the heart determine the outcome of cardiomyopathy syndrome in Atlantic salmon (Salmo salar)

Cardiomyopathy syndrome (CMS) caused by piscine myocarditis virus (PMCV) is a severe cardiac disease in Atlantic salmon (Salmo salar) and one of the leading causes of morbidity and mortality in the Norwegian aquaculture industry. Previous research suggest a variation in individual susceptibility to develop severe disease, however the role of the immune response in determining individual outcome of CMS is poorly understood particularly in cases where fish are also challenged by stress. The present study's aim was therefore to characterize cardiac transcriptional responses to PMCV infection in Atlantic salmon responding to infection under stressful conditions with a high versus low degree of histopathological damage. The study was performed as a large-scale controlled experiment of Atlantic salmon smolts from pre-challenge to 12 weeks post infection (wpi) with PMCV, during which fish were exposed to intermittent stressors. RNA sequencing (RNAseq) was used to compare the heart transcriptome of high responders (HR) with atrium histopathology score '4' and low responders (LR) with score '0.5' at 12 wpi. A high-throughput quantitative PCR (qPCR) analysis was used to compare immune gene transcription between individuals sampled at 6, 9 and 12 wpi. Based on RNAseq and qPCR results, RNAscope in situ hybridization (ISH) was performed for visualization of IFN-γ - and IFNb producing immune cells in affected heart tissue. Compared to LR, the transcription of 1592 genes was increased in HR at 12 wpi. Of these genes, around. 40 % were immune-related, including various chemokines, key antiviral response molecules, and genes. associated with a Th1 pro-inflammatory immune response. Further, the qPCR analysis confirmed. increased immune gene transcription in HR at both 9 and 12 wpi, despite a decrease in PMCV. transcription between these time points. Interestingly, increased IFNb transcription in HR suggests the. presence of high-quantity IFN secreting cells in the hearts of these individuals. Indeed, RNAscope. confirmed the presence of IFN-γ and IFNb-positive cells in the heart ventricle of HR but not LR. To conclude, our data indicate that in severe outcomes of PMCV infection various chemokines attract leucocytes to the salmon heart, including IFN-γ and IFNb-secreting cells, and that these cells play important roles in maintaining persistent antiviral responses and a sustained host immunopathology despite decreasing heart viral transcription.

TLR pathway signaling molecules in burbot (Lota lota): molecular characterization, basal expression, and their response to Poly(I:C)

Burbot (Lota lota), a fish species of economic and ecological significance found across northern hemisphere freshwater ecosystems, was the focus of this study. We characterized 19 Toll-like receptor (TLR) genes in burbot, tracing their expression patterns following pathogen exposure. TLR genes, crucial to the innate immune system, including TLR13-1/2/3, TLR2/2-2/2-3/2-4/2-5, and TLR22a/22b/22c/22d, were discovered to be tandemly repeated, signifying an evolution in the fish's immune system. Notably, different TLR subfamilies displayed tissue-specific expressions, with TLR1 primarily in spleen and head kidney, TLR13 in head kidney, trunk kidney, and heart, TLR22 in trunk kidney and liver, and TLR3 and TLR9 predominantly in spleen and head kidney, but also in trunk kidney. Further, we investigated the response of TLR genes in burbot to pathogen exposure using qRT-PCR. This involved measuring mRNA expressions of identified TLR genes in spleen and liver tissues after injecting Poly(I:C) to simulate a double-stranded RNA viral infection. The results revealed a time and tissue-specific expression pattern. Specifically, LoTLR3 reached peak expression in the spleen 12 h post-injection, declining thereafter, while TLR2 subfamily members only began expressing after 24 h. In the liver, activation of the TLR3-IRF7 and TLR3-IRF3 signaling pathways was noted. Integrating these results with transcriptomic data illuminated the pivotal role of TLR genes in the burbot's immune response. Such findings are vital in shaping future disease prevention and treatment strategies.

Characterization of early phases of cardiomyopathy syndrome pathogenesis in Atlantic salmon (Salmo salar L.) through various diagnostic methods

Since the first description of cardiomyopathy syndrome (CMS) in Atlantic salmon, in 1985, the disease caused by piscine myocarditisvirus (PMCV) has become a common problem in Atlantic salmon farming, not only in Norway, but also in other salmon farming countries like Scotland and Ireland. In the last years, CMS has been ranked as the most important salmon viral disease in Norway regarding both mortality and economic losses. Detailed knowledge of infection and pathogenesis is still lacking, a decade after the causal agent was first described, and there is a need for a wider range of methods/tools for diagnostic and research purposes. In this study, we compared the detection of PMCV- and CMS-related tissue lesions using previously used and well-known methods like histopathology and real-time RT-PCR to immunohistochemistry (IHC), a less used method, and a new method, RNAscope in situ hybridization. Tissue samples of three different cardiac compartments, mid-kidney and skin/muscle tissue were compared with non-lethal parallel samplings of blood and mucus. The development of pathological cardiac lesions observed in this experiment was in accordance with previous descriptions of CMS. Our results indicate a viremic phase 10- to 20-day post-challenge (dpc) preceding the cardiac lesions. In this early phase, virus could also be detected in relatively high amount in mid-kidney by real-time RT-PCR. Plasma and/or mid-kidney samples may, therefore, be candidates to screen for early-phase PMCV infection. The RNAscope in situ hybridization method showed higher sensitivity and robustness compared with the immunohistochemistry and may be a valuable support to histopathology in CMS diagnostics, especially in cases of untypical lesions or mixed infections.

Transcriptome Profiling of Atlantic Salmon (Salmo salar) Parr With Higher and Lower Pathogen Loads Following Piscirickettsia salmonis Infection

Salmonid rickettsial septicemia (SRS), caused by Piscirickettsia salmonis, is one of the most devastating diseases of salmonids. However, the transcriptomic responses of Atlantic salmon (Salmon salar) in freshwater to an EM-90-like isolate have not been explored. Here, we infected Atlantic salmon parr with an EM-90-like isolate and conducted time-course qPCR analyses of pathogen load and four biomarkers (campb, hampa, il8a, tlr5a) of innate immunity on the head kidney samples. Transcript expression of three of these genes (except hampa), as well as pathogen level, peaked at 21 days post-injection (DPI). Multivariate analyses of infected individuals at 21 DPI revealed two infection phenotypes [lower (L-SRS) and higher (H-SRS) infection level]. Five fish from each group (Control, L-SRS, and H-SRS) were selected for transcriptome profiling using a 44K salmonid microarray platform. We identified 1,636 and 3,076 differentially expressed probes (DEPs) in the L-SRS and H-SRS groups compared with the control group, respectively (FDR = 1%). Gene ontology term enrichment analyses of SRS-responsive genes revealed the activation of a large number of innate (e.g. “phagocytosis”, “defense response to bacterium”, “inflammatory response”) and adaptive (e.g. “regulation of T cell activation”, “antigen processing and presentation of exogenous antigen”) immune processes, while a small number of general physiological processes (e.g. “apoptotic process”, development and metabolism relevant) was enriched. Transcriptome results were confirmed by qPCR analyses of 42 microarray-identified transcripts. Furthermore, the comparison of individuals with differing levels of infection (H-SRS vs. L-SRS) generated insights into the biological processes possibly involved in disease resistance or susceptibility. This study demonstrated a low mortality (~30%) EM-90-like infection model and broadened the current understanding of molecular pathways underlying P. salmonis-triggered responses of Atlantic salmon, identifying biomarkers that may assist to diagnose and combat this pathogen.

Characterization of an outbreak of cardiomyopathy syndrome (CMS) in young Atlantic salmon, Salmo salar L

Cardiomyopathy syndrome (CMS) is the most common viral cardiac disease in Norwegian Atlantic salmon farming and typically affects large, market size fish. Only six months after seawater transfer, Atlantic salmon were diagnosed with CMS at a fish farm in the south-western part of Norway. Due to the unexpected young age and the remarkable large amounts of virus-specific RNA (Ct <10), the fish group was monitored with five additional samplings until slaughtered almost 10 months later. At three weeks after the first CMS diagnosis (weeks post-diagnosis, wpd) and at slaughter (39 wpd), more comprehensive samplings were performed of the study cage, with specific focus on three different cardiac compartments. The clinical, autopsy and histopathological findings at first diagnosis and at all succeeding samplings were similar to previous descriptions of typical CMS. A slightly elevated mortality was observed in the cage with diseased fish at the time of the first CMS diagnosis and continued throughout the study. The prevalence and load of PMCV-specific RNA in the fish remained high until slaughtering, with similar amounts in all sampled cardiac compartments. No fish from the other five cages at the site were diagnosed with CMS, until fish sampled from the last cage at the site were diagnosed 10 weeks after slaughtering of the study cage (49 wpd). Sequence analysis of the PMCV on the site showed that the outbreak virus was similar to PMCV variants previously sequenced from Norwegian field outbreaks. In conclusion, CMS in young Atlantic salmon had clinical signs and histopathological cardiac lesions typical for the disease, and diseased fish could be found in the study cage until slaughtering.

Genome-Wide Association Study of Piscine Myocarditis Virus (PMCV) Resistance in Atlantic Salmon (Salmo salar)

Cardiomyopathy syndrome is a severe, viral disease of Atlantic salmon that mostly affects farmed animals during their late production stage at sea. Caused by piscine myocarditis virus (PMCV), over the past few years outbreaks due to this disease have resulted in significant losses to the aquaculture industry. However, there is currently no vaccine that has proven effective against this virus. In this study, using a challenge model, we investigated the genetic variation for resistance to PMCV, by screening a large number of animals using a 55 K SNP array. In particular, we aimed to identify genetic markers that are tightly linked to higher disease resistance and can potentially be used in breeding programs. Using genomic information, we estimated a heritability of 0.51 ± 0.06, suggesting that resistance against this virus, to a great extent, is controlled by genetic factors. Through association analysis, we identified a significant quantitative trait locus (QTL) on chromosome 27, explaining approximately 57% of the total additive genetic variation. The region harboring this QTL contains various immune-related candidate genes, many of which have previously been shown to have a different expression profile between the naïve and infected animals. We also identified a suggestive association on chromosome 12, with the QTL linked markers located in 2 putatively immune-related genes. These results are of particular interest, as they can readily be implemented into breeding programs, can further assist in fine-mapping the causative mutations, and help in better understanding the biology of the disease and the immunological mechanisms underlying resistance against PMCV.

QTLs Associated with Resistance to Cardiomyopathy Syndrome in Atlantic Salmon

Cardiomyopathy syndrome (CMS) caused by piscine myocarditis virus is a major disease affecting the Norwegian Atlantic salmon industry. Three different populations of Atlantic salmon from the Mowi breeding program were used in this study. The first 2 populations (population 1 and 2) were naturally infected in a field outbreak, while the third population (population 3) went through a controlled challenged test. The aim of the study was to estimate the heritability, the genetic correlation between populations and perform genome-wide association analysis for resistance to this disease. Survival data from population 1 and 2 and heart atrium histology score data from population 3 was analyzed. A total of 571, 4312, and 901 fish from population 1, 2, and 3, respectively were genotyped with a noncommercial 55,735 Affymetrix marker panel. Genomic heritability ranged from 0.12 to 0.46 and the highest estimate was obtained from the challenge test dataset. The genetic correlation between populations was moderate (0.51–0.61). Two chromosomal regions (SSA27 and SSA12) contained single nucleotide polymorphisms associated with resistance to CMS. The highest association signal (P = 6.9751 × 10⁻²⁷) was found on chromosome 27. Four genes with functional roles affecting viral resistance (magi1, pi4kb, bnip2, and ha1f) were found to map closely to the identified quantitative trait loci (QTLs). In conclusion, genetic variation for resistance to CMS was observed in all 3 populations. Two important quantitative trait loci were detected which together explain half of the total genetic variance, suggesting strong potential application for marker-assisted selection and genomic predictions to improve CMS resistance.

Data-Driven Network Modeling as a Framework to Evaluate the Transmission of Piscine Myocarditis Virus (PMCV) in the Irish Farmed Atlantic Salmon Population and the Impact of Different Mitigation Measures

Cardiomyopathy syndrome (CMS) is a severe cardiac disease of Atlantic salmon caused by the piscine myocarditis virus (PMCV), which was first reported in Ireland in 2012. In this paper, we describe the use of data-driven network modeling as a framework to evaluate the transmission of PMCV in the Irish farmed Atlantic salmon population and the impact of different mitigation measures. Input data included live fish movement data from 2009 to 2017, population dynamics events and the spatial location of the farms. With these inputs, we fitted a network-based stochastic infection spread model. After assumed initial introduction of the agent in 2009, our results indicate that it took 5 years to reach a between-farm prevalence of 100% in late 2014, with older fish being most affected. Local spread accounted for only a small proportion of new infections, being more important for sustained infection in a given area. Spread via movement of subclinically infected fish was most important for explaining the observed countrywide spread of the agent. Of the targeted intervention strategies evaluated, the most effective were those that target those fish farms in Ireland that can be considered the most connected, based on the number of farm-to-farm linkages in a specific time period through outward fish movements. The application of these interventions in a proactive way (before the first reported outbreak of the disease in 2012), assuming an active testing of fish consignments to and from the top 8 ranked farms in terms of outward fish movement, would have yielded the most protection for the Irish salmon farming industry. Using this approach, the between-farm PMCV prevalence never exceeded 20% throughout the simulation time (as opposed to the simulated 100% when no interventions are applied). We argue that the Irish salmon farming industry would benefit from this approach in the future, as it would help in early detection and prevention of the spread of viral agents currently exotic to the country.

Genome-Wide Association Study Confirms Previous Findings of Major Loci Affecting Resistance to Piscine myocarditis virus in Atlantic Salmon (Salmo salar L.)

Cardiomyopathy syndrome is a viral disease of Atlantic salmon, mostly affecting fish during the late stages of production, resulting in significant losses to the industry. It has been shown that resistance to this disease has a strong genetic component, with quantitative trait loci (QTL) on chromosomes 27 (Ssa27) and Ssa12 to explain most of the additive genetic variance. Here, by analysing animals from a different year-class and a different population, we further aimed to confirm and narrow down the locations of these QTL. The data support the existence of the two QTL and suggest that the causative mutation on Ssa27 is most likely within the 10–10.5 Mbp segment of this chromosome. This region contains a cluster of major histocompatibility complex class I (MHC I) genes with the most strongly associated marker mapped to one of these loci. On Ssa12, the data confirmed the previous finding that the location of the causative mutation is within the 61.3 to 61.7 Mbp region. This segment contains several immune-related genes, but of particular interest are genes related to MHC II. Together, these findings highlight the likely key role of MHC genes in Atlantic salmon following infection with Piscine myocarditis virus (PMCV) and their potential impact on influencing the trajectory of this disease.

Estimating risk factors for the daily risk of developing clinical cardiomyopathy syndrome (CMS) on a fishgroup level

Cardiomyopathy syndrome (CMS) is a viral disease, causing significant mortality and decreased welfare in farmed salmon in the North Atlantic Ocean. In Norway, it has become the most important disease in animal husbandry, affecting more than 100 farms each year. Control of CMS is based on mitigation of risk factors, since no treatment or vaccine is available. However, little is known about how the disease spreads and develops, thus rendering disease control difficult for farmers and competent authorities. The objective of the present study was to identify risk factors leading to the development of clinical CMS, using data provided from the salmon producers. Daily production data from individual fishgroups in more than 120 salmon farms along the coast of Norway from fish put to sea in 2012-2014 was collected. The data included cause-specific mortalities, which was used to identify outbreaks of CMS and risk factors for disease. A model for describing the daily probability of outbreak of CMS in each fishgroup was developed. The model was run to find the most likely value for each of the parameters, given the observed outbreak data. From the data, we found that fish in the southern region of Norway have a much higher risk of developing CMS than fish in mid and west (parameter estimates (PE) 4.43 (CI: 2.54-7.04) vs. 3.27 and 2.58 (CI: 2.45-4.37 and 2.01-3.57). Further, across all regions, fish put to sea in the late fall develop CMS twice as often as fish put to sea in the early spring (PE 2.18-2.59; CI:1.54-4.6). Previous outbreaks of pancreas disease increased the risk of getting CMS with 3.36 (CI:2.97-3.78) in the west and 1.41 (CI: 1.24-1.63) in the mid regions and decreased the risk with 0.519 (CI: 0.456-0.611) in the south. Previous outbreaks of heart- and skeletal muscle inflammation increased the risk of CMS with 1.56-1.73 (CI:1.34-2.11) in the mid and south regions, and had no effect in the west. In addition, we found that fish groups originating from certain hatcheries had a higher risk of CMS than other fishgroups, independent on which farm they were farmed on. The risk of developing CMS also increased with the number of days at sea. The use of production data in the study gave the possibility to study disease development on a fish group level, and on a daily basis. Thus, the identification of risk factors provides new possibilities for control of disease.

Individual monitoring of immune response in Atlantic salmon Salmo salar following experimental infection with piscine myocarditis virus (PMCV), agent of cardiomyopathy syndrome (CMS)

Piscine myocarditis virus (PCMV) is a double-stranded RNA virus structurally similar to the Totiviridae family. PCMV is the causative agent of cardiomyopathy syndrome (CMS), a severe cardiac disease that affects farmed Atlantic salmon (Salmo salar). A recent study characterized the host immune response in infected salmon through a transcriptome immune profiling, which confirmed a high regulation of immune and anti-viral genes throughout infection with PCMV. Previously we developed a novel model based on repeated non-lethal blood sampling, enabling the individual monitoring of salmonids during an infection. In the present work, we used this model to describe the host immune response in the blood cells of Atlantic salmon after intramuscular infection with PCMV-containing tissue homogenate over a 77-day period. At the final stage heart samples were also collected to verify the PCMV load, the pathological impact of infection and to compare the transcript profiles to blood. The expression level of a range of key immune genes was determined in the blood and heart samples by real-time PCR. Results indicated selected immune genes (mx, cd8α and γip) were up-regulated in the heart tissue of infected animals at the terminal time point, in comparison to the non-infected fish. When analyzing the blood samples over the course of infection, a significant n up-regulation of mx gene was also observed. The time and number of peaks in the kinetics of expression was different between individuals. The PCMV load and CMS pathology was verified by real-time PCR and histopathology, respectively. No pathogen and no pathology could be detected during the course of the experiment except at the terminal stage (viral load by qPCR and pathology by histology). This study emphasizes the value of non-lethal monitoring for evaluating the health status of fish at early stages of infection and in the absence of clinical signs.

Antiviral defense in salmonids - Mission made possible?

Viral diseases represent one of the major threats for salmonid aquaculture. Survival from viral infections are highly dependent on host innate antiviral immune defense, where interferons are of crucial importance. Neutralizing antibodies and T cell effector mechanisms mediate long-term antiviral protection. Despite an immune cell repertoire comparable to higher vertebrates, farmed fish often fail to mount optimal antiviral protection. In the quest to multiply and spread, viruses utilize a variety of strategies to evade or escape the host immune system. Understanding the specific interplay between viruses and host immunity at depth is crucial for developing successful vaccination and treatment strategies in mammals. However, this knowledge base is still limited for pathogenic fish viruses. Here, we have focused on five RNA viruses with major impact on salmonid aquaculture: Salmonid alphavirus, Infectious salmon anemia virus, Infectious pancreatic necrosis virus, Piscine orthoreovirus and Piscine myocarditis virus. This review explore the protective immune responses that salmonids mount to these viruses and the existing knowledge on how the viruses counteract and/or bypass the immune response, including their IFN antagonizing effects and their mechanisms to establish persisting infections.

Wound healing in post-smolt Atlantic salmon (Salmo salar L.)

Skin biopsies (5 mm) taken from behind the dorsal fin on Atlantic salmon post-smolts were followed over a 2 month period. The healing process was dominated by hemostasis, acute inflammation, and epidermal repair the first 14 days post wounding (dpw), as shown through imaging, histological evaluation, and transcriptomics. Most of the immune genes showed decreased expression after two weeks, approaching the levels of intact skin, as also reflected in sections where reduced inflammation in the wound bed was observed. Transcriptional events suggest recruitment of lymphocytes to the wound site during the acute phase, with activation of humoral responses from 14 dpw and onward. From the histology, a more adherent mucus was observed that correlated with altered transcription of glycosyltransferases. This may indicate different properties and functions of the mucus during the wound healing process. Wound contraction started between 14 and 36 dpw. The occurrence of these events was concurrent with granulation tissue formation, melanocyte migration and up-regulation of genes involved in extracellular matrix formation. The presented description of the wound healing processes in Atlantic salmon gives insight into comparative ulcerative biology in mammals and fish and provides both novel and updated knowledge that can be applied for improved best operational practices for fish welfare in aquaculture.

Correlates of protective immunity for fish vaccines

Vaccination is one of the most effective disease control strategies that has contributed to the significant reduction of disease outbreaks and antibiotics usage in salmonid aquaculture. To date, licensing of fish vaccines is to a limited extent based on in vitro correlates of protection, as done for many mammalian vaccines. This is because the immunological mechanisms of vaccine protection have not been clearly elucidated for most fish vaccines. Herein, we provide an overview of the different steps required to establish correlates of protective immunity required to serve as benchmarks in optimizing vaccine production in aquaculture. We highlight the importance of optimizing challenge models needed to generate consistent results used during vaccine development as a basis for establishing immune correlates of protection. Data generated this far shows that antibodies are potentially the most reliable correlates of protective immunity for fish vaccines. Our findings also show that antigen dose can be optimized to serve as a correlate of protection for fish vaccines. Further, there is need to establish signatures of T-cell protective immunity when antibodies fail to serve as proxies of immune protection, particularly for vaccines against intracellular pathogens. We can anticipate that documentation of efficacy for future vaccines in aquaculture, particularly batch testing will be based on in vitro correlates of protective immunity.

Cardiomyopathy syndrome in Atlantic salmon Salmo salar L.: A review of the current state of knowledge

Cardiomyopathy syndrome (CMS) is a severe cardiac disease affecting Atlantic salmon Salmo salar L. The disease was first recognized in farmed Atlantic salmon in Norway in 1985 and subsequently in farmed salmon in the Faroe Islands, Scotland and Ireland. CMS has also been described in wild Atlantic salmon in Norway. The demonstration of CMS as a transmissible disease in 2009, and the subsequent detection and initial characterization of piscine myocarditis virus (PMCV) in 2010 and 2011 were significant discoveries that gave new impetus to the CMS research. In Norway, CMS usually causes mortality in large salmon in ongrowing and broodfish farms, resulting in reduced fish welfare, significant management-related challenges and substantial economic losses. The disease thus has a significant impact on the Atlantic salmon farming industry. There is a need to gain further basic knowledge about the virus, the disease and its epidemiology, but also applied knowledge from the industry to enable the generation and implementation of effective prevention and control measures. This review summarizes the currently available, scientific information on CMS and PMCV with special focus on epidemiology and factors influencing the development of CMS.

Emerging pathogens in the fish farming industry and sequencing-based pathogen discovery

The use of large scale DNA/RNA sequencing has become an integral part of biomedical research. Reduced sequencing costs and the availability of efficient computational resources has led to a revolution in how problems concerning genomics and transcriptomics are addressed. Sequencing-based pathogen discovery represents one example of how genetic data can now be used in ways that were previously considered infeasible. Emerging pathogens affect both human and animal health due to a multitude of factors, including globalization, a shifting environment and an increasing human population. Fish farming represents a relevant, interesting and challenging system to study emerging pathogens. This review summarizes recent progress in pathogen discovery using sequence data, with particular emphasis on viruses in Atlantic salmon (Salmo salar).

A conserved inhibitory role of suppressor of cytokine signaling 1 (SOCS1) in salmon antiviral immunity

The SOCS proteins are regulators of JAK/STAT signaling. A number of viral infections has been associated with SOCS upregulation. Here we report that SOCS1 mRNA expression is up-regulated in salmon alphavirus (SAV3) infected TO cells, while infectious pancreatic necrosis virus infection has a negligible effect. SAV3 infected salmon showed increased SOCS1 mRNA levels in heart correlating with increased viral transcripts. Together, the in vitro and in vivo data suggests that SAV3-induced SOCS1 expression may affect the outcome of the virus infection. Using CHSE-214 cells overexpressing SOCS1 we revealed increased SAV3 replication compared to control, suggesting that SOCS1 suppress the antiviral capacity of the cells. In IFNa1-treated cells, the suppression of viral replication was partially rescued by SOCS1 overexpression. The increased viral replication in SOCS1 transgene cells was likely a result of impaired IFN-signaling and the reduced expression of interferon-stimulated genes in the transgene cells underscores this.

Comparison of transcriptomic responses to pancreas disease (PD) and heart and skeletal muscle inflammation (HSMI) in heart of Atlantic salmon (Salmo salar L)

Pancreas disease (PD) and heart and skeletal muscle inflammation (HSMI) are viral diseases associated with SAV (salmonid alphavirus) and PRV (piscine reovirus), which induce systemic infections and pathologies in cardiac and skeletal muscle tissue of farmed Atlantic salmon (Salmo salar L), resulting in severe morbidity and mortality. While general features of the clinical symptoms and pathogenesis of salmonid viral diseases are relatively well studied, much less is known about molecular mechanisms associated with immunity and disease-specific changes. In this study, transcriptomic analyses of heart tissue from PD and HSMI challenged Atlantic salmon were done, focusing on the mature phases of both diseases at respectively 28-35 and 42-77 days post infection. A large number of immune genes was activated in both trials with prevalence of genes associated with early innate antiviral responses, their expression levels being slightly higher in PD challenged fish. Activation of the IFN axis was in parallel with inflammatory changes that involved diverse humoral and cellular factors. Adaptive immune response genes were more pronounced in fish with HSMI, as suggested by increased expression of a large number of genes associated with differentiation and maturation of B lymphocytes and cytotoxic T cells. A similar down-regulation of non-immune genes such as myofiber and mitochondrial proteins between diseases was most likely reflecting myocardial pathology. A suite of genes important for cardiac function including B-type natriuretic peptide and four neuropeptides displayed differential expression between PD and HSMI. Comparison of results revealed common and distinct features and added to the understanding of both diseases at their mature phases with typical clinical pictures. A number of genes that showed disease-specific changes can be of interest for diagnostics.

Effects of functional feeds on the lipid composition, transcriptomic responses and pathology in heart of Atlantic salmon (Salmo salar L.) before and after experimental challenge with Piscine Myocarditis Virus (PMCV)

Background

Cardiomyopathy syndrome (CMS) is a severe cardiac disease of Atlantic salmon (Salmo salar) recently associated with a double-stranded RNA virus, Piscine Myocarditis Virus (PMCV). The disease has been diagnosed in 75-85 farms in Norway each year over the last decade resulting in annual economic losses estimated at up to €9 million. Recently, we demonstrated that functional feeds led to a milder inflammatory response and reduced severity of heart lesions in salmon experimentally infected with Atlantic salmon reovirus, the causal agent of heart and skeletal muscle inflammation (HSMI). In the present study we employed a similar strategy to investigate the effects of functional feeds, with reduced lipid content and increased eicosapentaenoic acid levels, in controlling CMS in salmon after experimental infection with PMCV.

Results

Hepatic steatosis associated with CMS was significantly reduced over the time course of the infection in fish fed the functional feeds. Significant differences in immune and inflammatory responses and pathology in heart tissue were found in fish fed the different dietary treatments over the course of the infection. Specifically, fish fed the functional feeds showed a milder and delayed inflammatory response and, consequently, less severity of heart lesions at earlier and later stages after infection with PMCV. Decreasing levels of phosphatidylinositol in cell membranes combined with the increased expression of genes related with T-cell signalling pathways revealed new interactions between dietary lipid composition and the immune response in fish during viral infection. Dietary histidine supplementation did not significantly affect immune responses or levels of heart lesions.

Conclusions

Combined with the previous findings on HSMI, the results of the present study highlight the potential role of clinical nutrition in controlling inflammatory diseases in Atlantic salmon. In particular, dietary lipid content and fatty acid composition may have important immune-modulatory effects in Atlantic salmon that could be potentially beneficial in fish balancing the immune and tissue responses to viral infections.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-462) contains supplementary material, which is available to authorized users.

Cardiac responses to elevated seawater temperature in Atlantic salmon

Background

Atlantic salmon aquaculture operations in the Northern hemisphere experience large seasonal fluctuations in seawater temperature. With summer temperatures often peaking around 18-20°C there is growing concern about the effects on fish health and performance. Since the heart has a major role in the physiological plasticity and acclimation to different thermal conditions in fish, we wanted to investigate how three and eight weeks exposure of adult Atlantic salmon to 19°C, previously shown to significantly reduce growth performance, affected expression of relevant genes and proteins in cardiac tissues under experimental conditions.

Results

Transcriptional responses in cardiac tissues after three and eight weeks exposure to 19°C (compared to thermal preference, 14°C) were analyzed with cDNA microarrays and validated by expression analysis of selected genes and proteins using real-time qPCR and immunofluorescence microscopy. Up-regulation of heat shock proteins and cell signaling genes may indicate involvement of the unfolded protein response in long-term acclimation to elevated temperature. Increased immunofluorescence staining of inducible nitric oxide synthase in spongy and compact myocardium as well as increased staining of vascular endothelial growth factor in epicardium could reflect induced vascularization and vasodilation, possibly related to increased oxygen demand. Increased staining of collagen I in the compact myocardium of 19°C fish may be indicative of a remodeling of connective tissue with long-term warm acclimation. Finally, higher abundance of transcripts for genes involved in innate cellular immunity and lower abundance of transcripts for humoral immune components implied altered immune competence in response to elevated temperature.

Conclusions

Long-term exposure of Atlantic salmon to 19°C resulted in cardiac gene and protein expression changes indicating that the unfolded protein response, vascularization, remodeling of connective tissue and altered innate immune responses were part of the cardiac acclimation or response to elevated temperature.

Induced erythropoiesis during acute anemia in Atlantic salmon: a transcriptomic survey

Anemia is a common pathophysiological response to stressors, malnutrition and infections in salmonid fish. In order to improve our understanding of the molecular mechanisms and markers associated with induced erythropoiesis (EP) during acute anemia in Atlantic salmon (Salmo salar L.), we performed transcriptome analysis of fish injected with the hemolytic compound phenylhydrazine (PHZ). Treatment with a low dose of PHZ resulted in moderate but significant reduction of hematocrit (Hct) and increased transcription of cardiac erythropoietin (epo) at 2 days post challenge (dpc), and epo receptor (epor) in spleen from 2 to 4 dpc. Oligonucleotide microarrays were used to characterize the events of EP in the spleen, an important organ for expansive EP during acute erythropoietic stress in rodents, and these were compared to gene expression profiles of untreated mature red blood cells (RBC) in order to search for erythroid-specific genes. Splenic responses suggested a prevalence of protective mechanisms at the first stage, characterized by induced xenobiotic metabolism and responses to oxidative and protein stress. Erythroid-specific regulation was evident at 2 dpc and enhanced by 4 dpc, and gene expression profiles witnessed a rapid establishment of RBC phenotype although Hct levels remained low. A large group of genes showed a strong correlation to globins by expression profiles. In addition to epor this included genes of heme and iron metabolism, scavengers of free radicals and chaperones, channels and transporters, markers of erythrocytes, regulators of proliferation and cell cycle arrest and many genes with unidentified roles in RBC differentiation. Induced EP in spleen was characterized by specific features, such as upregulation of innate antiviral immune genes and sustained high expression of proapoptotic genes including caspases. Transcriptome changes suggested an association between EP and suppression of several developmental programs including adaptive immune responses. In conclusion, acute hemolysis and resulting anemia rapidly induced EP in the spleen of Atlantic salmon, which showed both common characteristics for all vertebrates as well as fish-specific properties.