Piscine Myocarditis Virus Research Articles

Unraveling the genomic landscape of piscine myocarditis virus (PMCV): mutation frequencies, viral diversity and evolutionary dynamics in Atlantic salmon

Abstract Over a decade since its discovery, piscine myocarditis virus (PMCV) remains a significant pathogen in Atlantic salmon aquaculture. Despite its significant impact, the genomic landscape, evolutionary dynamics, and virulence factors of PMCV are poorly understood. This study enhances the existing PMCV sequence dataset by adding 34 genome sequences and 202 new ORF3 sequences from clinical cardiomyopathy syndrome (CMS) cases in Norwegian aquaculture. Phylogenetic analyses, also including sequences from the Faroe Islands and Ireland reveal that PMCV sequences are highly conserved with distinct clustering by country of origin. Still, single CMS outbreaks display multiple PMCV variants, and although some clustering was seen by case origin, occasional grouping of sequences from different cases was also apparent. Temporal data from selected cases indicated increased sequence diversity in the population. We hypothesize that multiple bottlenecks and changing infection dynamics in the host population, with transfer to naïve individuals over time, represent a continuous selection pressure on the virus populations. No clear relation was found between PMCV variants and the severity of heart pathology. However, specific non-synonymous and synonymous mutations that might impact protein function and gene expression efficiency were identified. An additional factor that may impact PMCV replication is the presence of defective viral genomes, a novel finding for viruses of the order Ghabrivirales. This study provides new insights into PMCV genomic characteristics and evolutionary dynamics, highlighting the complex interplay of genetic diversity, virulence markers, and host-pathogen interactions, underscoring the epidemiological complexity of the virus.

Evaluating Atlantic Salmon (Salmo salar) as a Natural or Alternative Host for Piscine Myocarditis Virus (PMCV) Infection.

Cardiomyopathy syndrome (CMS) caused by piscine myocarditis virus (PMCV) has emerged with the rise of the aquaculture of Atlantic salmon (Salmo salar). The lack of cell culture cultivation has hampered the study of this infection. In this study, samples from naturally PMCV-infected Atlantic salmon from different commercial farms were collected and used. In situ hybridization (ISH) revealed intense staining of PMCV RNA in myocardial cells in the spongiform layer of the heart ventricle but almost no staining in the compact layer. In the kidneys, only sporadic staining was seen. Viral RNA was present in all organs, with the highest loads in the heart, kidney, and spleen. The high viral PMCV RNA loads in the heart were due to extensive viral mRNA transcription. The high ratio of viral mRNA to viral genomic dsRNA indicated active transcription but limited production of new viral particles. This suggests that the histopathological changes in the heart are caused by viral mRNA and corresponding viral proteins and not by virus particle formation. The production of full-length transcripts is regulated, with a reduction in the relative number of ORF3-containing transcripts at high transcription rates. Efforts to identify alternative hosts, such as fungi, were inconclusive, as fungal sequences were found inconsistently in the salmon tissue samples. The results of this study reinforce the need for further research to fully understand PMCV's life cycle and potential alternative hosts and its whereabouts when it is not infecting the hearts of the Atlantic salmon.

Most of the escaped farmed salmon entering a river during a 5-year period were infected with one or more viruses.

Disease interactions between farmed and wild populations have been poorly documented for most aquaculture species, in part due to the complexities to study this. Here, we tested 567 farmed Atlantic salmon escapees, captured in a Norwegian river during 2014-2018, for five viral infections that are prevalent in global salmonid aquaculture. Over 90% of the escapees were infected with one or more viruses. Overall prevalences were: 75.7% for piscine orthoreovirus (PRV-1), 43.6% for salmonid alphavirus (SAV), 31.2% for piscine myocarditis virus (PMCV), 1.2% for infectious pancreatic necrosis virus (IPNV) and 0.4% for salmon anaemia virus (ISAV). A significantly higher prevalence of PMCV infection was observed in immature compared to mature individuals. The prevalence of both SAV and PMCV infections was higher in fish determined by fatty acid profiling to be 'recent' as opposed to 'early' escapees that had been in the wild for a longer period of time. This is the first study to establish a time-series of viral infection status of escapees entering a river with a native salmon population. Our results demonstrate that farmed escapees represent a continuous source of infectious agents which could potentially be transmitted to wild fish populations.

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.

An Atypical Course of Cardiomyopathy Syndrome (CMS) in Farmed Atlantic Salmon (Salmo salar) Fed a Clinical Nutrition Diet.

Cardiomyopathy syndrome (CMS) poses a significant threat to farmed Atlantic salmon (Salmo salar), leading to high mortality rates during the seawater phase. Given that controlled experimental challenge trials with PMCV do not reproduce the mortality observed in severe field outbreaks of CMS, field trials on natural CMS outbreaks are warranted. This field study explored the impact of a clinical nutrition intervention, specifically a diet enriched with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), on a severe CMS outbreak in a commercial sea farm. CMS was diagnosed in a single sea cage with high mortality rates. Histopathological analysis, RT-qPCR in situ hybridization for virus detection, and fatty acid composition analysis were used to monitor the impact of disease and the inclusion of EPA and DHA in heart tissue. Following the implementation of clinical nutrition, a decline in mortality rates, regression of CMS-associated changes, and a significant reduction in piscine myocarditis virus (PMCV) RNA load were observed within the salmon population. Fatty acid composition analysis of heart samples demonstrated increased levels of EPA and DHA, reinforcing the association between dietary factors, viral load dynamics, and overall fish health. Although further validation is needed in future studies, as field trials may not be sufficient to establish causation, our results indicate that optimizing the EPA + DHA levels may prove beneficial in severe CMS outbreaks.

Characterization of a New Toti-like Virus in Sea Bass, Dicentrarchus labrax.

The European sea bass Dicentrarchus labrax is the main species reared in Mediterranean aquaculture. Its larval stage, which is very sensitive and highly affected by sanitary and environmental conditions, is particularly scrutinized in hatcheries. Recently, a Mediterranean sea bass farm had to deal with an abnormal increase in mortality, especially between 20 and 35 days post-hatching (dph). Biological investigations led to the observation of cytopathic effects on three different fish cell lines after almost 3 weeks of culture at 14 °C in contact with homogenized affected larvae, suggesting the presence of a viral agent. High-throughput sequencing revealed a 6818-nucleotide-long RNA genome with six putative ORFs, corresponding to the organization of viruses belonging to the Totiviridae family. This genome clustered with the newly described and suggested Pistolvirus genus, sharing 45.5% to 37.2% nucleotide identity with other piscine toti-like viruses such as Cyclopterus lumpus toti-like virus (CLuTLV) or piscine myocarditis virus (PMCV), respectively. Therefore, we propose to name this new viral agent sea bass toti-like virus (SBTLV). Specific real-time RT-PCR confirmed the presence of the viral genome in the affected larval homogenate from different production batches and the corresponding cell culture supernatant. Experimental infections performed on sea bass fingerlings did not induce mortality, although the virus could be detected in various organs and a specific immune response was developed. Additional studies are needed to understand the exact involvement of this virus in the mortality observed in hatcheries and the potential associated cofactors.

From viral load to survival: Unveiling new genetic links for resistance against PMCV in Atlantic Salmon (Salmo salar).

The infectious agent piscine myocarditis virus (PMCV) causes cardiomyopathy syndrome (CMS) and is responsible for substantial mortality and economic losses in the Atlantic salmon (Salmo salar) farming industry. Previous research has demonstrated that breeding for resistance against PMCV is an effective approach to mitigate the disease's impact. In this study, a new quantitative trait locus (QTL) is described on chromosome 23, together with previously described QTLs on chromosomes 12 and 27. The findings are based on two genome-wide association studies conducted on two different year-classes of Atlantic salmon of the Rauma strain. In this study, we utilized data from an experimental challenge trial with the viral load as the phenotype and a field outbreak of CMS with survival data as the phenotype. The estimated SNP-based heritability was 0.55 and 0.44 in the two studies, respectively. In the infection trial, the top associated SNP on chromosome 23 accounted for approximately 46% of the genetic and 25.53% of the phenotypic variations in the viral load. In the field outbreak, we identified a QTL on the same genomic region of chromosome 23. The most significantly associated marker on this chromosome explained 13.57% and 5.97% of the genetic and phenotypic variations. The QTL on chromosome 23 is in proximity to delta-5 fatty acyl desaturase and fatty acid desaturase 2 genes, both of which play a role in the production of polyunsaturated fatty acids. This proximity is particularly interesting as it offers valuable insights into enhancing our understanding of resistance against PMCV.

Potential origins of fish toti-like viruses in invertebrates.

The virus family Totiviridae had originally been considered to include only viruses which infected fungal and protist hosts, but since 2006 a growing number of viruses found in invertebrates and fish have been shown to cluster phylogenetically within this family. These Totiviridae-like, or toti-like, viruses do not appear to belong within any existing genera of Totiviridae, and whilst a number of new genus names have been suggested, none has yet been universally accepted. Within this growing number of toti-like viruses from animal hosts, there exists emerging viral threats particularly to aquaculture, namely Infectious myonecrosis virus in whiteleg shrimp and Piscine myocarditis virus (PMCV) in Atlantic salmon (Salmo salar). PMCV in particular continues to be an issue in salmon aquaculture as a number of questions remain unanswered about how the virus is transmitted and the route of entry into host fish. Using a phylogenetic approach, this study shows how PMCV and the other fish toti-like viruses probably have deeper origins in an arthropod host. Based on this, it is hypothesized that sea lice could be acting as a vector for PMCV, as seen with other RNA viruses in Atlantic salmon aquaculture and in the toti-like Cucurbit yellows-associated virus which is spread by the greenhouse whitefly Trialeurodes vaporariorum.

L-plastin levels are associated with mortality during cardiomyopathy syndrome in farmed Atlantic salmon (Salmo salar L.)

Cardiomyopathy syndrome (CMS) is a severe infectious cardiac disease affecting Atlantic salmon (Salmo salar L.). The disease is chief cause of sudden death in Norwegian fish farms and poses serious economical and fish welfare challenges to all salmon-producing countries. Although urgently needed, there are currently no treatments or effective preventive tools available. To achieve this, an in-depth understanding of the biological mechanisms leading to CMS is first required. Proteomic-based blood plasma analysis of inflammatory and immune response proteins recently identified L-plastin as an acute response protein in salmon. Here, we investigated whether L-plastin is associated with piscine myocarditis virus (PMCV) infection and mortality during CMS. By comparing heart and blood plasma samples from surviving and deceased fish from a confirmed CMS outbreak site, we observed that L-plastin expression was elevated in cardiac tissue while circulating free L-plastin levels were reduced in deceased fish. Interestingly, a higher molecular weight band positive for L-plastin was detected in plasma from deceased fish and correlated with L-plastin expression in the heart. In addition, cardiac L-plastin levels correlated with cardiac histopathological changes, mononuclear cell infiltration and necrosis of cardiomyocytes in the heart. Our findings set the base for further investigation of L-plastin as a potential biomarker for mortality risk during PMCV infection and for understanding the course of CMS in farmed salmon.

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.

Proteomic characterization of serum proteins from Atlantic salmon (Salmo salar L.) from an outbreak with cardiomyopathy syndrome

Cardiomyopathy syndrome (CMS), caused by piscine myocarditis virus (PMCV), is a serious challenge to Atlantic salmon (Salmo salar L.) aquaculture. Regrettably, husbandry techniques are the only tool to manage CMS outbreaks, and no prophylactic measures are available at present. Early diagnosis of CMS is therefore desirable, preferably with non-lethal diagnostic methods, such as serum biomarkers. To identify candidate biomarkers for CMS, the protein content of pools of sera (4 fish/pool) from salmon with a CMS outbreak (3 pools) and from clinically healthy salmon (3 pools) was compared using liquid chromatography-electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS). Overall, seven proteins were uniquely identified in the sera of clinically healthy fish, while 27 proteins were unique to the sera of CMS fish. Of the latter, 24 have been associated with cardiac disease in humans. These were grouped as leakage enzymes (creatine kinase, lactate dehydrogenase, glycogen phosphorylase and carbonic anhydrase); host reaction proteins (acute-phase response proteins-haptoglobin, fibrinogen, α2-macroglobulin and ceruloplasmin; and complement-related proteins); and regeneration/remodelling proteins (fibronectin, lumican and retinol). Clinical evaluation of the suitability of these proteins as biomarkers of CMS, either individually or as part of a panel, is a logical next step for the development of early diagnostic tools for CMS.

Establishment of a piscine myocarditis virus (PMCV) challenge model and testing of a plant-produced subunit vaccine candidate against cardiomyopathy syndrome (CMS) in Atlantic salmon Salmo salar

Cardiomyopathy syndrome (CMS) is a severe cardiac disease occurring in the grow-out sea phase of farmed Atlantic salmon with approximately 100 outbreaks annually in Norway. Piscine myocarditis virus (PMCV) is believed to be the causative agent of CMS. There is no vaccine available to control CMS, partially because PMCV withstands propagation in known cell cultures. In the present study, we selected the putative capsid protein of PMCV as the candidate antigen for immunization experiments and produced it in the plant Nicotiana benthamiana by transient expression. The recombinant PMCV antigen formed virus-like particles (VLPs). To evaluate the efficacy of the plant made VLP vaccine, a PMCV infection model was established. In an experimental salmon vaccination trial, the VLP vaccine triggered innate immunity, and indicative but not significant inhibition of viral replication in heart, spleen and kidney tissues was observed. Similarly, a reduction of inflammatory lesions in cardiomyocytes and subendocardial infiltration by mononuclear leukocytes were observed. Therefore, there was no difference in efficacy or immune response observed post the plant made PMCV VLP antigen vaccination.Taken together, this study has demonstrated that plant made VLP antigens should be investigated further as a possible platform for the development of PMCV antigens for a CMS vaccine.

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.

Lack of evidence of vertical transmission of piscine myocarditis virus in Atlantic salmon (Salmo salar L.).

Lack of evidence of vertical transmission of piscine myocarditis virus in Atlantic salmon (Salmo salar L.).

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.

Genetic diversity of piscine myocarditis virus in Atlantic salmon Salmo salar L. in Ireland

Piscine myocarditis virus (PMCV) is a double-stranded RNA virus which has been linked to cardiomyopathy syndrome (CMS) in Atlantic salmon (Salmo salar L.). The first recorded outbreak of CMS in Ireland occurred in 2012. Heart tissue samples were collected in the current study from farmed Atlantic salmon from various marine sites around Ireland, and the open reading frames (ORFs) 1 and 3 were amplified and sequenced in order to examine the genetic diversity of PMCV. Results showed PMCV to be largely homogenous in Irish samples, showing little genetic diversity. However, several amino acid positions within both ORF1 and ORF3 showed consistent variations unique to the Irish PMCV strains when compared with previously published Norwegian strains. The phylogeny generated in the present study suggests that PMCV may have been introduced into Ireland in two waves, both coming from the southern part of PMCV's range in Norway. In addition, over three-quarters of the PMCV strains which were sequenced came from fish not exhibiting any clinical signs of CMS, suggesting that either PMCV is evolving to become less virulent in Ireland or Irish Atlantic salmon are developing immunity to the 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.

Indications for a vertical transmission pathway of piscine myocarditis virus in Atlantic salmon (Salmo salar L.).

Losses due to cardiomyopathy syndrome (CMS) keep increasing in salmon‐producing countries in the North‐Atlantic. Recently, Piscine myocarditis virus (PMCV) has been detected in post‐smolts shortly after sea‐transfer, indicating a possible carry‐over from the hatcheries. In addition, there are reports of prevalences of PMCV as high as 70%–90% in certain groups of broodfish, and a recent outbreak of CMS in the Faroe Islands has been linked to the importation of eggs from a CMS‐endemic area. Thus, there is a need to investigate whether PMCV can be transmitted vertically from infected broodstock to their progeny. In the present study, samples from eggs, larvae, fingerlings and presmolt originating from PMCV‐positive broodstock from two commercial Atlantic salmon producers were tested for PMCV. The prevalence of PMCV in the broodstock was 98% in the hearts, 69% in the roe and 59% in the milt. Piscine myocarditis virus was detected in all stages of the progeny until and including the 40 g stage. Piscine myocarditis virus was also detected in presmolt sampled for tissue tropism. This provides farmers with several options for minimizing the risk of transfer of PMCV from broodstock to progeny, including screening of broodstock and aiming to use only those that are negative for PMCV or have low levels of virus.