Virulence of viral haemorrhagic septicaemia virus (VHSV) genotype III in rainbow trout

Determination of the minimum infective dose of viral hemorrhagic septicemia virus (VHSV) in juvenile olive flounder, Paralichthys olivaceus using an immersion challenge model

Viral hemorrhagic septicemia virus (VHSV) affects over 80 fish species, leading to viral hemorrhagic septicemia (VHS). Horizontal VHSV transmission is widely studied, with researchers utilizing various doses to establish infection models. Infected hosts shed the virus into the environment, elevating the risk of transmission to naïve fish within the same system. This study aimed to ascertain the minimum infective dose of VHSV in olive flounder (Paralichthys olivaceus). In olive flounder, the detection of VHSV within the kidney exhibited the highest infection rate on the third day among days 1, 3 and 5. Doses of 103.0 to 104.7 TCID50/ml were administered to juvenile olive flounder across three farms. Results showed resistance to infection below 103.4 TCID50/ml at 15 °C. While infection frequency varied by concentration, higher concentrations correlated with more infections. Nonetheless, viral copy numbers did not differ significantly among infected fish at varying concentrations. This study underscores the need for early VHSV management and contributes essential data for pathogenicity assessment and foundational knowledge.

Transcriptome Profiling of Oncorhynchus mykiss Infected with Low or Highly Pathogenic Viral Hemorrhagic Septicemia Virus (VHSV)

The rainbow trout (Oncorhynchus mykiss) is the most important produced species in freshwater within the European Union, usually reared in intensive farming systems. This species is highly susceptible to viral hemorrhagic septicemia (VHS), a severe systemic disease widespread globally throughout the world. Viral hemorrhagic septicemia virus (VHSV) is the etiological agent and, recently, three classes of VHSV virulence (high, moderate, and low) have been proposed based on the mortality rates, which are strictly dependent on the viral strain. The molecular mechanisms that regulate VHSV virulence and the stimulated gene responses in the host during infection are not completely unveiled. While some preliminary transcriptomic studies have been reported in other fish species, to date there are no publications on rainbow trout. Herein, we report the first time-course RNA sequencing analysis on rainbow trout juveniles experimentally infected with high and low VHSV pathogenic Italian strains. Transcriptome analysis was performed on head kidney samples collected at different time points (1, 2, and 5 days post infection). A large set of notable genes were found to be differentially expressed (DEGs) in all the challenged groups (e.s. trim63a, acod1, cox-2, skia, hipk1, cx35.4, ins, mtnr1a, tlr3, tlr7, mda5, lgp2). Moreover, the number of DEGs progressively increased especially during time with a greater amount found in the group infected with the high VHSV virulent strain. The gene ontology (GO) enrichment analysis highlighted that functions related to inflammation were modulated in rainbow trout during the first days of VHSV infection, regardless of the pathogenicity of the strain. While some functions showed slight differences in enrichments between the two infected groups, others appeared more exclusively modulated in the group challenged with the highly pathogenic strain.

Functional Nanomaterials Enhancing Electrochemical Biosensors as Smart Tools for Detecting Infectious Viral Diseases

Electrochemical biosensors are known as analytical tools, guaranteeing rapid and on-site results in medical diagnostics, food safety, environmental protection, and life sciences research. Current research focuses on developing sensors for specific targets and addresses challenges to be solved before their commercialization. These challenges typically include the lowering of the limit of detection, the widening of the linear concentration range, the analysis of real samples in a real environment and the comparison with a standard validation method. Nowadays, functional nanomaterials are designed and applied in electrochemical biosensing to support all these challenges. This review will address the integration of functional nanomaterials in the development of electrochemical biosensors for the rapid diagnosis of viral infections, such as COVID-19, middle east respiratory syndrome (MERS), influenza, hepatitis, human immunodeficiency virus (HIV), and dengue, among others. The role and relevance of the nanomaterial, the type of biosensor, and the electrochemical technique adopted will be discussed. Finally, the critical issues in applying laboratory research to the analysis of real samples, future perspectives, and commercialization aspects of electrochemical biosensors for virus detection will be analyzed.

Epitope mapping of the monoclonal antibody IP5B11 used for detection of viral haemorrhagic septicaemia virus facilitated by genome sequencing of carpione novirhabdovirus

The monoclonal antibody (mAb) IP5B11, which is used worldwide for the diagnosis of viral haemorrhagic septicaemia (VHS) in fish, reacts with all genotypes of VHS virus (VHSV). The mAb exceptionally also reacts with the carpione rhabdovirus (CarRV). Following next generation genome sequencing of CarRV and N protein sequence alignment including five kinds of fish novirhabdoviruses, the epitope recognized by mAb IP5B11 was identified. Dot blot analysis confirmed the epitope of mAb IP5B11 to be associated with the region N219 to N233 of the N protein of VHSV. Phylogenetic analysis identified CarRV as a new member of the fish novirhabdoviruses.

Generation of viperin-knockout zebrafish by CRISPR/Cas9-mediated genome engineering and the effect of this mutation under VHSV infection

Viperin is an important virus-induced protein in animals that negatively participates in RNA viral replication and transcription. The reactive machinery of viperin suggests that it produces a regulatory molecule ddhCTP, which may affect immune regulation. In this study, we investigated the expression pattern of viperin in larval and adult stages of zebrafish by whole-mount in situ hybridization and reverse transcription-quantitative PCR (RT-qPCR). To elucidate the function of viperin, we generated a zebrafish knockout model using the CRISPR/Cas9 method and evaluated the mutation's effects under viral hemorrhagic septicemia virus (VHSV) infections. In zebrafish larvae, viperin was expressed in the brain region, eye, and pharynx, which was confirmed by cryosectioning. In adult zebrafish, blood cells showed the highest levels of viperin expression. In 5 dpf fish challenged with VHSV, the expression of the viral NP protein was significantly enhanced in viperin^-/- compared to wild-type fish. In vitro VHSV propagation analysis indicated comparatively higher levels of virus propagation in viperin^-/- fish. Mortality analysis confirmed higher mortality rates, and interferon gene expression analysis showed a strong upregulation of interferon (ifn)φ1 and 3 gene in viperin^-/- fish infected with VHSV. This study describes the successful generation of a viperin-knockout model and the role of viperin during VHSV infections.

Molecular characterization, immune responses, and functional activities of manganese superoxide dismutase in disk abalone (Haliotis discus discus)

Superoxide dismutases (SODs) are metalloenzymes that convert superoxide radicals to H₂O₂ and O₂. Although SODs have been extensively studied in mammals and other species, comparative studies in invertebrates, such as abalones, are lacking. Here, we aimed to characterize manganese superoxide dismutase in disk abalone (Haliotis discus discus) (AbMnSOD) by assessing its transcriptional levels at different embryonic developmental stages. Additionally, the temporal expression of AbMnSOD in different abalone tissues in response to bacterial, viral, and pathogen-associated molecular pattern (PAMP) stimuli was investigated. SOD activity was measured at various recombinant protein concentrations via the xanthine oxidase/WST-1 system. Cell viability upon exposure to H₂O₂, wound healing ability, and subcellular localization were determined in AbMnSOD-transfected cells. AbMnSOD was 681 bp long and contained the SOD-A domain. AbMnSOD expression was higher at the trochophore stage than at the other stages. When challenged with immune stimulants, AbMnSOD showed the highest expression at 6 h post-injection (p.i.) for all stimulants except lipopolysaccharides. In the gills, the highest AbMnSOD expression was observed at 6 h p.i., except for the Vibrio parahaemolyticus challenge. Recombinant AbMnSOD showed concentration-dependent xanthine oxidase activity. Furthermore, AbMnSOD-transfected cells survived H₂O₂-induced apoptosis and exhibited significant wound gap closure. As expected, AbMnSOD was localized in the mitochondria of the cells. Our findings suggest that AbMnSOD is an essential antioxidant enzyme that participates in regulating developmental processes and defense mechanisms against oxidative stress in hosts.

Modifications of the nucleoprotein of viral haemorrhagic septicaemia virus showed gain of virulence in intraperitoneally infected rainbow trout

Viral haemorrhagic septicaemia virus (VHSV) is the cause of an important listed disease in European rainbow trout (Oncorhynchus mykiss) aquaculture and can be present in a wide range of fish species, including marine fish, which can act as viral reservoir. Recent studies revealed putative genetic virulence markers of VHSV to rainbow trout highlighting the roles of the nucleoprotein, phosphoprotein and non-virion protein. Using reverse genetics, we produced recombinant viruses by introducing parts of or the entire nucleoprotein from a high-virulent isolate VHSV into a low-virulent backbone. Furthermore, we also made recombinant viruses by introducing residue modifications in the nucleoprotein that seem to play a role in virulence. Rainbow trout challenged with these recombinant viruses (rVHSVs) by intraperitoneal injection (IP) developed clinical signs and showed lower survival when compared to the parental rVHSV whereas fish challenged by immersion did not show clinical signs except for the high-virulent control. The mutations did not influence the viral growth in cell culture. The recombinant viruses and parental recombinant were unable to replicate and show cytopathic effect in EPC cells whereas the high-virulent control was well adapted in all the fish cell lines tested. We showed evidence that corroborates with the hypothesis that the nucleoprotein has virulence motifs associated with VHSV virulence in rainbow trout.

Measures of diagnostic precision (repeatability and reproducibility) for three test methods designed to detect spring viremia of carp virus

Spring viremia of carp virus (SVCV) is a rhabdovirus of the Sprivivirus genus and the etiological agent of an internationally regulated aquatic animal disease in several fish species, including koi carp Cyprinus carpio L. The virus has a complex lifecycle with both acute and persistent stages of infection and can cause high mortality in affected populations. In this study, the diagnostic repeatability (within laboratory agreement) and reproducibility (between laboratory agreement) of 3 tests were investigated to assess their fitness as SVCV diagnostic tools. The tests, reverse transcription quantitative polymerase chain reaction (RT-qPCR) assays targeting either the SVCV glycoprotein (Q1G) or nucleoprotein (Q2N) genes and virus isolation by cell culture (VI), were performed in a blinded study with four Canadian laboratories. Test panels consisted of duplicate sets of 100 tissue samples collected from 3 SVCV prevalence populations of koi: a low-prevalence negative reference population (n = 20 fish) as well as moderate- (n = 50 fish) and high-prevalence (n = 30 fish) populations of koi experimentally infected with SVCV. The Q1G and Q2N tests were performed with kidney tissue in 3 laboratories and with brain tissue in 1 laboratory whereas pools of kidney, spleen and gill tissues were tested with the VI assay in 2 laboratories. Agreement of binary results was evaluated using the observed proportion of agreement, Cohen's kappa and Gwet's agreement coefficient (AC1) whereas the concordance correlation coefficient (ccc) and Bland Altman's limit of agreement were used to evaluate agreement of the RT-qPCR continuous data. Gwet's AC1 provided a more stable estimate of agreement than Cohen's kappa. Overall, high repeatability (AC1, 0.78-0.90) and reproducibility (AC1, 0.74-0.89) were observed for the Q1G and Q2N tests when kidney tissue was used. Lower agreement estimates of repeatability (AC1, 0.54-0.77) and reproducibility (AC1, 0.50-0.80) were obtained for the VI test. RT-qPCR reproducibility was low with kidney-brain tissue pairs (AC1, 0.09-0.46) and high with inter-test pairs of brain (AC1, 0.76-0.86) or kidney tissue (0.75-0.86). Tissue-specific differences in virus load affected test precision and informed final tissue selection. Repeatability (ccc, 0.94-0.97) and reproducibility (ccc, 0.91-0.97) estimates of agreement for paired continuous data from the RT-qPCR assays were similarly high with kidney tissue and lower with paired brain (ccc, 0.15-0.83) and kidney-brain tissues (ccc, 0.01-0.55). The high precision of Q1G and Q2N with kidney tissue suggests that the tests are performing similarly and are suitable candidates for assessment of their diagnostic accuracy.

Steps of the Replication Cycle of the Viral Haemorrhagic Septicaemia Virus (VHSV) Affecting Its Virulence on Fish

The viral haemorrhagic septicaemia virus (VHSV), a single-stranded negative-sense RNA novirhabdovirus affecting a wide range of marine and freshwater fish species, is a main concern for European rainbow trout (Oncorhynchus mykiss) fish farmers. Its genome is constituted by six genes, codifying five structural and one nonstructural proteins. Many studies have been carried out to determine the participation of each gene in the VHSV virulence, most of them based on genome sequence analysis and/or reverse genetics to construct specific mutants and to evaluate their virulence phenotype. In the present study, we have used a different approach with a similar aim: hypothesizing that a failure in any step of the replication cycle can reduce the virulence in vivo, we studied in depth the in vitro replication of VHSV in different cell lines, using sets of strains from different origins, with high, low and moderate levels of virulence for fish. The results demonstrated that several steps in the viral replication cycle could affect VHSV virulence in fish, including adsorption, RNA synthesis and morphogenesis (including viral release). Notably, differences among strains in any step of the replication cycle were mostly strain-specific and reflected only in part the in vivo phenotype (high and low virulent). Our data, therefore, support the need for further studies aimed to construct completely avirulent VHSV recombinants targeting a combination of genes rather than a single one in order to study the mechanisms of genes interplay and their effect on viral phenotype in vitro and in vivo.

The Viral Hemorrhagic Septicemia Virus (VHSV) Markers of Virulence in Rainbow Trout (Oncorhynchus mykiss)

Viral hemorrhagic septicemia virus (VHSV) is a highly contagious virus leading to high mortality in a large panel of freshwater and marine fish species. VHSV isolates originating from marine fish show low pathogenicity in rainbow trout. The analysis of several nearly complete genome sequences from marine and freshwater isolates displaying varying levels of virulence in rainbow trout suggested that only a limited number of amino acid residues might be involved in regulating the level of virulence. Based on a recent analysis of 55 VHSV strains, which were entirely sequenced and phenotyped in vivo in rainbow trout, several amino acid changes putatively involved in virulence were identified. In the present study, these amino acid changes were introduced, alone or in combination, in a highly-virulent VHSV 23–75 genome backbone by reverse genetics. A total of 35 recombinant VHSV variants were recovered and characterized for virulence in trout by bath immersion. Results confirmed the important role of the NV protein (R116S) and highlighted a major contribution of the nucleoprotein N (K46G and A241E) in regulating virulence. Single amino acid changes in these two proteins drastically affect virus pathogenicity in rainbow trout. This is particularly intriguing for the N variant (K46G) which is unable to establish an active infection in the fins of infected trout, the main portal of entry of VHSV in this species, allowing further spread in its host. In addition, salmonid cell lines were selected to assess the kinetics of replication and cytopathic effect of recombinant VHSV and discriminate virulent and avirulent variants. In conclusion, three major virulence markers were identified in the NV and N proteins. These markers explain almost all phenotypes (92.7%) observed in trout for the 55 VHSV strains analyzed in the present study and herein used for the backward validation of virulence markers. The identification of VHSV specific virulence markers in this species is of importance both to predict the in vivo phenotype of viral isolates with targeted diagnostic tests and to improve prophylactic methods such as the development of safer live-attenuated vaccines.

VHSV Single Amino Acid Polymorphisms (SAPs) Associated With Virulence in Rainbow Trout

The Viral Hemorrhagic Septicemia Virus (VHSV) is an OIE notifiable pathogen widespread in the Northern Hemisphere that encompasses four genotypes and nine subtypes. In Europe, subtype Ia impairs predominantly the rainbow trout industry causing severe rates of mortality, while other VHSV genotypes and subtypes affect a number of marine and freshwater species, both farmed and wild. VHSV has repeatedly proved to be able to jump to rainbow trout from the marine reservoir, causing mortality episodes. The molecular mechanisms regulating VHSV virulence and host tropism are not fully understood, mainly due to the scarce availability of complete genome sequences and information on the virulence phenotype. With the scope of identifying in silico molecular markers for VHSV virulence, we generated an extensive dataset of 55 viral genomes and related mortality data obtained from rainbow trout experimental challenges. Using statistical association analyses that combined genetic and mortality data, we found 38 single amino acid polymorphisms scattered throughout the complete coding regions of the viral genome that were putatively involved in virulence of VHSV in trout. Specific amino acid signatures were recognized as being associated with either low or high virulence phenotypes. The phylogenetic analysis of VHSV coding regions supported the evolution toward greater virulence in rainbow trout within subtype Ia, and identified several other subtypes which may be prone to be virulent for this species. This study sheds light on the molecular basis for VHSV virulence, and provides an extensive list of putative virulence markers for their subsequent validation.

Comparative analysis of marine and freshwater viral haemorrhagic septicaemia virus (VHSV) isolates antagonistic activity

Viral Haemorrhagic Septicaemia Virus (VHSV) isolates virulent to marine fish species can replicate in freshwater species, although producing little or no mortality. Conversely, isolates from freshwater fish do not cause disease in marine species. An inverse relationship between VHSV virulence and host mx gene up-regulation has been described for several fish species, suggesting that differences between the antagonistic activity exerted by these isolates might be involved in the outcome of infections. In this study, the antagonistic activity against the type I interferon system of two representative marine and freshwater VHSV isolates has been characterised using RTG-2 cells stably transfected with the luciferase gene under the control of the Senegalese sole mx (ssmx) promoter, RTG pssmx-luc cells. Both isolates exerted a dose-dependent negative effect on the activation of ssmx promoter, showing a notably different minimal viral dose to exert the antagonism. In particular, an inverse relationship between the minimal MOI required and the viral virulence to sole has been recorded, which suggests this parameter as a possible in vivo VHSV virulence marker. Furthermore, the quantification of the endogenous inf I, mx1 and mx3 mRNA has demonstrated differences between both isolates in their antagonistic activity. Besides, a different nv RNA kinetics, which seems to depend on specific cellular factors, has been recorded for both isolates. This knowledge could contribute to the development of efficient tools to fight against viral infections in fish farming. For that purpose, the RTG pssmx-luc cells may be a suitable in vitro tool to identify the molecular mechanisms underlying VHSV-host interactions.

Isolation and identification of a viral haemorrhagic septicaemia virus (VHSV) isolate from wild largemouth bass Micropterus salmoides in China

Fish rhabdoviruses are a family of viruses responsible for large-scale fish die-offs worldwide. Here, we reported the isolation and identification of a member of rhabdoviruses from wild largemouth bass (Micropterus salmoides) in the coastal area of the Pearl River Estuary, China. This virus isolate was identified as viral haemorrhagic septicaemia virus (VHSV) by specific RT-PCR. Furthermore, the virus (VHSVLB2018) was isolated by cell culture using fathead minnow cells and confirmed by RT-PCR. Electron microscopy showed the presence of bullet-shaped viral particles in the cytoplasm of infected cells. The complete sequencing of VHSVLB2018 confirmed that it was genome configuration typical of rhabdoviruses. Phylogenetic analysis based on whole-genome sequences and G gene nucleotides sequences revealed that VHSVLB2018 was assigned to VHSV genogroup Ⅳa. The pathogenicity of VHSVLB2018 was determined in infection experiments using specific pathogen-free largemouth bass juveniles. VHSVLB2018-infected fish showed typical clinical signs of VHSV disease, including darkened skin, petechial haemorrhages and pale enlarged livers, with the cumulative mortalities reached 63.3%-93.3% by 7 days post-infection. VHSVLB2018 was re-isolated from dead fish and confirmed by RT-PCR. Together, this is the first report of isolation and identification of a VHSV isolate from wild largemouth bass in China.

The Nucleoprotein and Phosphoprotein Are Major Determinants of the Virulence of Viral Hemorrhagic Septicemia Virus in Rainbow Trout

Viral hemorrhagic septicemia virus (VHSV), a fish rhabdovirus, infects several marine and freshwater fish species. There are many strains of VHSV that affect different fish, but some strains of one genetic subgroup have gained high virulence in rainbow trout (Oncorhynchus mykiss). To define the genetic basis of high virulence in trout, we used reverse genetics to create chimeric VHSVs in which viral nucleoprotein (N), P (phosphoprotein), or M (matrix protein) genes, or the N and P genes, were exchanged between a trout-virulent European VHSV strain (DK-3592B) and a trout-avirulent North American VHSV strain (MI03). Testing of the chimeric recombinant VHSV (rVHSV) by intraperitoneal injection in juvenile rainbow trout showed that exchanges of the viral P or M genes had no effect on the trout virulence phenotype of either parental strain. However, reciprocal exchanges of the viral N gene resulted in a partial gain of function in the chimeric trout-avirulent strain (22% mortality) and complete loss of virulence for the chimeric trout-virulent strain (2% mortality). Reciprocal exchanges of both the N and P genes together resulted in complete gain of function in the chimeric avirulent strain (82% mortality), again with complete loss of virulence in the chimeric trout-virulent strain (0% mortality). Thus, the VHSV N gene contains an essential determinant of trout virulence that is strongly enhanced by the viral P gene. We hypothesize that the host-specific virulence mechanism may involve increased efficiency of the viral polymerase complex when the N and P proteins have adapted to more efficient interaction with a host component from rainbow trout.IMPORTANCE Rainbow trout farming is a major food source industry worldwide that has suffered great economic losses due to host jumps of fish rhabdovirus pathogens, followed by evolution of dramatic increases in trout-specific virulence. However, the genetic determinants of host jumps and increased virulence in rainbow trout are unknown for any fish rhabdovirus. Previous attempts to identify the viral genes containing trout virulence determinants of viral hemorrhagic septicemia virus (VHSV) have not been successful. We show here that, somewhat surprisingly, the viral nucleocapsid (N) and phosphoprotein (P) genes together contain the determinants responsible for trout virulence in VHSV. This suggests a novel host-specific virulence mechanism involving the viral polymerase and a host component. This differs from the known virulence mechanisms of mammalian rhabdoviruses based on the viral P or M (matrix) protein.

Olfactory sensory neurons mediate ultrarapid antiviral immune responses in a TrkA-dependent manner

The nervous system regulates host immunity in complex ways. Vertebrate olfactory sensory neurons (OSNs) are located in direct contact with pathogens; however, OSNs' ability to detect danger and initiate immune responses is unclear. We report that nasal delivery of rhabdoviruses induces apoptosis in crypt OSNs via the interaction of the OSN TrkA receptor with the viral glycoprotein in teleost fish. This signal results in electrical activation of neurons and very rapid proinflammatory responses in the olfactory organ (OO), but dampened inflammation in the olfactory bulb (OB). CD8α⁺ cells infiltrate the OO within minutes of nasal viral delivery, and TrkA blocking, but not caspase-3 blocking, abrogates this response. Infiltrating CD8α⁺ cells were TCRαβ T cells with a nonconventional phenotype that originated from the microvasculature surrounding the OB and not the periphery. Nasal delivery of viral glycoprotein (G protein) recapitulated the immune responses observed with the whole virus, and antibody blocking of viral G protein abrogated these responses. Ablation of crypt neurons in zebrafish resulted in increased susceptibility to rhabdoviruses. These results indicate a function for OSNs as a first layer of pathogen detection in vertebrates and as orchestrators of nasal-CNS antiviral immune responses.

Olfactory sensory neurons mediate ultrarapid antiviral immune responses in a TrkA-dependent manner

The nervous system regulates host immunity in complex ways. Vertebrate olfactory sensory neurons (OSNs) are located in direct contact with pathogens; however, OSNs' ability to detect danger and initiate immune responses is unclear. We report that nasal delivery of rhabdoviruses induces apoptosis in crypt OSNs via the interaction of the OSN TrkA receptor with the viral glycoprotein in teleost fish. This signal results in electrical activation of neurons and very rapid proinflammatory responses in the olfactory organ (OO), but dampened inflammation in the olfactory bulb (OB). CD8α⁺ cells infiltrate the OO within minutes of nasal viral delivery, and TrkA blocking, but not caspase-3 blocking, abrogates this response. Infiltrating CD8α⁺ cells were TCRαβ T cells with a nonconventional phenotype that originated from the microvasculature surrounding the OB and not the periphery. Nasal delivery of viral glycoprotein (G protein) recapitulated the immune responses observed with the whole virus, and antibody blocking of viral G protein abrogated these responses. Ablation of crypt neurons in zebrafish resulted in increased susceptibility to rhabdoviruses. These results indicate a function for OSNs as a first layer of pathogen detection in vertebrates and as orchestrators of nasal-CNS antiviral immune responses.

The glycoprotein, non-virion protein, and polymerase of viral hemorrhagic septicemia virus are not determinants of host-specific virulence in rainbow trout

Background

Viral hemorrhagic septicemia virus (VHSV), a fish rhabdovirus belonging to the Novirhabdovirus genus, causes severe disease and mortality in many marine and freshwater fish species worldwide. VHSV isolates are classified into four genotypes and each group is endemic to specific geographic regions in the north Atlantic and Pacific Oceans. Most viruses in the European VHSV genotype Ia are highly virulent for rainbow trout (Oncorhynchus mykiss), whereas, VHSV genotype IVb viruses from the Great Lakes region in the United States, which caused high mortality in wild freshwater fish species, are avirulent for trout. This study describes molecular characterization and construction of an infectious clone of the virulent VHSV-Ia strain DK-3592B from Denmark, and application of the clone in reverse genetics to investigate the role of selected VHSV protein(s) in host-specific virulence in rainbow trout (referred to as trout-virulence).

Methods

Overlapping cDNA fragments of the DK-3592B genome were cloned after RT-PCR amplification, and their DNA sequenced by the di-deoxy chain termination method. A full-length cDNA copy (pVHSVdk) of the DK-3592B strain genome was constructed by assembling six overlapping cDNA fragments by using natural or artificially created unique restriction sites in the overlapping regions of the clones. Using an existing clone of the trout-avirulent VHSV-IVb strain MI03 (pVHSVmi), eight chimeric VHSV clones were constructed in which the coding region(s) of the glycoprotein (G), non-virion protein (NV), G and NV, or G, NV and L (polymerase) genes together, were exchanged between the two clones. Ten recombinant VHSVs (rVHSVs) were generated, including two parental rVHSVs, by transfecting fish cells with ten individual full-length plasmid constructs along with supporting plasmids using the established protocol. Recovered rVHSVs were characterized for viability and growth in vitro and used to challenge groups of juvenile rainbow trout by intraperitoneal injection.

Results

Complete sequence of the VHSV DK-3592B genome was determined from the cloned cDNA and deposited in GenBank under the accession no. KC778774. The trout-virulent DK-3592B genome (genotype Ia) is 11,159 nt in length and differs from the trout-avirulent MI03 genome (pVHSVmi) by 13% at the nucleotide level. When the rVHSVs were assessed for the trout-virulence phenotype in vivo, the parental rVHSVdk and rVHSVmi were virulent and avirulent, respectively, as expected. Four chimeric rVHSVdk viruses with the substitutions of the G, NV, G and NV, or G, NV and L genes from the avirulent pVHSVmi constructs were still highly virulent (100% mortality), while the reciprocal four chimeric rVHSVmi viruses with genes from pVHSVdk remained avirulent (0–10% mortality).

Conclusions

When chimeric rVHSVs, containing all the G, NV, and L gene substitutions, were tested in vivo, they did not exhibit any change in trout-virulence relative to the background clones. These results demonstrate that the G, NV and L genes of VHSV are not, by themselves or in combination, major determinants of host-specific virulence in trout.

Virulence marker candidates in N-protein of viral haemorrhagic septicaemia virus (VHSV): virulence variability within VHSV Ib clones

Four major genotypes of viral haemorrhagic septicaemia virus (VHSV), which have been isolated from many marine and freshwater fish species, are known to differ in virulence. While fast and low-cost genotyping systems based on monoclonal antibodies (MAbs) have been developed for typing of VHSV virulence, there is a need for supplementing the knowledge. In particular, 2 field isolates from viral haemorrhagic septicaemia (VHS) outbreaks in sea-reared rainbow trout Oncorhynchus mykiss in Sweden, SE-SVA-14 and SE-SVA-1033 (both genotype Ib), have yielded contradictory reactions. In the present study, upon cloning by limited dilution, both isolates appeared to be heterogeneous in terms of reactivity with nucleo (N)-protein-specific MAbs as well their gene sequences. Infection trials in rainbow trout further revealed differences in the virulence of these virus clones derived from the same primary isolate. Based on a comparative analysis of the entire genome of the clones tested, we suggest that the differences in virulence are tentatively linked to substitutions of amino acids (aa) in the N-protein region covered by aa 43-46 and aa position 168, or a combination of the two. The fact that such minor naturally occurring genetic differences affect the virulence implies that even low-virulent VHSV isolates in the marine environment should be considered as a potential threat for the trout farming industry. The described MAbs can represent useful tools for initial risk assessment of disease outbreaks in farmed trout by marine VHSV isolates.

Complete sequences of 4 viral hemorrhagic septicemia virus IVb isolates and their virulence in northern pike fry

Four viral hemorrhagic septicemia virus (VHSV) genotype IVb isolates were sequenced, their genetic variation explored, and comparative virulence assayed with experimental infections of northern pike Esox lucius fry. In addition to the type strain MI03, the complete 11183 bp genome of the first round goby Neogobius melanostomus isolate from the St. Lawrence River, and the 2013 and 2014 isolates from gizzard shad Dorosoma cepedianum die-offs in Irondequoit Bay, Lake Ontario and Dunkirk Harbor, Lake Erie were all deep sequenced on an Illumina platform. Mutations documented in the 11 yr since the MI03 index case from Lake St. Clair muskellunge Esox masquinongy showed 87 polymorphisms among the 4 isolates. Twenty-six mutations were non-synonymous and located at 18 different positions within the matrix protein, glycoprotein, non-virion protein, and RNA polymerase genes. The same 4 isolates were used to infect northern pike fry by a single 1 h bath exposure. Cumulative percent mortality varied from 42.5 to 62.5%. VHSV was detected in 57% (41/72) of the survivors at the end of the 21-d trial, suggesting that the virus was not rapidly cleared. Lesions were observed in many of the moribund and dead northern pike, such as hemorrhaging in the skin and fins, as well as hydrocephalus. Mean viral load measured from the trunk and visceral tissues of MI03-infected pike was significantly higher than the quantities detected in fish infected with the most recent isolates of genotype IVb, but there were no differences in cumulative mortality observed.

Atlantic salmon endothelial cells from the heart were more susceptible than fibroblasts from the bulbus arteriosus to four RNA viruses but protected from two viruses by dsRNA pretreatment

Heart diseases caused by viruses are major causes of Atlantic salmon aquaculture loss. Two Atlantic salmon cardiovascular cell lines, an endothelial cell line (ASHe) from the heart and a fibroblast cell line (BAASf) from the bulbus arteriosus, were evaluated for their response to four fish viruses, CSV, IPNV, VHSV IVa and VHSV IVb, and the innate immune agonist, double-stranded RNA mimic poly IC. All four viruses caused cytopathic effects in ASHe and BAASf. However, ASHe was more susceptible to all four viruses than BAASf. When comparing between the viruses, ASHe cells were found to be moderately susceptible to CSV and VHSV IVb, but highly susceptible to IPNV and VHSV IVa induced cell death. All four viruses were capable of propagating in the ASHe cell line, leading to increases in virus titre over time. In BAASf, CSV and IPNV produced more than one log increase in titre from initial infection, but VHSV IVb and IVa did not. When looking at the antiviral response of both cell lines, Mx proteins were induced in ASHe and BAASf by poly IC. All four viruses induced Mx proteins in BAASf, while only CSV and VHSV IVb induced Mx proteins in ASHe. IPNV and VHSV IVa suppressed Mx proteins expression in ASHe. Pretreatment of ASHe with poly IC to allow for Mx proteins accumulation protected the culture from subsequent infections with IPNV and VHSV IVa, resulting in delayed cell death, reduced virus titres and reduced viral proteins expression. These data suggest that endothelial cells potentially can serve as points of infections for viruses in the heart and that two of the four viruses, IPNV and VHSV IVa, have mechanisms to avoid or downregulate antiviral responses in ASHe cells. Furthermore, the high susceptibility of the ASHe cell line to IPNV and VHSV IVa can make it a useful tool for studying antiviral compounds against these viruses and for general detection of fish viruses.

The Promise of Whole Genome Pathogen Sequencing for the Molecular Epidemiology of Emerging Aquaculture Pathogens

Aquaculture is the fastest growing food-producing sector, and the sustainability of this industry is critical both for global food security and economic welfare. The management of infectious disease represents a key challenge. Here, we discuss the opportunities afforded by whole genome sequencing of bacterial and viral pathogens of aquaculture to mitigate disease emergence and spread. We outline, by way of comparison, how sequencing technology is transforming the molecular epidemiology of pathogens of public health importance, emphasizing the importance of community-oriented databases and analysis tools.