Molecular Evolution and Phylogeography of Co-circulating IHNV and VHSV in Italy

Dual RNA-seq analyses of viral hemorrhagic septicemia virus and olive flounder (Paralichthys olivaceus) interactions at low and high water temperatures

Transcriptomic studies in fish have considerably contributed to understanding the host responses under viral hemorrhagic septicemia virus (VHSV) infection. However, changes from the perspective of the pathogen and host-pathogen interactions have been relatively underestimated. Given that VHSV genes can be poly-adenylated during replication, this study investigated the global changes from both VHSV and host perspectives using the host transcriptomic data. To achieve this, we utilized transcriptomic data from VHSV infected flounder obtained at different water temperatures (13 °C and 20 °C). We collected the sequence reads belonging to VHSV through an bioinformatic pipeline developed especially for this study. The VHSV reads were used to construct the consensus reference genome and to investigate the expression of VHSV genes and the frequency of variants under different water temperatures. In addition, both linear and logarithmic scales of VHSV transcription levels, along with host transcriptomes, were used to understand pathogen-host interaction through weighted correlation network analysis (WGCNA). The results revealed that VHSV transcription can exceed 5% of host transcriptome during the infection. Single nucleotide variants (SNVs) appeared more frequently in 13 °C groups than in 20 °C groups. While VHSV can replicate at both 13 °C and 20 °C, host transcriptomic responses were notably different, with stronger immune responses and more frequent VHSV genetic changes observed at a lower temperature. This suggests that VHSV infection at low water temperatures significantly influences both the host transcriptional changes and pathogen. Through WGCNA, numerous genes in the module that correlated with VHSV reads on a linear scale were found to be related to cytoskeleton modulation and viral activity. By contrast, the gene module (black) correlated with VHSV reads on a logarithmic scale was strongly associated with host immune responses, such as TNF signaling, necroptosis, and the NF-kappa B signaling pathway. The dual RNA-seq approaches developed in this study will immensely enhance our understanding of host-pathogen interactions across different temperatures.

Heterologous Exchanges of Glycoprotein and Non-Virion Protein in Novirhabdoviruses: Assessment of Virulence in Yellow Perch (Perca flavescens) and Rainbow Trout (Oncorhynchus mykiss)

Infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV) are rhabdoviruses in two different species belonging to the Novirhabdovirus genus. IHNV has a narrow host range restricted to trout and salmon species, and viruses in the M genogroup of IHNV have high virulence in rainbow trout (Oncorhynchus mykiss). In contrast, the VHSV genotype IVb that invaded the Great Lakes in the United States has a broad host range, with high virulence in yellow perch (Perca flavescens), but not in rainbow trout. By using reverse-genetic systems of IHNV-M and VHSV-IVb strains, we generated six IHNV:VHSV chimeric viruses in which the glycoprotein (G), non-virion-protein (NV), or both G and NV genes of IHNV-M were replaced with the analogous genes from VHSV-IVb, and vice versa. These chimeric viruses were used to challenge groups of rainbow trout and yellow perch. The parental recombinants rIHNV-M and rVHSV-IVb were highly virulent in rainbow trout and yellow perch, respectively. Parental rIHNV-M was avirulent in yellow perch, and chimeric rIHNV carrying G, NV, or G and NV genes from VHSV-IVb remained low in virulence in yellow perch. Similarly, the parental rVHSV-IVb exhibited low virulence in rainbow trout, and chimeric rVHSV with substituted G, NV, or G and NV genes from IHNV-M remained avirulent in rainbow trout. Thus, the G and NV genes of either virus were not sufficient to confer high host-specific virulence when exchanged into a heterologous species genome. Some exchanges of G and/or NV genes caused a loss of host-specific virulence, providing insights into possible roles in viral virulence or fitness, and interactions between viral proteins.

The antiviral effects of TRIM23 and TRIM32 proteins in rainbow trout (Oncorhynchus mykiss)

TRIM proteins play a crucial antiviral effector role in the innate immune system of vertebrates. In this study, we found that TRIM proteins exhibited the highest expression levels in immune organs such as spleen and kidney during IHNV infection in rainbow trout, meanwhile, we successfully amplified TRIM23 and TRIM32 from diseased rainbow trout and analyzed their gene sequences, revealing that rainbow trout TRIM23 and TRIM32 proteins are closely related to Atlantic salmon and Chinook salmon; In this experiment, the TRIM23 and TRIM32 protein genes were resoundingly constructed as a recombinant plasmids and expressed in CHSE-214 cells. Upon transfected with the recombinant plasmid, followed by viral infection, significant decreasion in the copy numbers of the virus was observed, indicating that the TRIM23 and TRIM32 proteins of rainbow trout play an important role in inhibiting virus replication, with the TRIM32 role being the most pronounced. These results provide a basis for subsequent in-depth study of the antiviral effects of TRIM proteins, and provide new ideas for immune enhancers.

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.

Efficacy of DNA Vaccines in Protecting Rainbow Trout against VHS and IHN under Intensive Farming Conditions

Despite the negative impact of viral hemorrhagic septicemia (VHS) and infectious hematopoietic necrosis (IHN) on European rainbow trout farming, no vaccines are commercially available in Europe. DNA vaccines are protective under experimental conditions, but testing under intensive farming conditions remains uninvestigated. Two DNA vaccines encoding the glycoproteins (G) of recent Italian VHSV and IHNV isolates were developed and tested for potency and safety under experimental conditions. Subsequently, a field vaccination trial was initiated at a disease-free hatchery. The fish were injected intramuscularly with either the VHS DNA vaccine or with a mix of VHS and IHN DNA vaccines at a dose of 1 µg/vaccine/fish, or with PBS. At 60 days post-vaccination, fish were moved to a VHSV and IHNV infected facility. Mortality started 7 days later, initially due to VHS. After 3 months, IHN became the dominant cause of disease. Accordingly, both DNA vaccinated groups displayed lower losses compared to the PBS group during the first three months, while the VHS/IHN vaccinated group subsequently had the lowest mortality. A later outbreak of ERM caused equal disease in all groups. The trial confirmed the DNA vaccines to be safe and efficient in reducing the impact of VHS and IHN in farmed rainbow trout.

Alterations of the Mucosal Immune Response and Microbial Community of the Skin upon Viral Infection in Rainbow Trout (Oncorhynchus mykiss)

The skin is the largest organ on the surface of vertebrates, which not only acts as the first line of defense against pathogens but also harbors diverse symbiotic microorganisms. The complex interaction between skin immunity, pathogens, and commensal bacteria has been extensively studied in mammals. However, little is known regarding the effects of viral infection on the skin immune response and microbial composition in teleost fish. In this study, we exposed rainbow trout (Oncorhynchus mykiss) to infectious hematopoietic necrosis virus (IHNV) by immersion infection. Through pathogen load detection and pathological evaluation, we confirmed that IHNV successfully invaded the rainbow trout, causing severe damage to the epidermis of the skin. qPCR analyses revealed that IHNV invasion significantly upregulated antiviral genes and elicited strong innate immune responses. Transcriptome analyses indicated that IHNV challenge induced strong antiviral responses mediated by pattern recognition receptor (PRR) signaling pathways in the early stage of the infection (4 days post-infection (dpi)), and an extremely strong antibacterial immune response occurred at 14 dpi. Our 16S rRNA sequencing results indicated that the skin microbial community of IHNV-infected fish was significantly richer and more diverse. Particularly, the infected fish exhibited a decrease in Proteobacteria accompanied by an increase in Actinobacteria. Furthermore, IHNV invasion favored the colonization of opportunistic pathogens such as Rhodococcus and Vibrio on the skin, especially in the later stage of infection, leading to dysbiosis. Our findings suggest that IHNV invasion is associated with skin microbiota dysbiosis and could thus lead to secondary bacterial infection.

Increased virulence of Italian infectious hematopoietic necrosis virus (IHNV) associated with the emergence of new strains

Infectious hematopoietic necrosis virus (IHNV) is the causative agent of IHN triggering a systemic syndrome in salmonid fish. Although IHNV has always been associated with low levels of mortality in Italian trout farming industries, in the last years trout farmers have experienced severe disease outbreaks. However, the observed increasing virulence of IHNV is still based on empirical evidence due to the poor and often confounding information from the field. Virulence characterization of a selection of sixteen Italian isolates was performed through in vivo challenge of juvenile rainbow trout to confirm field evidence. The virulence of each strain was firstly described in terms of cumulative mortality and survival probability estimated by Kaplan-Meier curves. Furthermore, parametric survival models were applied to analyze the mortality rate profiles. Hence, it was possible to characterize the strain-specific mortality peaks and to relate their topology to virulence and mortality. Indeed, a positive correlation between maximum mortality probability and virulence was observed for all the strains. Results also indicate that more virulent is the strain, the earliest and narrowest is the mortality peak. Additionally, intra-host viral quantification determined in dead animals showed a significant correlation between viral replication and virulence. Whole-genome phylogeny conducted to determine whether there was a relation between virulence phenotype and IHNV genetics evidenced no clear clustering according to phenotype. Moreover, a root-to-tip analysis based on genetic distances and sampling date of Italian IHNV isolates highlighted a relevant temporal signal indicating an evolving nature of the virus, over time, with the more virulent strains being the more recent ones. This study provides the first systematic characterization of Italian IHNV's virulence. Overall results confirm field data and point out an abrupt increase in IHNV virulence, with strains from 2015-2019 showing moderate to high virulence in rainbow trout. Further investigations are needed in order to extensively clarify the relation between evolution and virulence of IHNV and investigate the genetic determinants of virulence of this viral species in rainbow trout.

Genomic and immunogenic changes of Piscine novirhabdovirus (Viral Hemorrhagic Septicemia Virus) over its evolutionary history in the Laurentian Great Lakes

A unique and highly virulent subgenogroup (-IVb) of Piscine novirhabdovirus, also known as Viral Hemorrhagic Septicemia Virus (VHSV), suddenly appeared in the Laurentian Great Lakes, causing large mortality outbreaks in 2005 and 2006, and affecting >32 freshwater fish species. Periods of apparent dormancy have punctuated smaller and more geographically-restricted outbreaks in 2007, 2008, and 2017. In this study, we conduct the largest whole genome sequencing analysis of VHSV-IVb to date, evaluating its evolutionary changes from 48 isolates in relation to immunogenicity in cell culture. Our investigation compares genomic and genetic variation, selection, and rates of sequence changes in VHSV-IVb, in relation to other VHSV genogroups (VHSV-I, VHSV-II, VHSV-III, and VHSV-IVa) and with other Novirhabdoviruses. Results show that the VHSV-IVb isolates we sequenced contain 253 SNPs (2.3% of the total 11,158 nucleotides) across their entire genomes, with 85 (33.6%) of them being non-synonymous. The most substitutions occurred in the non-coding region (NCDS; 4.3%), followed by the Nv- (3.8%), and M- (2.8%) genes. Proportionally more M-gene substitutions encoded amino acid changes (52.9%), followed by the Nv- (50.0%), G- (48.6%), N- (35.7%) and L- (23.1%) genes. Among VHSV genogroups and subgenogroups, VHSV-IVa from the northeastern Pacific Ocean has shown the fastest substitution rate (2.01x10^-3), followed by VHSV-IVb (6.64x10^-5) and by the VHSV-I, -II and-III genogroups from Europe (4.09x10^-5). A 2016 gizzard shad (Dorosoma cepedianum) from Lake Erie possessed the most divergent VHSV-IVb sequence. The in vitro immunogenicity analysis of that sample displayed reduced virulence (as did the other samples from 2016), in comparison to the original VHSV-IVb isolate (which had been traced back to 2003, as an origin date). The 2016 isolates that we tested induced milder impacts on fish host cell innate antiviral responses, suggesting altered phenotypic effects. In conclusion, our overall findings indicate that VHSV-IVb has undergone continued sequence change and a trend to lower virulence over its evolutionary history (2003 through present-day), which may facilitate its long-term persistence in fish host populations.

Molecular Epidemiology of Novirhabdoviruses Emerging in Iranian Trout Farms

Novirhabdoviruses cause large epizootics and economic losses of farmed trout. In this study, we surveyed Viral hemorrhagic septicemia virus and Infectious hematopoietic and necrosis virus (VHSV and IHNV) through both monitoring and investigation of clinical outbreaks reported by farmers in the regions with major rainbow trout production in Iran from 2015 to 2019. RT-PCR assays of the kidney samples and cell culture (EPC/FHM cells) samples confirmed the presence of the viruses, with 9 VHSV and 4 IHNV isolates, in both endemic and new areas of Iran. Sequence analysis of the G gene revealed that VHSV isolates belonged to genogroup Ia, and IHNV isolates were clustered into genogroup E, both typical for isolates from European countries. A haplotype analysis based on non-homologous amino acids of the G gene supports the emergence of two lineages of IHNV from clade 1 (E-1), as well as VHSV clade 2 (Ia-2) of the European genogroups, confirming that VHSV and IHNV isolates in Iran, have originated from Europe possibly via imported eggs.

Evolutionary trajectory of fish Piscine novirhabdovirus (=Viral Hemorrhagic Septicemia Virus) across its Laurentian Great Lakes history: Spatial and temporal diversification

Piscine novirhabdovirus = Viral Hemorrhagic Septicemia Virus (VHSV) first appeared in the Laurentian Great Lakes with large outbreaks from 2005 to 2006, as a new and novel RNA rhabdovirus subgenogroup (IVb) that killed >30 fish species. Interlude periods punctuated smaller more localized outbreaks in 2007, 2010, and 2017, although some fishes tested positive in the intervals. There have not been reports of outbreaks or positives from 2018, 2019, or 2020. Here, we employ a combined population genetics and phylogenetic approach to evaluate spatial and temporal evolutionary trajectory on its G-gene sequence variation, in comparison with whole-genome sequences (11,083 bp) from a subset of 44 individual isolates (including 40 newly sequenced ones). Our results show that IVb (N = 184 individual fish isolates) diversified into 36 G-gene haplotypes from 2003 to 2017, stemming from two originals ("a" and "b"). G-gene haplotypes "a" and "b" differed by just one synonymous single-nucleotide polymorphism (SNP) substitution, remained the most abundant until 2011, then disappeared. Group "a" descendants (14 haplotypes) remained most prevalent in the Upper and Central Great Lakes, with eight (51%) having nonsynonymous substitutions. Group "b" descendants primarily have occurred in the Lower Great Lakes, including 22 haplotypes, of which 15 (68%) contained nonsynonymous changes. Evolutionary patterns of the whole-genome sequences (which had 34 haplotypes among 44 isolates) appear congruent with those from the G-gene. Virus populations significantly diverged among the Upper, Central, and Lower Great Lakes, diversifying over time. Spatial divergence was apparent in the overall patterns of nucleotide substitutions, while amino acid changes increased temporally. VHSV-IVb thus significantly differentiated across its less than two decades in the Great Lakes, accompanied by declining outbreaks and virulence. Continuing diversification likely allowed the virus to persist at low levels in resident fish populations, and may facilitate its potential for further and future spread to new habitats and nonacclimated hosts.

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.

Health Surveillance of Wild Brown Trout (Salmo trutta fario) in the Czech Republic Revealed a Coexistence of Proliferative Kidney Disease and Piscine Orthoreovirus-3 Infection

The population of brown trout (Salmo trutta fario) in continental Europe is on the decline, with infectious diseases confirmed as one of the causative factors. However, no data on the epizootiological situation of wild fish in the Czech Republic are currently available. In this study, brown trout (n = 260) from eight rivers were examined for the presence of viral and parasitical pathogens. Salmonid alphavirus-2, infectious pancreatic necrosis virus, piscine novirhabdovirus (VHSV) and salmonid novirhabdovirus (IHNV) were not detected using PCR. Cell culturing showed no viruses as well, and serological analysis of 110 sera did not detect any specific antibodies against VHSV or IHNV. Fish from two rivers were positive for the presence of piscine orthoreovirus-3 (PRV-3), subtype PRV-3b. However, none of the PRV-3-positive fish showed gross pathologies typically associated with PRV infections. By far the most widespread pathogen was Tetracapsuloides bryosalmonae which was confirmed in each of the examined locations, with a prevalence of up to 65% and 100%, as established by immunohistochemistry and PCR, respectively. Furthermore, up to 43.8% of fish showed signs of proliferative kidney disease caused by T. bryosalmonae, suggesting that this parasite is a main health challenge for brown trout in the Czech Republic.

Comparative effects of Novirhabdovirus genes on modulating constitutive transcription and innate antiviral responses, in different teleost host cell types

BACKGROUND

Infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV) are highly contagious, pathogenic Novirhabdoviruses affecting fish and are thusly notifiable diseases with the World Organization for Animal Health. This study assessed the relative capacities of IHNV and VHSV genes to modulate host general transcription and explores the abilities of specific IHNV genes to interfere with the interferon pathway in heterogenous teleost cell-lines.

METHODS

Optimized protocols allowed for efficient transient transfections in EPC, BF-2, RTG-2 and RTgill-W1 cell lines of plasmids encoding IHNV (M genogroup) and VHSV (-IVb genotype) genes, including N, P, M, G and NV. Their impact on general cellular transcription was measured 48 hours post transfection (hpt) with luciferase constructs driven by a modified β-Actin promoter (pCAG). Their modulation of the innate antiviral immune response was characterized 72 hpt, using luciferase constructs measuring rainbow trout Type I IFN or MX-1 promoter augmentation, upon MAVS co-transfection.

RESULTS

M was generally confirmed as the strongest constitutive transcriptional suppressor while IHNV P, but not VHSV P, augmented constitutive transcription in fibroblastic cell types. Cell-specific effects were observed for viral G gene, with VHSV G exhibiting suppression of basal transcription in EPC and BF-2 but not in trout cells; while IHNV G was stimulatory in RTG-2, but inhibitory in RTgill-W1. NV consistently stimulated constitutive transcription, with higher augmentation patterns seen in fibroblastic compared to epithelial cells, and for IHNV NV compared to VHSV NV. The innate antiviral immune response, focusing on the IFN pathway, was silenced by IHNV M in all cell lines tested. IHNV N showed a dose-dependent suppression of type I IFN, but with minor effects on MX-1. IHNV P and G played minor IFN-inhibitory roles, consistent and dose-dependent only for G in rainbow trout cells. IHNV NV mediated a consistent stimulatory effect on either Type I IFN or MX-1, but much less pronounced in RTgill-W1.

CONCLUSIONS

This study extends our understanding of Novirhabdoviruses-host interaction, showing differential innate immune responses in heterogenous cell types. Viral regulators of innate immune signaling are identified, either as dose-dependent suppressors (such as M and N) or stimulators (mainly NV), indicating novel targets for the design of more efficient vaccination strategies.

Transient expression of the full-length glycoprotein from infectious hematopoietic necrosis virus in bean (Phaseolus vulgaris) leaves via agroinfiltration

The glycoprotein of infectious hematopoietic necrosis virus (IHNV), the causative agent of acute disease in salmonids, is the only structural protein of the virus that can induce protective immunity in the fish host. Here, the reliability of bean (Phaseolus vulgaris) plant for the production of this viral protein was examined by the transient expression method. Using the syringe agroinfiltration method, leaves of bean plants were transformed with the expression construct encoding the full-length of IHNV glycoprotein (IHNV-G) gene. Furthermore, the transformation efficacy of two infiltration buffers including PBS-A (PBS+acetosyringone) and MMS-A (MES buffer + MgSO₄  + sucrose + acetosyringone) was compared. The analysis of mRNA and dot-blot assay confirmed the transcription and translation of IHNV-G protein in bean leaves. Moreover, Western blotting verified the production of intact, full-length (∼57 kDa) IHNV-G protein in the agroinfiltrated plants. Of note, the production level of IHNV-G using MMS-A agroinfiltration buffer was approximately five times higher compared to PBS-A buffer (0.48 vs. 0.1% of total soluble protein), indicating the effect of infiltration buffer on the transient transformation efficiency. The recombinant protein was purified at the final yield of 0.35 μg/g of fresh leaf tissue, using nickel affinity chromatography. The present work is the first report describing the feasibility of the plant expression platform for the production of IHNV-G protein, which can be served as an oral vaccine against IHNV infection.

Genome and Phylogenetic Analysis of Infectious Hematopoietic Necrosis Virus Strain SNU1 Isolated in Korea

Infectious hematopoietic necrosis virus (IHNV), one of the most important pathogenic fish viruses, affects trout fisheries and causes considerable economic losses. Currently, in Korea, more studies on IHNV infection are being reported. However, relatively less data is available on Korean isolates than on those from other countries. Few studies have focused on gene sequence analyses of IHNV glycoprotein (G) gene and almost none have focused on other gene fragments. Therefore, considering the dearth of adequate phylogenetic and genomic studies on Korean IHNV strains because of the lack of data, our study aimed to provide sufficient relevant data by sequencing the complete genome of the IHNV strain SNU1, which was recently isolated from a Korean rainbow trout farm. Moreover, we focused on expanding the perspectives on the phylogenesis of IHNV isolates from Korea and other Asian countries. IHNV was isolated from pooled hematopoietic tissue samples using Epithelioma papulosum cyprinid (EPC) cells, and phylogenetic analysis and genome study were conducted using complete G, N, and nonvirion (NV) gene sequences. Our main achievements were the development of a phylogenetic analytical method based on the NV gene and complete genome sequence analysis of the IHNV strain SNU1, which was compared with other Asian isolate sequences.

Molecular evolution and selection pressure analysis of infectious hematopoietic necrosis virus (IHNV) revealed the origin and phylogenetic relationship of Iranian isolates in recent epidemics in Iran

Infectious hematopoietic necrosis virus (IHNV) is the causative agent for a lethal salmonid disease. In this study, we surveyed the IHNV's epidemiology, diversity and the origin of infection in Iran. Phylogenetic analysis revealed that Iranian isolates belonged to one of the two lineages of E genogroup. Subsequently, a combination of phylogenetic, antigenic and structural analysis was performed to investigate the evolution of E genogroup lineages. Site-specific analysis of the viral glycoprotein showed different co-evolving and positively selected sites in each lineage. Most of these sites were mapped to the predicted antigenic patches of the glycoprotein. Further characterization revealed E lineages can be differentiated, in part, by specific mutations at positions 91 and 130, which are located in the structurally flexible regions of the glycoprotein, suggesting a key adaptative role for these sites. These data may assist in better monitoring the emerging isolates in regions infected to IHNV from E genogroup.

Improvement of a diagnostic procedure in surveillance of the listed fish diseases IHN and VHS

Infectious haematopoietic necrosis (IHN) and viral haemorrhagic septicaemia (VHS) are OIE-listed and notifiable viral fish diseases which are controlled by eradication and surveillance programmes globally. The present study provides improved RT-qPCR procedures based on recently described OIE protocols. Improvements comprise the design of a new TaqMan® probe, replacing a TaqMan® MGB probe that turned out to show impaired binding. Reason for this is SNPs detected in the nucleoprotein N gene sequences of IHNV strains targeted by the RT-qPCR. Furthermore, the IHNV and VHSV RT-qPCR assays were realized as one-step and one-run procedures supplemented by an endogenous control system. The IHNV and VHSV RT-qPCR assays are characterized by a technical sensitivity of 19 and 190 gene equivalents (cRNA) and an analytical sensitivity of 2-7 and 13 TCID₅₀ /ml, respectively. For verification purposes, 105 IHNV and 165 VHSV isolates and several non-targeted viral and bacterial pathogens were included and returned adequate results. However, in field samples divergent results left 14 samples of 154 undetected for IHNV and one sample of 127 for VHSV using cell culture. The study shows that RT-qPCR assays ensure facilitated and reliable testing on IHNV and VHSV in eradication and surveillance programmes.

Phylogeography and evolution of infectious hematopoietic necrosis virus in China

Infectious hematopoietic necrosis virus (IHNV) is a well-known rhabdoviral pathogen of salmonid fish. In this study, a comprehensive analysis of 40 IHNV viruses isolated from thirteen fish farms in nine geographically dispersed Chinese provinces during 2012 to 2017 is presented. Identity of nucleotide and amino acid sequences among all the complete glycoprotein (G) genes from Chinese isolates was 98.0-100% and 96.7-100%, respectively. Coalescent phylogenetic analyses revealed that all the Chinese IHN virus characterized in this study were in a monophyletic clade that had a most recent common ancestor with the J Nagano (JN) subgroup within the J genogroup of IHNV. Within the Chinese IHNV clade isolates obtained over successive years from the same salmon fish farm clustered in strongly supported subclades, suggesting maintenance and diversification of virus over time within individual farms. There was also evidence for regional virus transmission within provinces, and some cases of longer distance transmission between distant provinces, such as Gansu and Yunnan. The data demonstrated that IHNV has evolved into a new subgroup in salmon farm environments in China, and IHNV isolates are undergoing molecular evolution within fish farms. We suggest that Chinese IHNV comprises a separate JC subgroup within the J genogroup of IHNV.

Low evolutionary rate of infectious pancreatic necrosis virus (IPNV) in Italy is associated with reduced virulence in trout

Infectious pancreatic necrosis virus (IPNV) is a naked double-stranded RNA virus with a bi-segmented genome that is classified within the family Birnaviridae, genus Aquabirnavirus. IPNV was first detected in Italian trout farms in the late 1970s and ultimately became endemic. To characterize the evolution of IPNV circulating in Italy, particularly whether there is a link between evolutionary rate and virulence, we obtained and analyzed the VP1 (polymerase) and the pVP2 (major capsid protein precursor) sequences from 75 IPNV strains sampled between 1978 and 2017. These data revealed that the Italian IPNV exhibit relatively little genetic variation over the sampling period, falling into four genetic clusters within a single genogroup (group 2 for VP1 and genogroup V for pVP2) and contained one example of inter-segment reassortment. The mean evolutionary rates for VP1 and pVP2 were estimated to be 1.70 and 1.45 × 10⁻⁴ nucleotide substitutions per site, per year, respectively, and hence significantly lower than those seen in other Birnaviruses. Similarly, the relatively low ratios of non-synonymous (d_N) to synonymous (d_S) nucleotide substitutions per site in both genes indicated that IPNV was subject to strong selective constraints, again in contrast to other RNA viruses infecting salmonids that co-circulate in the same area during the same time period. Notably, all the Italian IPNV harbored a proline at position 217 (P₂₁₇) and a threonine at position 221 (T₂₂₁) in pVP2, both of which are associated with a low virulence phenotype. We therefore suggest the lower virulence of IPNV may have resulted in reduced rates of virus replication and hence lower rates of evolutionary change. The data generated here will be of importance in understanding the factors that shape the evolution of Aquabirnaviruses in nature.

Molecular evolution and phylogeography of infectious hematopoietic necrosis virus with a focus on its presence in France over the last 30 years

Infectious hematopoietic necrosis virus (IHNV) is among the most important pathogens affecting the salmonid industry. Here, we investigated the molecular evolution and circulation of isolates from 11 countries or regions all over the world, with a special focus on the epidemiological situation in France. The phylogeography, time to the most recent common ancestor (TMRCA) and nucleotide substitution rate were studied using 118 full-length glycoprotein gene sequences isolated from 9 countries (5 genogroups) over a period of 47 years. The TMRCA dates back to 1943, with the L genogroup identified as the likely root (67 %), which is consistent with the first report of this pathogen in the USA. A Bayesian inference approach was applied to the partial glycoprotein gene sequences of 88 representative strains isolated in France over the period 1987-2015. The genetic diversity of these 88 sequences showed mean nucleotide and amino-acid identities of 97.1 and 97.8 %, respectively, and a d _N/d _S ratio (non-synonymous to synonymous mutations) of 0.25, indicating purifying selection. The French viral populations are divided into eight sub-clades and four individual isolates, with a clear spatial differentiation, suggesting the predominant role of local reservoirs in contamination. The atypical 'signatures' of some isolates underlined the usefulness of molecular phylogeny for epidemiological investigations that track the spread of IHNV.

Phylogeny of the infectious hematopoietic necrosis virus in European aquaculture

Infectious hematopoietic necrosis (IHN)-a highly lethal infectious salmonid disease-has caused substantial economic losses in the European production of rainbow trout (Oncorhynchus mykiss) since the late 1980s. The causal agent of IHN is the IHN virus (IHNV) introduced from overseas. However, until today, its phylogeographic spread in Europe remains poorly understood. We therefore sought to elucidate this unresolved topic by using the largest ever compiled dataset of European IHNV isolates (E isolates) (193 GenBank E isolates and 100 isolates from this study) for the complete glycoprotein (G) gene sequence. Our results clearly revealed that the active trout trade has left its traces in the E phylogeny. For example, the spread by trade of IHNV-infected trout was apparently the cause for the exposure of the E lineage to different local scenarios of selection and genetic drift, and therefore has led to the split of this lineage into various subordinated lineages. Accordingly, we also found evidence for E isolates being mixed Europe-wide by cross-border introduction events. Moreover, there were indications that this propagation of the E lineage within Europe corresponded with an extensive and rapid spread event, already during or shortly after its formation. Finally, in accordance with the high substitution rate of IHNV determined by previous studies, our dataset indicates that the mean period of occurrence of a single E haplotype is typically not longer than one calendar year.

Bivalent DNA vaccine induces significant immune responses against infectious hematopoietic necrosis virus and infectious pancreatic necrosis virus in rainbow trout

Infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV) are important pathogens of salmon and trout. An active bivalent DNA vaccine was constructed with the glycoprotein gene of Chinese IHNV isolate Sn1203 and VP2-VP3 gene of Chinese IPNV isolate ChRtm213. Rainbow trout (5 g) were vaccinated by intramuscular injection with 1.0 µg of the bivalent DNA vaccine and then challenged with an intraperitoneal injection of IHNV, IPNV, or both, at 30 and 60 days post-vaccination (d.p.v.). High protection rates against IHNV were observed, with 6% and 10% cumulative mortality, respectively, compared with 90-94% in the mock-vaccinated groups. IPNV loads (531-fold and 135-fold, respectively) were significantly reduced in the anterior kidneys of the vaccinated trout. Significant protection against co-infection with IHNV and IPNV was observed, with cumulative mortality rates of 6.67% and 3.33%, respectively, compared with 50.0% and 43.3%, respectively, in the mock-vaccinated groups. No detectable infective IHNV or IPNV was recovered from vaccinated trout co-infected with IHNV and IPNV. The bivalent DNA vaccine increased the expression of Mx-1 and IFN-γ at 4, 7, and 15 d.p.v, and IgM at 21 d.p.v., and induced high titres (≥160) of IHNV and IPNV neutralizing antibodies at 30 and 60 d.p.v.

Viral nervous necrosis in gilthead sea bream (Sparus aurata) caused by reassortant betanodavirus RGNNV/SJNNV: an emerging threat for Mediterranean aquaculture

Viral nervous necrosis (VNN) certainly represents the biggest challenge for the sustainability and the development of aquaculture. A large number of economically relevant fish species have proven to be susceptible to the disease. Conversely, gilthead sea bream has generally been considered resistant to VNN, although it has been possible to isolate the virus from apparently healthy sea bream and sporadically from affected larvae and postlarvae. Unexpectedly, in 2014–2016 an increasing number of hatcheries in Europe have experienced mass mortalities in sea bream larvae. Two clinical outbreaks were monitored over this time span and findings are reported in this paper. Despite showing no specific clinical signs, the affected fish displayed high mortality and histological lesions typical of VNN. Fish tested positive for betanodavirus by different laboratory techniques. The isolates were all genetically characterized as being reassortant strains RGNNV/SJNNV. A genetic characterization of all sea bream betanodaviruses which had been isolated in the past had revealed that the majority of the strains infecting sea bream are actually RGNNV/SJNNV. Taken together, this information strongly suggests that RGNNV/SJNNV betanodavirus possesses a particular tropism to sea bream, which can pose a new and unexpected threat to the Mediterranean aquaculture.

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.