Rapid detection and quantitation of viral hemorrhagic septicemia virus in experimentally challenged rainbow trout by real-time RT-PCR

Antiviral effects of umbelliferone against viral hemorrhagic septicemia virus in olive flounder (Paralichthys olivaceus)

Viral hemorrhagic septicemia virus (VHSV) poses a significant threat to global aquaculture, prompting ongoing efforts to identify potential drug candidates for disease prevention. Coumarin derivatives have recently emerged as a promising class of compounds effective against rhabdoviruses, which severely impact the aquaculture industry. In this study, we assessed the anti-VHSV activity of umbelliferone (7-hydroxycoumarin) in fathead minnow (FHM) cells and olive flounder Paralichthys olivaceus. Umbelliferone exhibited an EC50 of 100 μg/mL by reducing cytopathic effect, with a maximum cytotoxicity of 30.9 % at 750 μg/mL. Mechanistic analyses via a time-course plaque reduction assay revealed that direct incubation with the virus for 1 h resulted in 97.0 ± 1.8 % plaque reduction, showing excellent direct virucidal activity. Pretreatment for 4 h resulted in a 33.5 ± 7.8 % plaque reduction, which increased with longer incubation times. Cotreatment led to a 33.5 ± 2.9 % plaque reduction, suggesting interference with viral binding, whereas postinfection treatment proved less effective. Umbelliferone was prophylactically administered to the olive flounder through short-term (3 days) and long-term (14 days) medicated feeding, followed by a 4-day postinfection period. Short-term administration at 100 mg/kg body weight (bw)/day resulted in the highest relative percent survival (RPS) of 56 %, whereas long-term administration achieved a maximum RPS of 44 % at 30 mg/kg bw/day. Umbelliferone administration delayed mortality at these doses. Additionally, umbelliferone significantly inhibited the expression of the VHSV N gene during viral challenge, with no observed toxic effects in fish up to an administration dose of 30 mg/kg bw/day for 28 days. Our findings suggest that the protective mechanism of short-term administration of 100 mg umbelliferone against VHSV infection may involve the overexpression of TLR2, MDA5, STAT1, and NF-κB at 24 h postinfection (hpi). IL-8 and IFN II expression was upregulated, whereas TNF-α, IL-1β, and IFN I expression was suppressed at 24 hpi. The upregulation of ISG15 at 48 hpi may contribute to the inhibition of VHSV replication, whereas the downregulation of Caspase 3 expression at 96 hpi suggests a possible inhibition of virus-induced apoptosis at later infection stages. Overall, umbelliferone exhibited anti-VHSV activity through multiple mechanisms, with the added advantage of convenient administration via medicated feed.

Innate immune response of rainbow trout erythrocytes to spinycterins expressing a downsized viral fragment of viral haemorrhagic septicaemia virus

Recent studies have reported on the importance of RBCs in fish responses to viral infections and DNA vaccines. Surface-displaying recombinant bacterins (spinycterins) are a safe and adaptable prototype for viral vaccination of fish and represent an alternative method of aquaculture prophylaxis, since have been reported to enhance fish immune response. We evaluated the innate immune response of rainbow trout (Oncorhynchus mykiss) red blood cells (RBCs), head kidney, and spleen to spinycterins expressing a fragment of the glycoprotein G of viral haemorrhagic septicemia virus (VHSV), one of the most devastating world-wide diseases in farmed salmonids. We first selected an immunorelevant downsized viral fragment of VHSV glycoprotein G (frg16252-450). Then, spinycterins expressing frg16252-450 fused to Nmistic anchor-motif (Nmistic+frg16252-450) were compared to spinycterins expressing frg16252-450 internally without the anchor motif. Nmistic+frg16252-450 spinycterins showed increased attachment to RBCs in vitro and modulated the expression of interferon- and antigen presentation-related genes in RBCs in vitro and in vivo, after intravenous injection. In contrast, the head kidney and spleen of fish injected with frg16252-450, but not Nmistic+frg16252-450, spinycterins demonstrated upregulation of interferon and antigen-presenting genes. Intravenous injection of Nmistic+frg16252-450 spinycterins resulted in a higher innate immune response in RBCs while frg16252-450 spinycterins increased the immune response in head kidney and spleen. Although more studies are required to evaluate the practicality of using spinycterins as fish viral vaccines, these results highlight the important contribution of RBCs to the fish innate immune response to antiviral prophylactics.

In vitro and in vivo evaluation of the antiviral activity of arctigenin, ribavirin, and ivermectin against viral hemorrhagic septicemia virus infection

Viral hemorrhagic septicemia virus (VHSV) causes a severe and often lethal infection in olive flounder (Paralichthys olivaceus) in Korea, resulting in mass mortality and substantial economic loss. As a potential prevention strategy for infectious viral diseases, this study aimed to evaluate the antiviral activity of three compounds (arctigenin [ARG], ribavirin [RBV], and ivermectin [IVM]) against VHSV infection in vitro and in vivo. In epithelioma papulosum cyprini cells, the expression of both VHSV glycoprotein (G) and nucleoprotein (N) genes were significantly suppressed by the three compounds in a dose-dependent manner (P < 0.05). Also, cell morphology and viability were maintained at the following concentrations: ARG 1.5 mg/L, RBV 2.5 mg/L, and IVM 10 mg/L. The fish that were treated with RBV (8.33 mg/kg) and IVM (0.25 mg/kg) before VHSV infection and those treated with IVM (0.25 mg/kg) after VHSV infection showed significant improvements in the survival rate, a reduction in the viral shedding rate, and downregulation of viral gene expression compared to those seen in fish with naïve VHSV infections. Furthermore, among the innate immune genes studied, persistent expression of Mx and upregulation of tumor necrosis factor-α gene expression in VHSV-infected fish treated with RBV and IVM revealed that these compounds might induce an immunostimulatory effect as one of their antiviral activities. Overall, this study supports the use of RBV and IVM as antiviral agents to control VHSV infections in olive flounder.

Characterization of Nervous Necrosis Virus (NNV) Nonstructural Protein B2 and Its Enhancement on Virus Proliferation

The nerve necrosis virus (NNV), a pathogen of viral nervous necrosis disease in several important mariculture economic fish species, causes economic loss. Its nonstructural protein B2 encoded by the sub-genomic RNA3 affects the amplification of the virus. In this study, the B2 protein was recombinantly expressed, the polyclonal antibodies were produced and the dynamics of the B2 protein and genomes were measured in vivo and in vitro after NNV infection. Then, the effects of the overexpressed B2 protein on virus proliferation were investigated. The results showed that the polyclonal antibodies can recognize the B2 protein in both SSN-1 cells and the brain/eye of the grouper. The RNA3 expression significantly increased at 12 h and kept rising till the end of the experiment; it was 10^6.9 copies/μL at 120 h. The B2 protein could be first detected at 3 h post-infection, which was earlier than the capsid protein was first detected (12 h post-infection). The B2 protein can be detected in the brain, eye and heart on day 3 and the copy number of genomes reached a maximum at 6 d post-infection. There was a low expression of NNV genomes in the liver, spleen and kidney, and no virus was detected in the gill, stomach and intestine. In the meantime, the B2 protein was successfully expressed in GF-1 cells and significantly enhanced virus proliferation, which produced an earlier cytopathic effect and higher cell death rates after 3 d post-infection than the control. In conclusion, the B2 protein acts as an early expressed protein during virus replication and proliferation and is involved in the early infection of NNV. The results may provide insight into the early stage of virus infection and prevention of the disease.

How modular protein nanoparticles may expand the ability of subunit anti-viral vaccines: The spring viremia carp virus (SVCV) case

Spring viremia of carp (SVC) remains as a vaccine orphan disease mostly affecting juvenile specimens. Young fish are especially difficult to vaccinate and oral administration of vaccine combined with food would be the election system to minimise stress and the vaccination costs associated to injection. However, administration of prophylactics with food pellets faces off several drawbacks mainly related with vaccine degradation and weak protection correlates of oral vaccines. Here we present a platform based on recombinant proteins (subunit vaccines) manufactured as highly resistant nanostructured materials, and providing excellent levels of protection against SVC virus in a preliminary i.p injection challenge. The G3 domain of SVCV glycoprotein G was overexpressed in E. coli together with IFNγ and the modular protein was purified from bacterial aggregates (inclusion bodies) as highly organised nanostructured biomaterial (nanopellets, NP). These SVCV-IFN^NP were taken up by zebrafish cells leading to the enhanced expression of different antiviral and IFN markers (e.g vig1, mx, lmp2 or ifngr1 among others) in zebrafish liver cells (ZFL). To monitor if SVCV^NP and SVCV-IFN^NP can be taken up by intestinal epithelia and can induce antiviral response we performed experiments with SVCV^NP and SVCV-IFN^NP in 3 days post fertilization (dpf) zebrafish larvae. Both, SVCV^NP and SVCV-IFN^NP were taken up and accumulated in the intestine without signs of toxicity. The antiviral response in larvae showed a different induction pattern: SVCV-IFN^NP did not induce an antiviral response while SVCV^NP showed a good antiviral induction. Interestingly ZF4, an embryonic derived cell line, showed an antiviral response like ZFL cells, although the lmp2 and ifngr1 (markers of the IFNγ response) were not overexpressed. Experiments with adult zebrafish indicated an excellent level of protection against a SVCV model infection where SVCV-IFN^NP vaccinated fish reached 20% cumulative mortality while control fish reached over 80% cumulative mortality.

A Novel Specific Single-Chain Variable Fragment Diagnostic System for Viral Hemorrhagic Septicemia Virus

Viral hemorrhagic septicemia virus (VHSV), one of the most important viral marine pathogens worldwide, has a broad range of hosts, such as members of the families Salmonidae and Paralichthyidae. In addition to being highly contagious, VHSV causes high lethality. The transmission of VHSV can be both vertical and horizontal. In fish, the resolution of VHSV infection is challenging. Thus, early diagnosis of VHSV infections is critical, especially in fish farms that have a high population of juvenile fish. Serological methods are commonly used to detect viral antigens. However, limited serological methods are available for marine viruses. In this study, a VHSV-specific single-chain variable fragment (scFv), E5, was selected using the yeast surface display and phage display systems. scFv, a type of recombinant antibody, comprises a variable heavy chain ([Formula: see text]) and a variable light chain ([Formula: see text]) connected by a polypeptide linker. An scFv clone was selected from the VHSV glycoprotein-expressing yeast cells using the bio-panning method. The scFv-encoding gene was subcloned and expressed in the Escherichia coli expression system. The binding affinity of the expressed and purified scFv protein was determined using an enzyme-linked immunosorbent assay and western blotting. Thus, this study reported a method to identify VHSV-specific scFv using bio-panning that can be utilized to develop a diagnostic system for other viruses.

Genome-wide association study of VHSV-resistance trait in Paralichthys olivaceus

In flounder aquaculture, selective breeding plays a vital role in the development of disease-resistant traits and animals with high growth rates. Moreover, superior animals are required to achieve high profits. Unlike growth-related traits, disease-resistant experiments need to be conducted in a controlled environment, as the improper measurement of traits often leads to low genetic correlation and incorrect estimation of breeding values. In this study, viral hemorrhagic septicemia virus (VHSV) resistance was studied using a genome-wide association study (GWAS), and the genetic parameters were estimated. Genotyping was performed using a high-quality 70 K single nucleotide polymorphism (SNP) Affymetrix® Axiom® myDesign™ Genotyping Array of olive flounder. A heritability of ∼0.18 for resistance to VHSV was estimated using genomic information of the fish. According to the GWAS, significant SNPs were detected in chromosomes 21, 24, and contig AGQT02032065.1. Three SNPs showed significance at the genome-wide level (p < 1 × 10^-6), while others showed significance above the suggestive cutoff (p < 1 × 10^-4). The 3% phenotypic variation was explained by the highest significant SNP, named AX-419319631. Of the important genes for disease resistance, SNPs were associated with plcg1, epha4, clstn2, pik3cb, hes6, meis3, prx6, cep164, siae, and kirrel3b. Most of the genes associated with these SNPs have been previously reported with respect to viral entry, propagation, and immune mechanisms. Therefore, our study provides helpful information regarding VHSV resistance in olive flounder, which can be used for breeding applications.

Immunomodulatory Lectin-like Peptides for Fish Erythrocytes-Targeting as Potential Antiviral Drug Delivery Platforms

One of the challenges of science in disease prevention is optimizing drug and vaccine delivery. Until now, many strategies have been employed in this sector, but most are quite complex and labile. To overcome these limitations, great efforts are directed to coupling drugs to carriers, either of natural or synthetic origin. Among the most studied cell carriers are antigen-presenting cells (APCs), however, red blood cells (RBCs) are positioned as attractive carriers in drug delivery due to their abundance and availability in the body. Furthermore, fish RBCs have a nucleus and have been shown to have a strong involvement in modulating the immune response. In this study, we evaluated the binding of three peptides to rainbow trout RBCs, two lectin-like peptides and another derived from Plasmodium falciparum membrane protein, in order to take advantage of this peptide-RBCs binding to generate tools to improve the specificity, efficacy, immunostimulatory effect, and safety of the antiviral therapeutic or prophylactic administration systems currently used.

Antiviral Function of NKEF against VHSV in Rainbow Trout

Simple Summary

An antioxidant protein has been identified in a sample of erythrocytes exposed to a fish virus. We evaluated the role of this protein as an antiviral molecule in fish. Through silencing and overexpression assays we determined the antiviral effect of this protein in the infectivity of the virus. In conclusion, this antioxidant protein may be a potential target for new therapeutic strategies against viral infections.

Abstract

Natural killer enhancing factor (NKEF) belongs to the peroxiredoxin family of proteins, a group of antioxidants that has been extensively studied in mammals. Recently, we identified NKEF in the immunoprecipitated proteome of rainbow trout red blood cells (RBCs) exposed to viral hemorrhagic septicemia virus (VHSV). In the present study, we evaluated the role of NKEF in the antiviral response of rainbow trout against VHSV by examining the expression profile of NKEF in VHSV-exposed RBCs and rainbow trout gonad-2 (RTG-2) cell line. We found an in vitro correlation between decreased VHSV replication and increased NKEF expression after RBCs were exposed to VHSV, however this was not found in RTG-2 cells where the infection highly increased and nkef transcripts remained almost unchanged. In addition, siRNA silencing of the nkef gene in rainbow trout RBCs and RTG-2 cells resulted in increased VHSV replication. We also found a correlation between nkef gene silencing and a decrease in the expression of genes related to type 1 interferon (IFN1) pathway. These findings indicated that NKEF is involved in the antiviral mechanisms of rainbow trout RBCs against VHSV and thus support its antiviral role and implication in the modulation of their immune response. Finally, overexpression of NKEF in an EPC cell line significantly reduced VHSV infectivity and was coupled to an increment in IFN1-related genes. In conclusion, NKEF may be a potential target for new therapeutic strategies against viral infections.

Induction of foxp3 during the Crosstalk between Antigen Presenting Like-Cells MHCII+CD83+ and Splenocytes CD4+IgM- in Rainbow Trout

Simple Summary

In aquatic biological models, the communication between cells from the immune system remains poorly characterized. In this work, to determine the gene expression of master transcriptional factors that coordinate the polarization of T cells, co-cultures of rainbow trout splenocytes are analyzed after stimulation with Interferon-gamma and/or Piscirickettsia salmonis. The results showed an upregulation of foxp3 compared to the other transcriptional factors, suggesting a potential communication between cells in the spleen, which may induce a Treg phenotype.

Abstract

In fish, the spleen is one of the major immune organs in the animal, and the splenocytes could play a key role in the activation and modulation of the immune response, both innate and adaptive. However, the crosstalk between different types of immune cells in the spleen has been poorly understood. In this work, an in vitro strategy is carried out to obtain and characterize mononuclear splenocytes from rainbow trout, using biomarkers associated with lymphocytes (CD4 and IgM) and antigen-presenting cells (CD83 and MHC II). Using these splenocytes, co-cultures of 24 and 48 h are used to determine the gene expression of master transcriptional factors that coordinate the polarization of T cells (t-bet, gata3, and foxp3). The results show a proportional upregulation of foxp3 (compared to t-bet and gata3) in co-cultures (at 24 h) of IFNγ-induced splenocytes with and without stimulation of Piscirickettsia salmonis proteins. In addition, foxp3 upregulation was established in co-cultures with IFNγ-induced cells and in cells only stimulated previously with P. salmonis proteins at 48 h of co-culture. These results show a potential communication between antigen-presenting-like cells and lymphocyte in the spleen, which could be induced towards a Treg phenotype.

Stability of viral haemorrhagic septicaemia virus, infectious hematopoietic necrosis virus and cyprinid herpesvirus 3 in various water samples

Rainbow trout (Oncorhynchus mykiss) and common carp (Cyprinus carpio) are the two most common species in traditional fish farming in Germany. Their aquaculture is threatened upon others by viruses that can cause a high mortality. Therefore, this work focuses on three viruses-viral haemorrhagic septicaemia virus, infectious hematopoietic necrosis virus and cyprinid herpesvirus 3 (CyHV-3)-that endanger these species. To prevent their spread and contain further outbreaks, it is essential to know how long they can outlast in environmental waters and what affects their infectivity outside the host. Hence, the stability of the target viruses in various water matrices was examined and compared in this work. In general, all three viruses were quite stable within sterile water samples (showing mostly ≤1 log reduction after 96 hr) but were inactivated faster and to a higher extent (up to five log steps within 96 hr) in unsterile environmental water samples. The inactivation of the viruses correlated well with the increasing bacterial load of the samples, suggesting that bacteria had the greatest effect on their stability in the examined samples. In comparison, CyHV-3 seemed to be the most sensitive and maintained its infectivity for the shortest period.

Oral immunization of trout fry with recombinant Lactococcus lactis NZ3900 expressing G gene of viral hemorrhagic septicaemia virus (VHSV)

This study has investigated the ability of Lactococcus lactic (NZ3900) carried G gene of viral haemorrhagic septicaemia virus (VHSV) under nisin-controlled gene expression (NICE) system in rainbow trout (O.Mykiss). Two groups of trout fry (7 ± 0.65 g) were immunized with 1 × 10¹⁰ cfu/g and 1 × 10⁸ cfu/g recombinant L. lactis NZ3900, two groups of fish were fed 1 × 10¹⁰ cfu/g and 1 × 10⁸ cfu/g L. lactis vector free, and one group was fed by the basal diet as a control. Oral immunization was done on days 1-7 and boosting was performed on days 15-21. The relative expression of IFN-1 and MX-1 genes significantly increased in head kidney of vaccinated fish depend on vaccine dosage compared to the control group. Fish in vaccinated group also showed elevated VHSV-specific antibody levels compared to the control groups. Relative percent survival (RPS), under virulent isolate VHSV challenge were estimated 62%, 78% for 10⁸ cfu/g 10¹⁰ cfu/g feed vaccinated groups 21 days post-vaccination, while groups fed similar doses of L. lactis vector free illustrated 22% and 27% RPSs, respectively. The significant reduction of viral loads (transcript levels of N gene) were detected in the immunized groups. Increased weight gain and decreased feed consumption in vaccinated group attributed to the probiotic effect were also observed. In conclusion, our results demonstrate the ability of recombinant L. lactis as oral vaccine against VHS in rainbow trout, which can be considered as effective method against different fish pathogens.

Presence of viral haemorrhagic septicaemia virus (VHSV) in the environment of virus-contaminated fish farms and processing plants

After the first outbreak of viral haemorrhagic septicaemia virus (VHSV) in Finnish brackish water rainbow trout Oncorhynchus mykiss farms, infection spread rapidly between the farms. The infrastructure of fish farming did not take into account spreading of infectious fish diseases. To show the presence of VHSV in the environment, we tested seawater, sediment and wild blue mussels Mytilus edulis from VHSV-infected fish farms, and liquid waste from a processing plant that handled infected rainbow trout. Additionally, blue mussels were bath-challenged with VHSV (exposed to cultivated virus or naturally infected rainbow trout). To detect VHSV, virus isolation in cell culture and real-time reverse transcriptase polymerase chain reaction (qRT-PCR) were used. The virus or viral RNA was detected in sea water and in liquid waste from processing plants during wintertime when water temperature is close to 0°C and sunlight is sparse. VHSV did not appear to replicate in blue mussels in our study. Therefore, blue mussels were not considered relevant carriers of VHSV. However, traces of viral RNA were detected up to 29 d post challenge in mussels. Contact with water from processing plants handling VHSV-infected fish populations increases the risk of the disease spreading to susceptible fish populations, especially during cold and dark times of the year.

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.

Integrated Transcriptomic and Proteomic Analysis of Red Blood Cells from Rainbow Trout Challenged with VHSV Point Towards Novel Immunomodulant Targets

Teleost red blood cells (RBCs) are nucleated and therefore can propagate cellular responses to exogenous stimuli. RBCs can mount an immune response against a variety of fish viruses, including the viral septicemia hemorrhagic virus (VHSV), which is one of the most prevalent fish viruses resulting in aquaculture losses. In this work, RBCs from blood and head kidney samples of rainbow trout challenged with VHSV were analyzed via transcriptomic and proteomic analyses. We detected an overrepresentation of differentially expressed genes (DEGs) related to the type I interferon response and signaling in RBCs from the head kidney and related to complement activation in RBCs from blood. Antigen processing and presentation of peptide antigen was overrepresented in RBCs from both tissues. DEGs shared by both tissues showed an opposite expression profile. In summary, this work has demonstrated that teleost RBCs can modulate the immune response during an in vivo viral infection, thus implicating RBCs as cell targets for the development of novel immunomodulants.

Potential Role of Rainbow Trout Erythrocytes as Mediators in the Immune Response Induced by a DNA Vaccine in Fish

In recent years, fish nucleated red blood cells (RBCs) have been implicated in the response against viral infections. We have demonstrated that rainbow trout RBCs can express the antigen encoded by a DNA vaccine against viral hemorrhagic septicemia virus (VHSV) and mount an immune response to the antigen in vitro. In this manuscript, we show, for the first time, the role of RBCs in the immune response triggered by DNA immunization of rainbow trout with glycoprotein G of VHSV (GVHSV). Transcriptomic and proteomic profiles of RBCs revealed genes and proteins involved in antigen processing and presentation of exogenous peptide antigen via MHC class I, the Fc receptor signaling pathway, the autophagy pathway, and the activation of the innate immune response, among others. On the other hand, GVHSV-transfected RBCs induce specific antibodies against VHSV in the serum of rainbow trout which shows that RBCs expressing a DNA vaccine are able to elicit a humoral response. These results open a new direction in the research of vaccination strategies for fish since rainbow trout RBCs actively participate in the innate and adaptive immune response in DNA vaccination. Based on our findings, we suggest the use of RBCs as target cells or carriers for the future design of novel vaccine strategies.

Kinetics of infectious virus and viral RNA copy number in the blood of olive flounder infected with viral hemorrhagic septicemia virus (VHSV)

Viral hemorrhagic septicemia (VHS) is a cold-water disease caused by viral hemorrhagic septicemia virus (VHSV) at an optimal temperature of 9 °C-15 °C. VHSV isolation and detection have been accomplished by using a number of diagnostic methods such as cell culture and qRT-PCR. Spleen and kidney have been reported as the main target organs of VHSV-infection; however, how VHSV spreads throughout the fish body has not been clearly studied. The purpose of this study was 1) to investigate viral titer and viral RNA copy number in the blood of VHSV-infected olive flounder at 10 °C and 13 °C; 2) to compare VHSV titer and viral RNA copy numbers in blood from fish exposed to the virus by two different challenges. VHSV titer at 10 °C was higher than at 13 °C in blood samples of injection challenged group. Whereas, similar titer was observed at 10 °C and 13 °C in the blood samples of the immersion challenged group. At 10 °C, copy numbers of VHSV-N gene in blood of immersion challenged group increased slightly in comparison to injection challenged group. At 13 °C, similar patterns were observed between the injection and immersion challenged groups. Also, higher titer and copy number were observed in fish blood compared to tested organs from our previous study. Our results indicate that VHSV genome existed in fish blood at earlier time points after infection, and the blood may contribute to the spread of the virus in whole fish body. In addition, VHSV diagnosis by qRT-PCR from fish blood samples, not requiring sacrificing the host fish can be valuable to collect the kinetic information of viral infection.

Rainbow Trout Red Blood Cells Exposed to Viral Hemorrhagic Septicemia Virus Up-Regulate Antigen-Processing Mechanisms and MHC I&II, CD86, and CD83 Antigen-presenting Cell Markers

Nucleated teleost red blood cells (RBCs) are known to express molecules from the major histocompatibility complex and peptide-generating processes such as autophagy and proteasomes, but the role of RBCs in antigen presentation of viruses have not been studied yet. In this study, RBCs exposed ex vivo to viral hemorrhagic septicemia virus (VHSV) were evaluated by means of transcriptomic and proteomic approaches. Genes and proteins related to antigen presentation molecules, proteasome degradation, and autophagy were up-regulated. VHSV induced accumulation of ubiquitinated proteins in ex vivo VHSV-exposed RBCs and showed at the same time a decrease of proteasome activity. Furthermore, induction of autophagy was detected by evaluating LC3 protein levels. Sequestosome-1/p62 underwent degradation early after VHSV exposure, and it may be a link between ubiquitination and autophagy activation. Inhibition of autophagosome degradation with niclosamide resulted in intracellular detection of N protein of VHSV (NVHSV) and p62 accumulation. In addition, antigen presentation cell markers, such as major histocompatibility complex (MHC) class I & II, CD83, and CD86, increased at the transcriptional and translational level in rainbow trout RBCs exposed to VHSV. In summary, we show that nucleated rainbow trout RBCs can degrade VHSV while displaying an antigen-presenting cell (APC)-like profile.

IFIT5 Participates in the Antiviral Mechanisms of Rainbow Trout Red Blood Cells

Viral hemorrhagic septicemia virus (VHSV) infection appears to be halted in rainbow trout nucleated red blood cells (RBCs). Diverse mechanisms are thought to be related to the antiviral immune response of rainbow trout RBCs to VHSV. However, the specific rainbow trout RBC proteins that interact directly with VHSV are still unknown. In an attempt to identify VHSV-RBC protein interactions, we characterized the immunoprecipitated (IP) proteome of RBCs exposed to VHSV using an antibody against the N protein of VHSV. The IP proteomic characterization identified 31 proteins by mass spectrometry analysis. Among them, we identified interferon-induced protein with tetratricopeptide repeats 5 (IFIT5), a protein belonging to a family of proteins that are induced after the production of type I interferon. Importantly, IFIT5 has been implicated in the antiviral immune response. We confirmed the participation of IFIT5 in the rainbow trout RBC antiviral response by examining the expression profile of IFIT5 in RBCs after VHSV exposure at transcriptional and protein levels. We detected a correlation between the highest IFIT5 expression levels and the decline in VHSV replication at 6 h post-exposure. In addition, silencing ifit5 resulted in a significant increase in VHSV replication in RBCs. Moreover, an increase in VHSV replication was observed in RBCs when the IFIT5 RNA-binding pocket cavity was modulated by using a natural compound from the SuperNatural II database. We performed a proximity ligation assay and detected a significant increase in positive cells among VHSV-exposed RBCs compared to unexposed RBCs, indicating protein-protein colocalization between IFIT5 and the glycoprotein G of VHSV. In summary, these results suggest a possible role of IFIT5 in the antiviral response of RBCs against VHSV.

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.

Wild fish are negligible transmitters of viral haemorrhagic septicaemia virus (VHSV) genotype Id in the VHS restriction zone in Finland

Wild fish were suspected to be the source of reinfection by viral haemorrhagic septicaemia virus (VHSV) in Finnish brackish water rainbow trout farms located in a restriction zone regarding viral haemorrhagic septicaemia (VHS) comprising the entire Province of Åland, Baltic Sea, in the 2000s. Altogether, 1636 wild fish of 17 different species living in the vicinity of infected fish farms were screened for VHSV during the years 2005-2008. Additionally, 2 uninfected wild fish species as well as farmed whitefish were introduced into a VHS-positive fish farm to test whether they became infected by VHSV from the clinically diseased rainbow trout. Wild fish did not test positive for VHSV on any occasion. In contrast, whitefish introduced to a VHS-positive farm were infected with VHSV genotype Id and started to replicate the virus for a short time during the trial. Whitefish are farmed together with, or in the vicinity of, farmed rainbow trout in the study area and, according to this study, are a possible source of the recurring infection in the restriction area. A sprivivirus was isolated from all fish species in the infection trial without causing mortality in the test groups.

Development of an in-situ hybridization assay using riboprobes for detection of viral haemorrhagic septicemia virus (VHSV) mRNAs in a cell culture model

An in situ hybridization (RNA-ISH) assay has been developed and optimized to detect viral haemorrhagic septicemia virus (VHSV), an OIE listed piscine rhabdovirus, in infected fish cells using fathead minnow (FHM) as a model cell line. Two antisense riboprobes (RNA probes) targeting viral transcripts from a fragment of nucleoprotein (N) and glycoprotein (G) genes were generated by reverse transcription polymerase chain reaction (RT-PCR) using VHSV specific primers followed by a transcription reaction in the presence of digoxigenin dUTP. The synthesized RNA probes were able to detect viral mRNAs in formalin fixed VHSV infected FHM cells at different time points post inoculation (pi). To correlate the signal intensity, a time dependent quantitation of the viral mRNA transcript and infectivity titer was done by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) and 50% tissue culture infectivity dose (TCID₅₀), respectively, from the infected cells and culture supernatants. Further, we compared the diagnostic sensitivity of ISH assay with immunocytochemistry (ICC). Both the riboprobes used in the ISH assay detected VHSV as early as 6 hpi in the FHM cells inoculated with a multiplicity of infection (moi) of 2. Also, the signal detection in ISH was at an early stage in comparison to ICC, wherein, signal was first detected at 12 hpi. Our results clearly highlight that current ISH assay can be of value as a diagnostic tool to localize and detect VHSV in conjunction with conventional virus isolation in cell culture.

Rainbow Trout Erythrocytes ex vivo Transfection With a DNA Vaccine Encoding VHSV Glycoprotein G Induces an Antiviral Immune Response

Fish red blood cells (RBCs), are integral in several biologic processes relevant to immunity, such as pathogen recognition, pathogen binding and clearance, and production of effector molecules and cytokines. So far, one of the best strategies to control and prevent viral diseases in aquaculture is DNA immunization. DNA vaccines (based on the rhabdoviral glycoprotein G [gpG] gene) have been shown to be effective against fish rhabdoviruses. However, more knowledge about the immune response triggered by DNA immunization is necessary to develop novel and more effective strategies. In this study, we investigated the role of fish RBCs in immune responses induced by DNA vaccines. We show for the first time that rainbow trout RBCs express gpG of viral hemorrhagic septicaemia virus (VHSV) (GVHSV) when transfected with the DNA vaccine ex vivo and modulate the expression of immune genes and proteins. Functional network analysis of transcriptome profiling of RBCs expressing GVHSV revealed changes in gene expression related to G-protein coupled receptor (GPCR)-downstream signaling, complement activation, and RAR related orphan receptor α (RORA). Proteomic profile functional network analysis of GVHSV-transfected RBCs revealed proteins involved in the detoxification of reactive oxygen species, interferon-stimulated gene 15 (ISG15) antiviral mechanisms, antigen presentation of exogenous peptides, and the proteasome. Conditioned medium of GVHSV-transfected RBCs conferred antiviral protection and induced ifn1 and mx gene expression in RTG-2 cells infected with VHSV. In summary, rainbow trout nucleated RBCs could be actively participating in the regulation of the fish immune response to GVHSV DNA vaccine, and thus may represent a possible carrier cells for the development of new vaccine approaches.

Shape-Shifted Red Blood Cells: A Novel Red Blood Cell Stage?

Primitive nucleated erythroid cells in the bloodstream have long been suggested to be more similar to nucleated red cells of fish, amphibians, and birds than the red cells of fetal and adult mammals. Rainbow trout Ficoll-purified red blood cells (RBCs) cultured in vitro undergo morphological changes, especially when exposed to stress, and enter a new cell stage that we have coined shape-shifted RBCs (shRBCs). We have characterized these shRBCs using transmission electron microscopy (TEM) micrographs, Wright–Giemsa staining, cell marker immunostaining, and transcriptomic and proteomic evaluation. shRBCs showed reduced density of the cytoplasm, hemoglobin loss, decondensed chromatin in the nucleus, and striking expression of the B lymphocyte molecular marker IgM. In addition, shRBCs shared some features of mammalian primitive pyrenocytes (extruded nucleus surrounded by a thin rim of cytoplasm and phosphatidylserine (PS) exposure on cell surface). These shRBCs were transiently observed in heat-stressed rainbow trout bloodstream for three days. Functional network analysis of combined transcriptomic and proteomic studies resulted in the identification of proteins involved in pathways related to the regulation of cell morphogenesis involved in differentiation, cellular response to stress, and immune system process. In addition, shRBCs increased interleukin 8 (IL8), interleukin 1 β (IL1β), interferon ɣ (IFNɣ), and natural killer enhancing factor (NKEF) protein production in response to viral hemorrhagic septicemia virus (VHSV). In conclusion, shRBCs may represent a novel cell stage that participates in roles related to immune response mediation, homeostasis, and the differentiation and development of blood cells.

Infectious pancreatic necrosis virus triggers antiviral immune response in rainbow trout red blood cells, despite not being infective

Background: Some fish viruses, such as piscine orthoreovirus and infectious salmon anemia virus, target red blood cells (RBCs), replicate inside them and induce an immune response. However, the roles of RBCs in the context of infectious pancreatic necrosis virus (IPNV) infection  have not been studied yet.

Methods: Ex vivo rainbow trout RBCs were obtained from peripheral blood, Ficoll purified and exposed to IPNV in order to analyze infectivity and immune response using RT-qPCR, immune fluorescence imaging, flow cytometry and western-blotting techniques.

Results: IPNV could not infect RBCs; however, IPNV increased the expression of the INF1-related genes ifn-1, pkr and mx genes. Moreover, conditioned media from IPNV-exposed RBCs conferred protection against IPNV infection in CHSE-214 fish cell line.

Conclusions: Despite not being infected, rainbow trout RBCs could respond to IPNV with increased expression of antiviral genes. Fish RBCs could be considered as mediators of the antiviral response and therefore targets of new strategies against fish viral infections. Further research is ongoing to completely understand the molecular mechanism that triggers this antiviral response in rainbow trout RBCs.

Identification of diverse defense mechanisms in rainbow trout red blood cells in response to halted replication of VHS virus

Background: It has been described that fish nucleated red blood cells (RBCs) generate a wide variety of immune-related gene transcripts when viruses highly replicate inside them and are their main target cell. The immune response and mechanisms of fish RBCs against viruses targeting other cells or tissues has not yet been explored and is the objective of our study. Methods: Rainbow trout RBCs were obtained from peripheral blood, ficoll purified and exposed to Viral Haemorrhagic Septicaemia virus (VHSV). Immune response was evaluated by means of RT-qPCR, flow cytometry, immunofluorescence and isobaric tag for relative and absolute quantification (iTRAQ) protein profiling. Results: VHSV N gene transcripts incremented early postexposure and were drastically decreased after 6 hours postexposure (hpe). The expression of type I interferon ( ifn1) gene was significantly downregulated at early postexposure (3 hpe), together with a gradual downregulation of interferon-inducible mx and pkr genes until 72 hpe. Type I IFN protein was downregulated and interferon-inducible Mx protein was maintained at basal levels. Co-culture assays of RBCs, previously exposed to UV-inactivated VHSV, and TSS (stromal cell line from spleen) revealed IFN crosstalk between both cell types. On the other hand, anti-microbial peptide β-defensin 1 and neutrophil chemotactic factor interleukin 8 were slightly upregulated in VHSV-exposed RBCs. iTRAQ profiling revealed that VHSV exposure can induce a global protein downregulation in rainbow trout RBCs, mainly related to RNA stability and proteasome pathways. Antioxidant/antiviral response is also suggested to be involved in the response of rainbow trout RBCs to VHSV. Conclusions: A variety of mechanisms are proposed to be implicated in the antiviral response of rainbow trout RBCs against VHSV halted infection. Ongoing research is focused on understanding the mechanisms in detail.

Identification of diverse defense mechanisms in trout red blood cells in response to VHSV halted viral replication

Background: It has been described that fish nucleated red blood cells (RBCs) generate a wide variety of immune-related gene transcripts when viruses highly replicate inside them and are their main target cell. The immune response and mechanisms of fish RBCs against viruses targeting other cells or tissues has not yet been explored and is the objective of our study. Methods: Trout RBCs were obtained from peripheral blood, ficoll purified and exposed to Viral Haemorrhagic Septicaemia virus (VHSV). Immune response was evaluated by means of RT-qPCR, flow cytometry, immunofluorescence and isobaric tag for relative and absolute quantification (iTRAQ) protein profiling Results: VHSV N gene transcripts incremented early postexposure and were drastically decreased after 6 hours postexposure (hpe). The expression of the type I interferon ( ifn1) gene was significantly downregulated at early postexposure (3 hpe), together with a gradual downregulation of interferon-inducible mx and pkr genes until 72 hpe. Type I IFN protein was downregulated and interferon-inducible Mx protein was maintained at basal levels. Co-culture assays of RBCs with TSS (stromal cell line from spleen) revealed the IFN crosstalk between both cell types. On the other hand, anti-microbial peptide β-defensin 1 and neutrophil chemotactic factor interleukin 8 were slightly upregulated in VHSV-exposed RBCs Isobaric tag for relative and absolute quantification (iTRAQ) revealed that VHSV exposure can induce a global protein downregulation in trout RBCs, mainly related to RNA stability and proteasome pathways. The antioxidant/antiviral response is also suggested to be involved in the response of trout RBCs to VHSV. Conclusions: A variety of mechanisms are proposed to be implicated in the antiviral response of trout RBCs against VHSV halted infection. Ongoing research is focused on understanding the mechanisms in detail. To our knowledge, this is the first report that implicates fish RBCs in the antiviral response against viruses not targeting RBCs.

Viral haemorrhagic septicaemia virus (VHSV Id) infections are detected more consistently using syndromic vs. active surveillance

The eradication of viral haemorrhagic septicaemia virus (VHSV Id) from Finnish brackish-water rainbow trout Oncorhynchus mykiss farms located in the restriction zone in the Province of Åland, Baltic Sea, failed several times in the 2000s. The official surveillance programme was often unable to find VHSV-positive populations, leading to the misbelief in the fish farming industry that virus eradication could be achieved. The ability of 3 other surveillance programmes to detect infected fish populations was compared with the official programme. One programme involved syndromic surveillance based on the observation of clinical disease signs by fish farmers, while 2 programmes comprised active surveillance similar to the official programme, but included increased sampling frequencies and 2 additional tests. The syndromic surveillance concentrated on sending in samples for analysis when any sign of a possible infectious disease at water temperatures below 15°C was noticed. This programme clearly outperformed active surveillance. A real-time reverse transcriptase-polymerase chain reaction method proved to be at least as sensitive as virus isolation in cell culture in detecting acute VHSV infections. An ELISA method was used to test fish serum for antibodies against VHSV. The ELISA method may be a useful tool in VHSV eradication for screening populations during the follow-up period, before declaring an area free of infection.

Restricted replication of viral hemorrhagic septicemia virus (VHSV) in a birnavirus-carrier cell culture

Viral hemorrhagic septicemia virus (VHSV) and infectious pancreatic necrosis virus (IPNV) are economically important pathogens of the salmonid aquaculture industry. In previous work we demonstrated that a cell line persistently infected with IPNV (EPC^IPNV) exhibited antiviral activity against superinfection with the heterologous virus VHSV. This work extends our study by analyzing the replication of VHSV in the IPNV-persistently infected cells. At early and late stages of infection VHSV RNA synthesis, as well as VHSV-induced syncytia formation, were examined in EPC^IPNV cultures. During the course of VHSV infection the accumulation of VHSV RNA is inhibited in EPC^IPNV cells. Typical VHSV-induced membrane fusion at the late stages of infection is also absent in the IPNV carrier cultures. VHSV binding and fusion to EPC^IPNV cells did not appear to be impaired, but a potent inhibitory effect on VHSV RNA synthesis is exerted at early times of infection in the IPNV carrier culture. In conclusion, the EPC^IPNV cells are considered to be a useful system to study viral interference as well to analyze the mechanisms underlying the phenomenon of superinfection exclusion.

Temporary protection of rainbow trout gill epithelial cells from infection with viral haemorrhagic septicaemia virus IVb

The branchial epithelium is not only a primary route of entry for viral pathogens, but is also a site of viral replication and subsequent shedding may also occur from the gill epithelium. This study investigated the potential of agents known to stimulate innate immunity to protect rainbow trout epithelial cells (RTgill-W1) from infection with VHSV IVb. RTgill-W1 cells were pretreated with poly I:C, FuGENE(®) HD + poly I:C, lipopolysaccharide (LPS), LPS + poly I:C or heat-killed VHSV IVb and then infected with VHSV IVb 4 days later. Cytopathic effect (CPE) was determined at 2, 3, 4, 7 and 11 days post-infection. Virus in cells and supernatant was detected using quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). All of the treatments delayed the onset of CPE (per cent of monolayer destruction), compared with untreated controls; however, killed VHSV or poly I:C combined with LPS was the most effective. Similarly, the detection of viral RNA in the supernatant was delayed, and the quantity was significantly (P < 0.05) reduced by all treatments with the exception of LPS alone (4 days). Unlike many of the other treatments, pretreatment of RTgill-W1 with heat-killed VHSV did not upregulate interferon 1, 2 or MX 1 gene expression.

Turbot (Scophthalmus maximus) vs. VHSV (Viral Hemorrhagic Septicemia Virus): A Review

Turbot (Scophthalmus maximus) is a very valuable fish species both in Europe and China. The culture of this flatfish is well-established but several bacteria, viruses, and parasites can produce mortality or morbidity episodes in turbot farms. Viral Hemorrhagic Septicemia Virus (VHSV) is one of the most threatening pathogens affecting turbot, because neither vaccines nor treatments are commercially available. Although the mortality in the turbot farms is relatively low, when this virus is detected all the stock have to be destroyed. The main goals that need to be improved in order to reduce the incidence of this disease is to know what are the strategies or molecules the host use to fight the virus and, in consequence, try to potentiate this response using different ways. Certain molecules can be selected as potential antiviral treatments because of their high protective effect against VHSV. On the other hand, the use of resistance markers for selective breeding is one of the most attractive approaches. This review englobes all the investigation concerning the immune interaction between turbot and VHSV, which until the last years was very scarce, and the knowledge about VHSV-resistance markers in turbot. Nowadays, the availability of abundant transcriptomic information and the recent sequencing of the turbot genome open the door to a more exhaustive and profuse investigation in these areas.

Piscine orthoreovirus (PRV) in red and melanised foci in white muscle of Atlantic salmon (Salmo salar)

Melanised focal changes (black spots) are common findings in the white skeletal muscle of seawater-farmed Atlantic salmon (Salmo salar). Fillets with melanised focal changes are considered as lower quality and cause large economic losses. It has been suggested that red focal changes (red spots) precede the melanised focal changes. In the present work, we examined different populations of captive and wild salmon for the occurrence of both types of changes, which were investigated for the presence of different viruses by immunohistochemistry and RT-qPCR. The occurrence of red or melanised foci varied significantly between the populations, from none in wild fish control group, low prevalence of small foci in fish kept in in-house tanks, to high prevalence of large foci in farm-raised salmon. Large amounts of Piscine orthoreovirus (PRV) antigen were detected in all foci. No other viruses were detected. Red focal changes contained significantly higher levels of PRV RNA than apparently non-affected areas in white muscle of the same individuals. Some changes displayed a transient form between a red and melanised pathotype, indicating a progression from an acute to a chronic manifestation. We conclude that PRV is associated with the focal pathological changes in the white muscle of farmed Atlantic salmon and is a premise for the development of focal melanised changes.

Real-time RT-PCR for detection, identification and absolute quantification of viral haemorrhagic septicaemia virus using different types of standards

In the present study, 2 systems of real-time RT-PCR-one based on SYBR Green and the other on TaqMan-were designed to detect strains from any genotype of viral haemorrhagic septicaemia virus (VHSV), with high sensitivity and repeatability/reproducibility. In addition, the method was optimized for quantitative purposes (qRT-PCR), and standard curves with different types of reference templates were constructed and compared. Specificity was tested against 26 isolates from 4 genotypes. The sensitivity of the procedures was first tested against cell culture isolation, obtaining a limit of detection (LD) of 100 TCID50 ml-1 (100-fold below the LD using cell culture), at a threshold cycle value (Ct) of 36. Sensitivity was also evaluated using RNA from crude (LD = 1 fg; 160 genome copies) and purified virus (100 ag; 16 copies), plasmid DNA (2 copies) and RNA transcript (15 copies). No differences between both chemistries were observed in sensitivity and dynamic range. To evaluate repeatability and reproducibility, all experiments were performed in triplicate and on 3 different days, by workers with different levels of experience, obtaining Ct values with coefficients of variation always <5. This fact, together with the high efficiency and R2 values of the standard curves, encouraged us to analyse the reliability of the method for viral quantification. The results not only demonstrated that the procedure can be used for detection, identification and quantification of this virus, but also demonstrated a clear correlation between the regression lines obtained with different standards, which will help scientists to compare sensitivity results between different studies.

Detection and surveillance of viral hemorrhagic septicemia virus using real-time RT-PCR. I. Initial comparison of four protocols

Eight laboratories worked collectively to evaluate 4 real-time RT-PCR (rRT-PCR) protocols targeting viral hemorrhagic septicemia virus (VHSV) being considered for deployment to a USA laboratory testing network. The protocols utilized previously published primers and probe sets developed for detection and surveillance of VHSV. All participating laboratories received and followed a standard operating protocol for extraction and for each of the rRT-PCR assays. Performance measures specifically evaluated included limit of detection (defined as the smallest amount of analyte in which 95% of the samples are classified as positive), analytical specificity, assay efficiency across genotype representatives, within- and between-plate variation within a laboratory, and variation between laboratories using the same platform, between platforms, and between software versions. This evaluation clearly demonstrated that the TaqMan®-based assay developed by Jonstrup et al. (2013; J Fish Dis 36:9-23) produced the most consistent analytical performance characteristics for detecting all genotypes of VHSV across the 8 participating laboratories.

Accurate detection and quantification of the fish viral hemorrhagic Septicemia virus (VHSv) with a two-color fluorometric real-time PCR assay

Viral Hemorrhagic Septicemia virus (VHSv) is one of the world's most serious fish pathogens, infecting >80 marine, freshwater, and estuarine fish species from Eurasia and North America. A novel and especially virulent strain - IVb - appeared in the Great Lakes in 2003, has killed many game fish species in a series of outbreaks in subsequent years, and shut down interstate transport of baitfish. Cell culture is the diagnostic method approved by the USDA-APHIS, which takes a month or longer, lacks sensitivity, and does not quantify the amount of virus. We thus present a novel, easy, rapid, and highly sensitive real-time quantitative reverse transcription PCR (qRT-PCR) assay that incorporates synthetic competitive template internal standards for quality control to circumvent false negative results. Results demonstrate high signal-to-analyte response (slope = 1.00±0.02) and a linear dynamic range that spans seven orders of magnitude (R(2) = 0.99), ranging from 6 to 6,000,000 molecules. Infected fishes are found to harbor levels of virus that range to 1,200,000 VHSv molecules/10(6) actb1 molecules with 1,000 being a rough cut-off for clinical signs of disease. This new assay is rapid, inexpensive, and has significantly greater accuracy than other published qRT-PCR tests and traditional cell culture diagnostics.

Comparative susceptibility among three stocks of yellow perch, Perca flavescens (Mitchill), to viral haemorrhagic septicaemia virus strain IVb from the Great Lakes

The Great Lakes strain of viral haemorrhagic septicaemia virus IVb (VHSV-IVb) is capable of infecting a wide number of naive species and has been associated with large fish kills in the Midwestern United States since its discovery in 2005. The yellow perch, Perca flavescens (Mitchill), a freshwater species commonly found throughout inland waters of the United States and prized for its high value in sport and commercial fisheries, is a species documented in several fish kills affiliated with VHS. In the present study, differences in survival after infection with VHSV IVb were observed among juvenile fish from three yellow perch broodstocks that were originally derived from distinct wild populations, suggesting innate differences in susceptibility due to genetic variance. While all three stocks were susceptible upon waterborne exposure to VHS virus infection, fish derived from the Midwest (Lake Winnebago, WI) showed significantly lower cumulative % survival compared with two perch stocks derived from the East Coast (Perquimans River, NC and Choptank River, MD) of the United States. However, despite differences in apparent susceptibility, clinical signs did not vary between stocks and included moderate-to-severe haemorrhages at the pelvic and pectoral fin bases and exophthalmia. After the 28-day challenge was complete, VHS virus was analysed in subsets of whole fish that had either survived or succumbed to the infection using both plaque assay and quantitative PCR methodologies. A direct correlation was identified between the two methods, suggesting the potential for both methods to be used to detect virus in a research setting.

Expression of the marA, soxS, acrB and ramA genes related to the AcrAB/TolC efflux pump in Salmonella enterica strains with and without quinolone resistance-determining regions gyrA gene mutations

Several studies have been conducted in recent years to elucidate the structure, function and significance of AcrB, MarA, SoxS and RamA in Salmonella enterica. In this study, the relative quantification of acrB, soxS, marA and ramA genes expression was evaluated in 14 strains of S. enterica, with or without accompanying mutations in the quinolone resistance-determining regions of the gyrA gene, that were exposed to ciprofloxacin during the exponential growth phase. The presence of ciprofloxacin during the log phase of bacterial growth activated the genes marA, soxS, ramA and acrB in all S. enterica strains analyzed in this study. The highest expression levels for acrB were observed in strains with gyrA mutation, and marA showed the highest expression in the strains without mutation. Considering only the strains with ciprofloxacin minimum inhibitory concentration values < 0.125 μg/mL (sensitive to ciprofloxacin), the most expressed gene in the strains both with and without mutations was acrB. In the strains with ciprofloxacin minimum inhibitory concentration values ≥ 0.125 μg/mL (low susceptibility), with and without mutations in gyrA, the most expressed gene was marA. In this study, we observed that strains resistant to nalidixic acid may express genes associated with the efflux pump and the expression of the AcrAB-TolC pump genes seems to occur independently of mutations in gyrA.

Development and validation of a novel Taqman-based real-time RT-PCR assay suitable for demonstrating freedom from viral haemorrhagic septicaemia virus

Viral haemorrhagic septicaemia (VHS) is a serious disease in several fish species. VHS is caused by the rhabdovirus viral haemorrhagic septicaemia virus (VHSV). To prevent spreading of the pathogen, it is important to use a fast, robust, sensitive and specific diagnostic tool to identify the infected fish. Traditional diagnosis based on isolation in cell culture followed by identification using, for example, ELISA is sensitive and specific but slow. By switching to RT-PCR for surveillance and diagnosis of VHS the time needed before a correct diagnosis can be given will be considerably shortened and the need for maintaining expensive cell culture facilities reduced. Here we present the validation, according to OIE guidelines, of a sensitive and specific Taqman-based real-time RT-PCR. The assay detects all isolates in a panel of 79 VHSV isolates covering all known genotypes and subtypes, with amplification efficiencies of approximately 100%. The analytical and diagnostic specificity of the real-time RT-PCR is close to 1, and the analytical and diagnostic sensitivity is comparable with traditional cell-based methods. In conclusion, the presented real-time RT-PCR assay has the necessary qualities to be used as a VHSV surveillance tool on par with cell culture assays.

Ex vivo transfection of trout pronephros leukocytes, a model for cell culture screening of fish DNA vaccine candidates

DNA vaccination opened a new era in controlling and preventing viral diseases since DNA vaccines have shown to be very efficacious where some conventional vaccines have failed, as it occurs in the case of the vaccines against fish novirhabdoviruses. However, there is a big lack of in vitro model assays with immune-related cells for preliminary screening of in vivo DNA vaccine candidates. In an attempt to solve this problem, rainbow trout pronephros cells in early primary culture were transfected with two plasmid DNA constructions, one encoding the green fluorescent protein (GFP) and another encoding the viral haemorrhagic septicaemia virus (VHSV) glycoprotein G (G(VHSV)) - the only viral antigen which has conferred in vivo protection. After assessing the presence of GFP- and G(VHSV)-expressing cells, at transcription and protein levels, the immune response in transfected pronephros cells was evaluated. At 24h post-transfection, G(VHSV) up-regulated migm and tcr transcripts expression, suggesting activation of B and T cells, as well, a high up-regulation of tnfα gene was observed. Seventy-two hours post-transfection, we detected the up-regulation of mx and tnfα genes transcripts and Mx protein which correlated with the induction of an anti-VHSV state. All together we have gathered evidence for successful transfection of pronephros cells with pAE6G, which correlates with in vivo protection results, and is less time-consuming and more rapid than in vivo assays. Therefore, this outcome opens the possibility to use pronephros cells in early primary culture for preliminary screening fish DNA vaccines as well as to further investigate the function that these cells perform in fish immune response orchestration after DNA immunisation.

Protection and antibody response induced by intramuscular DNA vaccine encoding for viral haemorrhagic septicaemia virus (VHSV) G glycoprotein in turbot (Scophthalmus maximus)

Turbot (Scophthalmus maximus) is a high-value farmed marine flatfish with growing demand and production levels in Europe susceptible to turbot-specific viral haemorrhagic septicaemia virus (VHSV) strains. To evaluate the possibility of controlling the outbreaks of this infectious disease by means of DNA vaccination, the gpG of a VHSV isolated from farmed turbot (VHSV(860)) was cloned into an expression plasmid containing the human cytomegalovirus (CMV) promoter (pMCV1.4-G(860)). In our experimental conditions, DNA immunised turbots were more than 85% protected against VHSV(860) lethal challenge and showed both VHSV-gpG specific and neutralizing antibodies. To our knowledge this is the first report showing the efficacy of turbot genetic immunisation against a VHSV. Work is in progress to determine the contribution of innate and adaptive immunity to the protective response elicited by the immunization.

Toll-like receptors and interferon associated immune factors in viral haemorrhagic septicaemia virus-infected olive flounder (Paralichthys olivaceus)

Pattern recognition receptor (PRR) toll-like receptors (TLRs), antiviral agent interferon (IFN) and the effector IFN stimulated genes (ISGs) play pivotal role in antiviral innate immunity of a host. The present in-vivo experiment was conducted to investigate the role of these innate immune factors in early phase as well as during recovery of viral haemorrhagic septicaemia virus (VHSV) infection by quantitative real-time reverse transcriptase polymerase chain reaction. A less lethal VHSV infection was generated in olive flounder (Paralichthys olivaceus) and was sampled at 3, 6, and 12h post infection (hpi), and 1, 2, 4, and 7 days post infection (dpi). At 3 hpi, the VHSV N gene was detected in three out of five fish and all five fish showed a relative fold increase of TLR 2, TLR 7, interleukin 8 (IL 8), IFN regulatory factor 3 (IRF 3), IRF 7, and ISG 15. Viral copies rapidly increased at 12 hpi then remained high until 2 dpi. When viral copy numbers were high, a higher expression of immune genes IL 1β, IRF 3, IRF 7, Type I IFN, ISG 15 and Mx was observed. Viral copies were drastically reduced in 4 and 7 dpi fish, and also the immune response was considerably reduced but remained elevated, except for ISG 15 which found equal to control in 7 dpi fish. A high degree of correlation was observed between immune genes and viral copy number in each of the sampled fish at 12 hpi. A fish with ascites sampled at 7 dpi displayed high viral copy but under-expressed immune genes except for Mx. When viral copies were high at 1 and 2 dpi, both TLR 2 and TLR 7 were down-regulated, perhaps indicating immune suppression by the virus. The quick and prolonged elevated expression of the immune genes indicates their crucial role in survival of host against VHSV.

Mytilus galloprovincialis myticin C: a chemotactic molecule with antiviral activity and immunoregulatory properties

Previous research has shown that an antimicrobial peptide (AMP) of the myticin class C (Myt C) is the most abundantly expressed gene in cDNA and suppressive subtractive hybridization (SSH) libraries after immune stimulation of mussel Mytilus galloprovincialis. However, to date, the expression pattern, the antimicrobial activities and the immunomodulatory properties of the Myt C peptide have not been determined. In contrast, it is known that Myt C mRNA presents an unusual and high level of polymorphism of unidentified biological significance. Therefore, to provide a better understanding of the features of this interesting molecule, we have investigated its function using four different cloned and expressed variants of Myt C cDNA and polyclonal anti-Myt C sera. The in vivo results suggest that this AMP, mainly present in hemocytes, could be acting as an immune system modulator molecule because its overexpression was able to alter the expression of mussel immune-related genes (as the antimicrobial peptides Myticin B and Mytilin B, the C1q domain-containing protein MgC1q, and lysozyme). Moreover, the in vitro results indicate that Myt C peptides have antimicrobial and chemotactic properties. Their recombinant expression in a fish cell line conferred protection against two different fish viruses (enveloped and non-enveloped). Cell extracts from Myt C expressing fish cells were also able to attract hemocytes. All together, these results suggest that Myt C should be considered not only as an AMP but also as the first chemokine/cytokine-like molecule identified in bivalves and one of the few examples in all of the invertebrates.

Development and validation of a reverse transcription quantitative PCR for universal detection of viral hemorrhagic septicemia virus

Viral hemorrhagic septicemia virus (VHSV) infects over 70 fish species inhabiting marine, brackish or freshwater environments throughout the Northern Hemisphere. Over its geographic range, 4 VHSV genotypes and multiple subtypes exist. Here, we describe the development and validation of a rapid, sensitive and specific real-time reverse transcription quantitative PCR assay (RT-qPCR) that amplifies sequence from representative isolates of all VHSV genotypes (I, II, III and IV). The pan-specific VHSV RT-qPCR assay reliably detects 100 copies of VHSV nucleoprotein RNA without cross-reacting with infectious hematopoietic necrosis virus, spring viremia of carp virus or aquatic birnavirus. Test performance characteristics evaluated on experimentally infected Atlantic salmon Salmo salar L. revealed a diagnostic sensitivity (DSe) > or = 93% and specificity (DSp) = 100%. The repeatability and reproducibility of the procedure was exceptionally high, with 93% agreement among test results within and between 2 laboratories. Furthermore, proficiency testing demonstrated the VHSV RT-qPCR assay to be easily transferred to and performed by a total of 9 technicians representing 4 laboratories in 2 countries. The assay performed equivalent to the traditional detection method of virus isolation via cell culture with the advantage of faster turnaround times and high throughput capacity, further suggesting the suitability of the use of this VHSV RT-qPCR in a diagnostic setting.

Viral load of various tissues of rainbow trout challenged with viral haemorrhagic septicaemia virus at various stages of disease

Market-sized rainbow trout Oncorhynchus mykiss were challenged by waterborne exposure to viral haemorrhagic septicaemia virus (VHSV isolate of genogroup Ia). Fish were sampled at 4 stages of infection (before onset of clinical signs, clinically affected fish, mortalities and survivors) and the viral load determined in (1) internal organs, (2) muscle tissue and (3) brain and gill tissue. Virus levels were determined by virus titration and real-time RT-PCR. VHSV was detected by either method in the majority of fish before onset of clinical signs and in the survivor group as well as in all fish in the clinically affected fish and mortality groups. Mean virus amounts per mg of tissue determined by virus titration (TCID50) or real-time RT-PCR (copy number) were > 10(4) in preclinical fish, > 10(3.8) in clinically affected fish, > 10(3.9) in mortalities and > 10(1.2) in survivors. Virus levels tended to be highest in the internal organs of subclinical and clinically affected fish and in brain and gill tissue of survivors. The results demonstrate that significant levels of VHSV can be found in tissues of rainbow trout that may be marketed for human consumption, which may have relevance for the biosecurity of VHS-free areas.

In vitro analysis of the factors contributing to the antiviral state induced by a plasmid encoding the viral haemorrhagic septicaemia virus glycoprotein G in transfected trout cells

We have found out that transfection of the RTG-2 cell line with the viral haemorrhagic septicaemia virus (VHSV) glycoprotein G (G(VHSV))-coding plasmid induces an anti-VHSV state, similar to that induced by poly I:C. Taking the advantage of the constitutive expression of toll-like receptor 9 gene (tlr9) in RTG-2 cells, we have investigated whether this antiviral state was induced by the cytosine-phosphodiester-guanine (CpG) motifs present in the plasmid DNA, by the endogenous expression of G(VHSV) protein or by both elements. For that, we have analysed the expression profile of the rainbow trout tlr9 and several genes related to TLR9-mediated immune response in the absence or presence of a lysosomotropic drug that specifically blocks TLR9-CpG DNA interaction. The results suggested that the high levels of cell protection conferred by a plasmid encoding G(VHSV) gene are due to G(VHSV) rather than to the CpG motifs within plasmid DNA. Therefore, plasmid DNA might not play a key role in the immune response elicited by DNA vaccines or perhaps other receptors instead TLR9 could be implicated in CpG motifs recognition and signalling. In addition, since RTG-2 cells express tlr9 gene, this cell line could be a good tool for screening TLR9 agonists, such as the immunomodulatory oligonucleotides (IMOs), as fish DNA vaccine adjuvants.

Pepscan mapping of viral hemorrhagic septicemia virus glycoprotein G major lineal determinants implicated in triggering host cell antiviral responses mediated by type I interferon

Surface glycoproteins of enveloped virus are potent elicitors of type I interferon (IFN)-mediated antiviral responses in a way that may be independent of the well-studied genome-mediated route. However, the viral glycoprotein determinants responsible for initiating the IFN response remain unidentified. In this study, we have used a collection of 60 synthetic 20-mer overlapping peptides (pepscan) spanning the full length of glycoprotein G (gpG) of viral hemorrhagic septicemia virus (VHSV) to investigate what regions of this protein are implicated in triggering the type I IFN-associated immune responses. Briefly, two regions with ability to increase severalfold the basal expression level of the IFN-stimulated mx gene and to restrict the spread of virus among responder cells were mapped to amino acid residues 280 to 310 and 340 to 370 of the gpG protein of VHSV. In addition, the results obtained suggest that an interaction between VHSV gpG and integrins might trigger the host IFN-mediated antiviral response after VHSV infection. Since it is known that type I IFN plays an important role in determining/modulating the protective-antigen-specific immune responses, the identification of viral glycoprotein determinants directly implicated in the type I IFN induction might be of special interest for designing new adjuvants and/or more-efficient and cost-effective viral vaccines as well as for improving our knowledge on how to stimulate the innate immune system.

Distribution of an invasive aquatic pathogen (viral hemorrhagic septicemia virus) in the Great Lakes and its relationship to shipping

Viral hemorrhagic septicemia virus (VHSV) is a rhabdovirus found in fish from oceans of the northern hemisphere and freshwaters of Europe. It has caused extensive losses of cultured and wild fish and has become established in the North American Great Lakes. Large die-offs of wild fish in the Great Lakes due to VHSV have alarmed the public and provoked government attention on the introduction and spread of aquatic animal pathogens in freshwaters. We investigated the relations between VHSV dispersion and shipping and boating activity in the Great Lakes by sampling fish and water at sites that were commercial shipping harbors, recreational boating centers, and open shorelines. Fish and water samples were individually analyzed for VHSV using quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and cell culture assays. Of 1,221 fish of 17 species, 55 were VHSV positive with highly varied qRT-PCR titers (1 to 5,950,000 N gene copies). The detections of VHSV in fish and water samples were closely associated and the virus was detected in 21 of 30 sites sampled. The occurrence of VHSV was not related to type of site or shipping related invasion hotspots. Our results indicate that VHSV is widely dispersed in the Great Lakes and is both an enzootic and epizootic pathogen. We demonstrate that pathogen distribution information could be developed quickly and is clearly needed for aquatic ecosystem conservation, management of affected populations, and informed regulation of the worldwide trade of aquatic organisms.