Betanodavirus of marine and freshwater fish: distribution, genomic organization, diagnosis and control measures

Nervous necrosis virus induced vacuolization is a Rab5- and actin-dependent process

Cytoplasmic vacuolization is commonly induced by bacteria and viruses, reflecting the complex interactions between pathogens and the host. However, their characteristics and formation remain unclear. Nervous necrosis virus (NNV) infects more than 100 global fish species, causing enormous economic losses. Vacuolization is a hallmark of NNV infection in host cells, but remains a mystery. In this study, we developed a simple aptamer labelling technique to identify red-spotted grouper NNV (RGNNV) particles in fixed and live cells to explore RGNNV-induced vacuolization. We observed that RGNNV-induced vacuolization was positively associated with the infection time and virus uptake. During infection, most RGNNV particles, as well as viral genes, colocalized with vacuoles, but not giant vacuoles > 3 μm in diameter. Although the capsid protein (CP) is the only structural protein of RGNNV, its overexpression did not induce vacuolization, suggesting that vacuole formation probably requires virus entry and replication. Given that small Rab proteins and the cytoskeleton are key factors in regulating cellular vesicles, we further investigated their roles in RGNNV-induced vacuolization. Using live cell imaging, Rab5, a marker of early endosomes, was continuously located in vacuoles bearing RGNNV during giant vacuole formation. Rab5 is required for vacuole formation and interacts with CP according to siRNA interference and Co-IP analysis. Furthermore, actin formed distinct rings around small vacuoles, while vacuoles were located near microtubules. Actin, but not microtubules, plays an important role in vacuole formation using chemical inhibitors. These results provide valuable insights into the pathogenesis and control of RGNNV infections.

Novel lytic bacteriophage AhFM11 as an effective therapy against hypervirulent Aeromonas hydrophila

Several farmed fish species, including carps, tilapia, salmon, and catfish, have experienced significant economic losses in aquaculture due to motile Aeromonas septicemia caused by Aeromonas hydrophila. In the present study, a novel lytic bacteriophage infecting hypervirulent Aeromonas hydrophila (vAh) was isolated and characterized. This is the first report of a phage against vAh. Phage AhFM11 demonstrated lytic activity against both vAh strains and the A. hydrophila reference strain ATCC 35654. The AhFM11 genome was sequenced and assembled, comprising 168,243 bp with an average G + C content of 41.5%. The genome did not harbor any antibiotic resistance genes. Genomic information along with transmission electron microscopy revealed that phage AhFM11 belongs to the Straboviridae family. Therapeutic application of monophage AhFM11 in fish showed 100% survival in injection, 95% in immersion and 93% in oral feeding of phage top-coated feed. Fish and chicken meat spiked with A. hydrophila and phage showed significant reduction of A. hydrophila. These findings support that phage AhFM11 can be used as a biocontrol agent against vAh as an alternative to antibiotics.

The susceptibility of shi drum juveniles to betanodavirus increases with rearing densities in a process mediated by neuroactive ligand-receptor interaction

Nervous necrosis virus (NNV) is one of the greatest threats to Mediterranean aquaculture, infecting more than 170 fish species and causing mortalities up to 100% in larvae and juveniles of susceptible species. Intensive aquaculture implies stressed conditions that affect the welfare of fish and their ability to fight against infections. In fact, a higher susceptibility to NNV has been related to poor welfare conditions. In order to analyze the physiological link between stressed conditions and increased susceptibility to NNV, as well as its possible role in the pathogenesis of this disease, we reared shi drum (Umbrina cirrosa) juveniles (30.7 ± 3.10 g body weight), which are expected to be asymptomatic upon NNV infection, at three stocking densities (2, 15, and 30 kg/m3) for 27 days and subsequently challenged them with NNV. We firstly characterized the stressed conditions of the specimens before and after infection and recorded the mortalities, demonstrating that stressed specimens reared at 30 kg/m3 suffered mortalities. However, the viral loads in different tissues were similar in all experimental groups, allowing horizontal and vertical transmission of the virus from asymptomatic specimens. All of these data suggest that shi drum tolerates wide ranges of culture densities, although high densities might be a setback for controlling NNV outbreaks in this species. In an attempt to understand the molecular pathways orchestrating this susceptibility change in stressed conditions, we performed a transcriptomic analysis of four tissues under mock- and NNV-infected conditions. In addition to the modification of the exceptive pathways such as cell adhesion, leukocyte migration, cytokine interaction, cell proliferation and survival, and autophagy, we also observed a heavy alteration of the neuroactive ligand-receptor pathway in three of the four tissues analyzed. Our data also point to some of the receptors of this pathway as potential candidates for future pharmacological treatment to avoid the exacerbated immune response that could trigger fish mortalities upon NNV infection.

RNA-Seq Virus Fraction in Lake Baikal and Treated Wastewaters

In this study, we analyzed the transcriptomes of RNA and DNA viruses from the oligotrophic water of Lake Baikal and the effluent from wastewater treatment plants (WWTPs) discharged into the lake from the towns of Severobaikalsk and Slyudyanka located on the lake shores. Given the uniqueness and importance of Lake Baikal, the issues of biodiversity conservation and the monitoring of potential virological hazards to hydrobionts and humans are important. Wastewater treatment plants discharge treated effluent directly into the lake. In this context, the identification and monitoring of allochthonous microorganisms entering the lake play an important role. Using high-throughput sequencing methods, we found that dsDNA-containing viruses of the class Caudoviricetes were the most abundant in all samples, while Leviviricetes (ssRNA(+) viruses) dominated the treated water samples. RNA viruses of the families Nodaviridae, Tombusviridae, Dicitroviridae, Picobirnaviridae, Botourmiaviridae, Marnaviridae, Solemoviridae, and Endornavirida were found in the pelagic zone of three lake basins. Complete or nearly complete genomes of RNA viruses belonging to such families as Dicistroviridae, Marnaviridae, Blumeviridae, Virgaviridae, Solspiviridae, Nodaviridae, and Fiersviridae and the unassigned genus Chimpavirus, as well as unclassified picorna-like viruses, were identified. In general, the data of sanitary/microbiological and genetic analyses showed that WWTPs inadequately purify the discharged water, but, at the same time, we did not observe viruses pathogenic to humans in the pelagic zone of the lake.

Silencing Asian Seabass gab3 Inhibits Nervous Necrosis Virus Replication

The nervous necrosis virus (NNV) causes the viral nervous necrosis (VNN) disease in aquatic animals and has been a major threat in aquaculture. Thus, it is essential for the development of a prevention method to minimize economic losses caused by NNV such as the identification of NNV resistance genes and application of these genes in molecular breeding to increase disease resistance. gab3 is an important NNV resistance gene in Asian seabass. However, the mechanism of gab3 in NNV resistance has not been elucidated. In this study, knockdown of gab3 in NNV-infected Asian seabass cells resulted in a significant decrease in viral RNA and virus titers. Knockout of gab3 in zebrafish led to an increased survival rate and resistant time after NNV infection. Cellular localization of the GAB3 and NNV by immunofluorescence staining showed that the GAB3 was translocated from the nucleus to the cytoplasm, and finally reached the cell membrane of SB cells after 48 h post NNV infection. Our study suggests that gab3 plays an important role in NNV replication and silencing gab3 can inhibit virus replication.

Cromileptes altivelis microRNA Transcriptome Analysis upon Nervous Necrosis Virus (NNV) Infection and the Effect of cal-miR-155 on Cells Apoptosis and Virus Replication

MicroRNAs (miRNAs) could regulate various biological processes. Nervous necrosis virus (NNV) is one of the primary germs of the Humpback grouper (Cromileptes altivelis), a commercial fish of great importance for Asian aquaculture. However, there is limited available information on the host-virus interactions of C. altivelis. miRNAs have been shown to play key roles in the host response to infection by a variety of pathogens. To better understand the regulatory mechanism of miRNAs, we constructed miRNA transcriptomes and identified immune-related miRNAs of C. altivelis spleen in response to NNV infection. Reads from the three libraries were mapped onto the Danio rerio reference genome. As a result, a total of 942 mature miRNAs were determined, with 266 known miRNAs and 676 novel miRNAs. Among them, thirty-two differentially expressed miRNAs (DEmiRs) were identified compared to the PBS control. These DEmiRs were targeted on 895 genes, respectively, by using miRanda v3.3a. Then, 14 DEmiRs were validated by qRT-PCR and showed consistency with those obtained from high-throughput sequencing. In order to study the relationship between viral infection and host miRNA, a cell line from C. altivelis brain (CAB) was used to examine the expressions of five known DEmiRs (miR-132-3p, miR-194a, miR-155, miR-203b-5p, and miR-146) during NNV infection. The results showed that one miRNA, cal-miRNA-155, displayed significantly increased expression in response to the virus infection. Subsequently, it was proved that overexpression of cal-miR-155 enhanced cell apoptosis with or without NNV infection and inhibited virus replication in CAB cells. Oppositely, the cal-miRNA-155 inhibitor markedly suppressed apoptosis in CAB cells. The results of the apoptosis-related genes mRNA expression also showed the regulation of cal-miR-155 on the apoptosis process in CAB cells. These findings verify that miR-155 might exert a function as a pro-apoptotic factor in reply to NNV stimulation in CAB cells and help us further study the molecular mechanisms of the pathogenesis of NNV in C. altivelis.

Isolation and identification of a new strain of nervous necrosis virus from the big-belly seahorse Hippocampus abdominalis

Background

Betanodaviruses, members of the Nodaviridae family, are the causative agents of viral nervous necrosis in fish, resulting in great economic losses worldwide.

Methods

In this study, we isolated a virus strain named seahorse nervous necrosis virus (SHNNV) from cultured big-belly seahorses Hippocampus abdominalis in Xiamen city, Fujian Province, China. Virus isolation, PCR detection, phylogenetic analysis, qRT-PCR, fluorescence in situ hybridization and histology were used for virus identification and analysis of virus histopathology. Furthermore, an artificial infection experiment was conducted for virulence testing.

Results

Brain and eye tissue homogenates of diseased big-belly seahorses were inoculated onto a grouper spleen (GS) cell monolayer at 28 °C. Tissue homogenates induced obvious cytopathic effects in GS cells. PCR and sequencing analyses revealed that the virus belonged to Betanodavirus and shared high sequence identity with red-spotted grouper nervous necrosis virus isolates. qRT-PCR and fluorescence in situ hybridization revealed that SHNNV mainly attacked the brain and eye. Histopathological examination revealed that the virus led to cytoplasmic vacuolation in the brain and retinal tissues. Infection experiments confirmed that SHNNV was highly infectious, causing massive death in big-belly seahorses.

Conclusion

A novel seahorse betanodavirus from the big-belly seahorse cultured in China was discovered. This finding will contribute to the development of efficient strategies for disease management in aquaculture.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-022-01837-8.

High-density lipoproteins negatively regulate innate immunity and facilitate red-spotted grouper nervous necrosis virus entry via scavenger receptor B type 1

Lipid metabolism plays an important role in viral infections, and it can directly or indirectly affect various stages of viral infection in cells. As an important component of lipid metabolism, high-density lipoprotein (HDL) plays crucial roles in inflammation, immunity, and viral infections. Scavenger receptor B type 1 (SR-B1), a receptor of HDL, cannot be ignored in the regulation of lipid metabolism. Here, we investigate, for the first time, the role of Epinephelus coioides SR-B1 (Ec-SR-B1) in red-spotted grouper nervous necrosis virus (RGNNV) infection. Our results indicate that Ec-SR-B1 could promote RGNNV infection. We also demonstrate that Ec-SR-B1 could facilitate viral entry and interact with capsid protein (CP) of RGNNV. As the natural ligand of SR-B1, HDL significantly increased RGNNV entry in a dose-dependent manner. However, we observed no effect of HDL on Ec-SR-B1 expression. The results of the micro-scale thermophoresis assay did not reveal an association between HDL and CP, suggesting that RGNNV does not enter target cells by using HDL as a ligand to bind to its receptor. In addition, block lipid transport-1, a compound that inhibits HDL-mediated cholesterol transfer, reduced the HDL-induced enhancement of RGNNV infection, indicating a role for lipid transfer in facilitating RGNNV entry. Furthermore, HDL inhibited the expression of pro-inflammatory factors and antiviral genes in a dose-dependent manner. These findings suggest that the HDL-induced enhancement of RGNNV entry involves the complex interplay between Ec-SR-B1, HDL, and RGNNV, as well as the regulation of innate antiviral responses by HDL. In summary, we highlight the crucial role of HDL in RGNNV entry, identify a possible molecular connection between RGNNV and lipoprotein metabolism, and indicate the role of Ec-SR-B1 in RGNNV infection.

Development and Evaluation of a Bicistronic DNA Vaccine against Nervous Necrosis Virus in Pearl Gentian Grouper (Epinephelus lanceolatus × Epinephelus fuscoguttatus)

Nervous necrosis virus (NNV) can cause enormous economic losses in mariculture. Vaccines are promising ways to control the disease. In this study: the interferon regulatory factor 3 (IRF3) gene of pearl gentian grouper was cloned and functionally analyzed; then a bicistronic DNA vaccine encoding both capsid protein (CP) and IRF3 was constructed; then the cellular, humoral, and local immune responses in the grouper after immunization were investigated; and then the protective effects after the NNV challenge were investigated. The results showed that the vaccine successfully expressed CP and IRF3. After immunization, the lymphocytes were recruited at the injection site in the muscles. The percentage of sIgM+ lymphocytes in the head, kidney, and spleen significantly increased and peaked at 28.8 ± 3.1% and 42.6 ± 4.2% at the 3rd to 4th weeks. Six immune-related genes were significantly up-regulated. In the meantime, the total antibodies, anti-NNV specific antibodies, and neutralizing antibody titers in serum increased. After the challenge with 10⁵, 10⁶ or 10⁷ TCID₅₀/fish, the relative percent survival rate was 81.25%, 73.91%, and 66.67%, respectively. In 10⁶ TCID₅₀/fish groups, the percentages of sIgM+ lymphocytes, antibodies, and the viral load were investigated. In conclusion, the bicistronic vaccine significantly induced humoral and cellular responses in pearl gentian grouper and provided effective protection against NVV infection.

Pathogenicity of Different Betanodavirus RGNNV/SJNNV Reassortant Strains in European Sea Bass

European sea bass (Dicentrarchus labrax) is an important farmed marine species for Mediterranean aquaculture. Outbreaks of betanodavirus represent one of the main infectious threats for this species. The red-spotted grouper nervous necrosis virus genotype (RGNNV) is the most widely spread in Southern Europe, while the striped jack nervous necrosis virus genotype (SJNNV) has been rarely detected. The existence of natural reassortants between these genotypes has been demonstrated, the RGNNV/SJNNV strain being the most common. This study aimed to evaluate the pathogenicity of different RGNNV/SJNNV strains in European sea bass. A selection of nine European reassortants together with parental RGNNV and SJNNV strains were used to perform in vivo experimental challenges via intramuscular injection. Additional in vivo experimental challenges were performed by bath immersion in order to mimic the natural infection route of the virus. Overall, results on survival rates confirmed the susceptibility of European sea bass to reassortants and showed different levels of induced mortalities. Results obtained by RT-qPCR also highlighted high viral loads in asymptomatic survivors, suggesting a possible reservoir role of this species. Our findings on the comparison of complete genomic segments of all reassortants have shed light on different amino acid residues likely involved in the variable pathogenicity of RGNNV/SJNNV strains in European sea bass.

Amantadine, a promising therapeutic agent against viral encephalopathy and retinopathy

Outbreaks of viral encephalopathy and retinopathy (VER) in marine and freshwater species severely devastate the aquaculture worldwide. The causative agent of VER is nervous necrosis virus (NNV), which mainly infects the early developmental stages of fish, limiting the effectiveness of vaccines. To counter this case, the anti-NNV potentials of nine drugs with broad-spectrum antiviral activity were explored using ribavirin as a positive drug. Toxicity of the selected drugs to SSN-1 cells and grouper was firstly evaluated to determine the safety concentrations. For screening in vitro, amantadine and oseltamivir phosphate can relieve the cytopathic effects and inhibit NNV replication with the 90% inhibitory concentrations (IC₉₀ ) of 38.74 and 106.75 mg/L, respectively. Amantadine has a stronger anti-NNV activity than ribavirin at the with- and post-NNV infection stages, indicating that it is a potential therapeutic agent against VER by acting directly on NNV. Similarly, amantadine also has a strong anti-NNV activity in vivo with the IC₉₀ of 27.91 mg/L at the 7 days post-infection, while that was 73.25 mg/L for ribavirin. Following exposure to amantadine (40 mg/L) and ribavirin (100 mg/L) for 7 days, the survival rates of NNV-infected grouper were increased to 44% and 39%, respectively. The maximum amantadine content (11.88 mg/Kg) in grouper brain was reached following exposure for 24 hr, and amantadine can be quickly excreted from fish, reducing the risk of drug residue. Results so far indicated that amantadine is a promising therapeutic agent against NNV in aquaculture, providing an effective strategy for VER control at the early developmental stages of fish.

Update on the Inactivation Procedures for the Vaccine Development Prospects of a New Highly Virulent RGNNV Isolate

Viral nervous necrosis (VNN) caused by the nervous necrosis virus (NNV) affects a broad range of primarily marine fish species, with mass mortality rates often seen among larvae and juveniles. Its genetic diversification may hinder the effective implementation of preventive measures such as vaccines. The present study describes different inactivation procedures for developing an inactivated vaccine against a new NNV isolate confirmed to possess deadly effects upon the European seabass (Dicentrarchus labrax), an important Mediterranean farmed fish species that is highly susceptible to this disease. First, an NNV isolate from seabass adults diagnosed with VNN was rescued and the sequences of its two genome segments (RNA1 and RNA2) were classified into the red-spotted grouper NNV (RGNNV) genotype, closely clustering to the highly pathogenic 283.2009 isolate. The testing of different inactivation procedures revealed that the virus particles of this isolate showed a marked resistance to heat (for at least 60 °C for 120 min with and without 1% BSA) but that they were fully inactivated by 3 mJ/cm² UV-C irradiation and 24 h 0.2% formalin treatment, which stood out as promising NNV-inactivation procedures for potential vaccine candidates. Therefore, these procedures are feasible, effective, and rapid response strategies for VNN control in aquaculture.

An investigation on protective effects of the new killed vaccine against nervous necrosis virus (NNV) using histopathology and immunohistochemistry approach on the brain and eye tissues of Acipenser stellatus Pallas 1771

The objective of this study was to analyze the efficiency of the killed vaccine against nervous necrosis virus on Acipenser stellutus. Heat inactivated VNN vaccine was administrated in 7 g juveniles of Acipenser stellutus as a laboratory model and it was included in three different adjuvants that were used as injection and immersion forms with different doses. Ten groups consisting of 30 A. stellutus fish in each group (group 1-4 with 3 replications, others with no replicate) were divided totally into 18 aquariums. Two steps of vaccination were done with a one-month interval and after that, all treatments and control groups were challenged by the virulent VNN virus. The mortality rate of immersion and injection groups were 12.9% and 19.8% respectively, compared to 100% mortality in the control group. Histopathology and immunohistochemistry findings were evaluated. According to the mortality rate one month after challenging, a low range mortality of 12.5% was seen in group 2 with no pathological lesion and negative IHC test in the brain and eye tissues, whereas 100% of the control group (unvaccinated group) died with severe vacuolation in the brain and eye tissues and also positive IHC test. The correlation assay between these results concluded that the immersion form with 75% of aquatic-specific Montanide IMS 1312 Seppic adjuvant made better immunization with no pathological sign or forming the complex of antigen-antibody in IHC assay. These findings are important because of the impossibility of injection in the larval stage and also due to the occurrence of the disease in the first stage of sturgeon life which could cause high mortality in susceptible fish in the larval stage.

Efficient RNA virus targeting via CRISPR-CasRx in fish

The emergence of the CRISPR-Cas system as a technology has transformed our ability to modify nucleic acids, and the CRISPR-Cas13 system has been used to target RNA. CasRx is a small sized type VI-D effector (Cas13d) with RNA knockdown efficiency that may have an interference effect on RNA viruses. However, the RNA virus-targeting activity of CasRx still needs to be verified in vivo in vertebrates. In this study, we successfully engineered a highly effective CasRx system for fish virus interference. We designed synthetic mRNA coding for CasRx and used CRISPR RNAs to guide it to target the grouper nervous necrosis virus (RGNNV). This technique resulted in significant interference with virus infections both in vitro and in vivo. These results indicate that CRISPR/CasRx can be used to engineer interference against RNA viruses in fish, which provides a potential novel mechanism for RNA-guided immunity against other RNA viruses in vertebrates. Importance RNA viruses are most important viral pathogens infecting vertebrates and mammals. RNA virus populations are highly dynamic due to short generation times, large population sizes, and high mutation frequencies. Therefore, it is difficult to find a widely effective ways to inhibit RNA viruses. Therefore, we urgently need to develop effective antiviral methods. CasRx is a small sized type VI-D effector (Cas13d) with RNA knockdown efficiency that can have an interference effect on RNA viruses. Nervous necrosis virus (NNV), a non-enveloped positive-strand RNA virus, is one of the most serious viral pathogens infecting more than 40 cultured fish species resulting in huge economic losses worldwide. Here, we establish a novel efective CasRx system for RNA virus interference using NNV and grouper (Epinephelus coioices) as model. Our data show that CasRx have the most robust for RNA virus interference applications in fish and demonstrate its suitability for studying key questions relating to virus biology.

Grouper TRAF4, a Novel, CP-Interacting Protein That Promotes Red-Spotted Grouper Nervous Necrosis Virus Replication

Tumor necrosis factor receptor-associated factors (TRAFs) play important roles in the biological processes of immune regulation, the inflammatory response, and apoptosis. TRAF4 belongs to the TRAF family and plays a major role in many biological processes. Compared with other TRAF proteins, the functions of TRAF4 in teleosts have been largely unknown. In the present study, the TRAF4 homologue (EcTRAF4) of the orange-spotted grouper was characterized. EcTRAF4 consisted of 1413 bp encoding a 471-amino-acid protein, and the predicted molecular mass was 54.27 kDa. EcTRAF4 shares 99.79% of its identity with TRAF4 of the giant grouper (E. lanceolatus). EcTRAF4 transcripts were ubiquitously and differentially expressed in all the examined tissues. EcTRAF4 expression in GS cells was significantly upregulated after stimulation with red-spotted grouper nervous necrosis virus (RGNNV). EcTRAF4 protein was distributed in the cytoplasm of GS cells. Overexpressed EcTRAF4 promoted RGNNV replication during viral infection in vitro. Yeast two-hybrid and coimmunoprecipitation assays showed that EcTRAF4 interacted with the coat protein (CP) of RGNNV. EcTRAF4 inhibited the activation of IFN3, IFN-stimulated response element (ISRE), and nuclear factor-κB (NF-κB). Overexpressed EcTRAF4 also reduced the expression of interferon (IFN)-related molecules and pro-inflammatory factors. Together, these results demonstrate that EcTRAF4 plays crucial roles in RGNNV infection.

Betanodavirus infection in Kuhlia rupestris and Ambassis marianus and isolation of betanodavirus from infected pond water

This is the first report of betanodavirus infection in 2 species of finfish, Kuhlia rupestris (jungle perch) and Ambassis marianus (estuary perchlet). This report also describes isolation of betanodavirus from infected pond water using the SSN-1 cell line. Histopathology of K. rupestris larvae revealed vacuolation in the eye and brain, which was confirmed using betanodavirus-specific immunohistochemistry. The eye and brain from A. marianus and betanodavirus isolated from pond water were confirmed using real-time PCR and Sanger sequencing. High throughput sequencing was used to obtain betanodavirus sequences from paraffin blocks containing infected K. rupestris. The phylogenetic analysis of betanodavirus RNA1 and RNA2 sequences from all 3 sources were associated with the red-spotted grouper nervous necrosis virus (RGNNV) genotype. The RNA1 nucleotide sequence from jungle perch showed 100% identity with the betanodavirus water isolate and 99.37% identity with A. marianus. Furthermore, we have demonstrated the usefulness of combining recovery of viable virus from environmental samples through fish cell line culture with PCR testing as a means of validating the efficacy of chlorination to eradicate betanodavirus from the pond environment.

Modelling the effect of vaccination on transmission dynamics of nervous necrosis virus in grouper larvae Epinephelus coioides

Nervous necrosis virus (NNV) infection in susceptible grouper larvae has been reported to cause high mortalities, leading to great economic losses in aquaculture industry. Although the effects of NNV vaccines on grouper have been broadly investigated, vaccination strategies have not been fully established. To this end, we introduced the parsimonious epidemiological models that explored the assessment of key epidemiological parameters and how they changed when vaccinations showed the effects. We showed that the models capture the published cumulative mortality data accurately. We estimated a basic reproduction number R₀  = 2.44 for NNV transmission in grouper larvae without vaccination. To effectively control NNV transmission by vaccination, a model for disease control was also generalized to attain the goals of controlled reproduction number less than 1. Our results indicated that at least 60% of grouper population needed to be immunized for ~75 min. Our data-driven modelling approach that links the transmission dynamics of NNV and vaccination strategies for grouper has the potential to support evidence-based planning and adaptation of integrated control measures. We encourage that the epidemiology-based framework introduced here can be further implemented for establishing effective vaccination and mitigation actions aimed at controlling diseases in fish farming practices.

Metabolic profiles of fish nodavirus infection in vitro: RGNNV induced and exploited cellular fatty acid synthesis for virus infection

Red-spotted grouper nervous necrosis virus (RGNNV), the causative agent of viral nervous necrosis disease, has caused high mortality and heavy economic losses in marine aquaculture worldwide. However, changes in host cell metabolism during RGNNV infection remain largely unknown. Here, the global metabolic profiling during RGNNV infection and the roles of cellular fatty acid synthesis in RGNNV infection were investigated. As the infection progressed, 71 intracellular metabolites were significantly altered in RGNNV-infected cells compared with mock-infected cells. The levels of metabolites involved in amino acid biosynthesis and metabolism were significantly decreased, whereas those that correlated with fatty acid synthesis were significantly up-regulated during RGNNV infection. Among them, tryptophan and oleic acid were assessed as the most crucial biomarkers for RGNNV infection. In addition, RGNNV infection induced the formation of lipid droplets and re-localization of fatty acid synthase (FASN), indicating that RGNNV induced and required lipogenesis for viral infection. The exogenous addition of palmitic acid (PA) enhanced RGNNV infection, and the inhibition of FASN and acetyl-CoA carboxylase (ACC) significantly decreased RGNNV replication. Additionally, not only inhibition of palmitoylation and phospholipid synthesis, but also destruction of fatty acid β-oxidation significantly decreased viral replication. These data suggest that cellular fatty acid synthesis and mitochondrial β-oxidation are essential for RGNNV to complete the viral life cycle. Thus, it has been demonstrated for the first time that RGNNV infection in vitro overtook host cell metabolism and, in that process, cellular fatty acid synthesis was an essential component for RGNNV replication.

Development of a Novel Lateral Flow Biosensor Combined With Aptamer-Based Isolation: Application for Rapid Detection of Grouper Nervous Necrosis Virus

Nervous necrosis virus (NNV) has infected more than 50 fish species worldwide, and has caused serious economic losses in the aquaculture industries. However, there is no effective antiviral therapy. The development of a rapid and accurate point-of-care diagnostic method for the prevention and control of NNV infection is urgently required. Commonly used methods for NNV detection include the cell culture-based assay, antibody-based assay and polymerase chain reaction (PCR)-based assay. However, these methods have disadvantages as they are time-consuming and complex. In the present study, we developed a simple and sensitive aptamer-based lateral flow biosensor (LFB) method for the rapid detection of red-spotted grouper nervous necrosis virus (RGNNV). An aptamer is a single-stranded nucleotide, which can specifically bind to the target and has many advantages. Based on a previously selected aptamer, which specifically bound to the coat protein of RGNNV (RGNNV-CP), two modified aptamers were used in this study. One aptamer was used for magnetic bead enrichment and the other was used for isothermal strand displacement amplification (SDA). After amplification, the product was further tested by the LFB, and the detection results were observed by the naked eye within 5 min with high specificity and sensitivity. The LFB method could detect RGNNV-CP protein as low as 5 ng/mL or 5 × 103 RGNNV-infected GB (grouper brain) cells. Overall, it is the first application of a LFB combined with aptamer in the rapid diagnosis of virus from aquatic animals, which provides a new option for virus detection in aquaculture.

Current status and future directions of fish vaccines employing virus-like particles

In most breeding schemes, fish are cultured in enclosed spaces, which greatly increases the risk of outbreaks where the onset of infectious diseases can cause massive mortality and enormous economic losses. Vaccination is the most effective and long-term measure for improving the basic make-up of a fish farm. As the relationship between antibody and antigen is similar to that between screw and nut, similarity in the shape or nature of the vaccine antigen to the original pathogen is important for achieving a satisfactory/good/excellent antibody response with a vaccine. Virus-like particles (VLPs) best fulfil this requirement as their tertiary structure mimics that of the native virus. For this reason, VLPs have been attracting attention as next-generation vaccines for humans and animals, and the effects of various types of VLP vaccines on humans and livestock have been examined. Recent studies of VLP-based fish vaccines indicate that these vaccines are promising, and raise hopes of extending their use in the near future. In this review, the structural properties and immunogenicity of VLP-based vaccines against fish viruses such as infectious pancreatic necrosis virus (IPNV), salmonid alphavirus (SAV), nervous necrosis virus (NNV) and iridovirus are introduced/summarized. The NNV VLP vaccine is the most-studied VLP-based vaccine against fish viruses. Therefore, the current status of NNV VLP research is highlighted in this review, which deals with the advantages of using VLPs as vaccines, and the expression systems for producing them. Moreover, the need for lyophilized VLPs and oral VLP delivery is discussed. Finally, future directions for the development of VLP vaccines in the fish vaccine field are considered.

A multi-omic analysis of orange-spotted grouper larvae infected with nervous necrosis virus identifies increased adhesion molecules and collagen synthesis in the persistent state

Grouper (Epinephelus coioides) is an important commercial maricultural fish, which suffers from nervous necrosis virus (NNV) infection. The molecular mechanisms underlying the pathogenesis of the viral infection are not clear. In this study, we combined deep RNA sequencing and label-free mass spectrum for the first time to analyze the transcriptomic and proteomic profiles in infected/dead, infected/survival (persistent), and infection-free (control)orange-spotted groupers in the larval stage. Further analyses showed that the transcriptome and proteome changed dramatically among the three distinct groups, especially differentially-expressed genes in the infected/dead and infected/survival larvae enriched for pathways related to immune response. Notably, the overlapped genes between transcriptomes and proteomes identified that genes related to collagen synthesis and adhesion molecules were enhanced in the persistent (infected/survival) stage, which might contribute to suppressing the acute and lethal immune responses upon NNV infection. These transcriptomic and proteomic datasets enable the investigation of molecular mechanisms underlying NNV infection, thus may help further development of molecular breeding in marine fishery.

The Old and the New on Viral Diseases in Sturgeon

Although sturgeon production by aquaculture has increased worldwide, a major factor limiting its expansion are infectious diseases, although few data about viral diseases are available however. This review provides a rapid overview of viral agents detected and described to date. Following a general introduction on viral diseases are four sections arranged by virus classification: sturgeon nucleocytoplasmic large DNA viruses, herpesviruses, white sturgeon adenovirus 1, and other viruses. Molecular diagnosis is currently the best tool to detect viral diseases, since cell culture isolation is not yet applicable for the detection of most sturgeon viruses.

Susceptibility of betanodavirus in a newly established vertebra-derived cell line from Mosquitofish (Gambusia affinis)

In the present study, a new cell line from the vertebra of mosquitofish Gambusia affinis was successfully established and characterized. The cell line is named as bone Gambusia affinis (BGA) and subcultured for more than 55 passages in Leibovitz's/L15 medium supplemented with 15% FBS at 28°C. The cell line has a modal chromosome number of 48. Molecular characterization of the partial sequence of the coi gene confirmed the origin of the BGA cell line from mosquitofish. These cells exhibited epithelial morphology confirmed by the cytokeratin marker. The BGA cells showed mineralization of their extracellular matrix when stained with alizarin red and von Kossa stain. BGA cells were found to be susceptible to RGNNV and SJNNV strains of betanodavirus (NNV) showing cytopathic effect with multiple vacuolations in the cells. The RT-PCR confirmed the betanodavirus infections in BGA cells. The SEM micrograph showed the morphological changes observed in the cell during virus infection. The in vivo challenge experiment also showed the viral replicating efficiency in the Gambusia affinis with increasing viral titre. Thus, our present results show that the BGA cell line is a useful tool for isolating betanodavirus and could be used to investigate bone cell differentiation and extracellular matrix mineralization.

Lumpfish (Cyclopterus lumpus, Linnaeus) is susceptible to viral nervous necrosis: Result of an experimental infection with different genotypes of Betanodavirus

In recent years, the use of cleaner fish for biological control of sea lice has increased considerably. Along with this, a number of infectious diseases have emerged. The aim of this study was to investigate the susceptibility of lumpfish (Cyclopterus lumpus) to Betanodavirus since it was detected in asymptomatic wild wrasses in Norway and Sweden. Three betanodaviruses were used to challenge lumpfish: one RGNNV genotype and two BFNNV genotypes. Fish were injected and monitored for 4 weeks. Brain samples from clinically affected specimens, from weekly randomly selected fish and survivors were subjected to molecular testing, viral isolation, histopathology and immunohistochemistry. Reduced survival was observed but was attributed to tail-biting behaviour, since no nervous signs were observed throughout the study. Betanodavirus RNA was detected in all samples, additionally suggesting an active replication of the virus in the brain. Viral isolation confirmed molecular biology results and revealed a high viral titre in BFNNV-infected groups associated with typical lesions in brains and eyes of survivor fish. We concluded that lumpfish are susceptible to Betanodavirus, as proven by the high viral titre and brain lesions detected, but further studies are necessary to understand if Betanodavirus can cause clinical disease in this species.

Comparative genome analysis reveals a distinct influence of nucleotide composition on virus-host species-specific interaction of prawn-infecting nodavirus

Discovery of species-specific interaction between the host and virus has drawn the interest of many researchers to study the evolution of the newly emerged virus. Comparative genome analysis provides insights of the virus functional genome evolution and the underlying mechanisms of virus-host interactions. The analysis of nucleotide composition signified the evolution of nodavirus towards host specialization in a host-specific mutation manner. GC-rich genome of betanodavirus was significantly deficient in UpA and UpU dinucleotides composition, whilst the AU-rich genome of gammanodavirus was deficient in CpG dinucleotide. The capsid of MrNV and PvNV of gammanodavirus retains the highest abundance of adenine and uracil at the second codon position, respectively, which were found to be very distinctive from the other genera. ENC-GC3 plot inferred the influence of natural selection and mutational pressure in shaping the evolution of MrNV RdRp and capsid, respectively. Furthermore, CAI/eCAI analysis predicts a comparable adaptability of MrNV in squid, Sepia officinalis than its natural host, Macrobrachium rosenbergii. Thus, further study is warranted to investigate the capacity of MrNV replication in S. officinalis owing to its high codon adaptation index.

Risk factors associated with viral nervous necrosis in hybrid groupers in Malaysia and the high similarity of its causative agent nervous necrosis virus to reassortant red-spotted grouper nervous necrosis virus/striped jack nervous necrosis virus strains

Background and Aim:

Viral nervous necrosis (VNN) is a serious disease of several marine fish species. VNN causes 100% mortality in the larval stages, while lower losses have been reported in juvenile and adult fish. This study aimed to detect the occurrence of VNN while identifying its associated risk factors and the genotypes of its causative agent in a hybrid grouper hatchery in Malaysia.

Materials and Methods:

A batch of newly hatched hybrid grouper fry (Epinephelus fuscoguttatus × Epinephelus lanceolatus) were followed from the larval stage to market size. Samples of the hybrid groupers, water, live feed, and artificial fish pellets were collected periodically from day 0 to 180 in the hybrid grouper hatchery. Reverse transcription-polymerase chain reaction (RT-PCR) and nested PCR amplifications were carried out on VNN-related sequences. The phylogenetic tree including the sampled causative agent of VNN was inferred from the coat protein genes from all known Betanodavirus species using Molecular Evolutionary Genetics Analysis (MEGA). Pearson’s correlation coefficient values were calculated to determine the strength of the correlation between the presence of VNN in hybrid grouper samples and its associated risk factors.

Results:

A total of 113 out of 146 pooled and individual samples, including hybrid grouper, water, and artificial fish pellet samples, demonstrated positive results in tests for the presence of VNN-associated viruses. The clinical signs of infection observed in the samples included darkened skin, deformation of the backbone, abdominal distension, skin lesions, and fin erosion. VNN was present throughout the life stages of the hybrid groupers, with the first detection occurring at day 10. VNN-associated risk factors included water temperature, dissolved oxygen content, salinity, ammonia level, fish size (adults more at risk than younger stages), and life stage (age). Detection of VNN-associated viruses in water samples demonstrated evidence of horizontal transmission of the disease. All the nucleotide sequences found in this study had high nucleotide identities of 88% to 100% to each other, striped jack nervous necrosis virus (SJNNV), and the reassortant strain red-spotted grouper NNV/SJNNV (RGNNV/SJNNV) isolate 430.2004 (GenBank accession number JN189932.1) (n=26). The phylogenetic analysis showed that quasispecies was present in each VNN-causing virus-positive sample, which differed based on the type of sample and life stage.

Conclusion:

This study was the first to confirm the existence of a reassortant strain (RGNNV/SJNNV) in hybrid groupers from Malaysia and Southeast Asia. However, the association between the mode of transmission and the risk factors of this virus needs to be investigated further to understand the evolution and potential new host species of the reassortant strain.

Transcriptome comparative analysis of immune tissues from asymptomatic and diseased Epinephelus moara naturally infected with nervous necrosis virus

Epinephelus moara is an economically important fish in Southeast Asian countries but is suffering from nervous necrosis virus (NNV) infection. A deeper understanding of the host-NNV interaction mechanisms makes sense for disease control, however, at present, the pathogenesis of natural NNV infection and the resistance mechanism in host remains poorly understood. In this study, asymptomatic and diseased E. moara with clinical symptoms of viral nervous necrosis (VNN) from a grouper farm were both detected with a positive RT-PCR signal of NNV, then transcriptome sequencing of their immune tissues (liver, spleen and kidney) were performed for comparation analysis. The de novo assemblies yielded 53,789 unigenes which had a length varied from 201 to 19,675 bp and a N50 length of 2115 bp, and 29,451 unigenes were functionally annotated, with 83, 250 and 5632 unigenes being differentially expressed in liver, spleen and kidney respectively. KEGG pathway enrichment analysis of the DEGs showed many DEGs were enriched in immune related pathways. Although the expression of class I major histocompatibility complex (MHC) was significantly higher in three immune tissues of the diseased grouper, many immune related genes, including humoral immune molecules (such as antibodies), the cellular mediated cytotoxic molecules (such as perforin) and some adhesion related genes were down regulated in the diseased grouper. Our results provided many unigenes that might play important roles in NNV resistance for further research. Furthermore, a total of 8666 unigenes containing 11,623 simple sequence repeats (SSRs) were identified, which provided useful information for screening molecular markers associated with NNV resistance in E. moara.

Detection of Betanodavirus in experimentally infected European seabass (Dicentrarchus labrax, Linnaeus 1758) using non-lethal sampling methods

One of the major disease threats affecting the Mediterranean aquaculture industry is viral encephalopathy and retinopathy (VER). The target organs for Betanodavirus detection are the brain and eyes, obtained through lethal sampling. This study aimed to evaluate the efficacy and suitability of non-lethal samples for detecting Betanodavirus in European seabass (Dicentrarchus labrax). European seabass juveniles were infected with Betanodavirus, by either an intramuscular injection or immersion (10⁷ TCID₅₀ /ml and 10⁶ TCID₅₀ /ml, respectively), and samples collected 7, 15 and 30 days post-infection (dpi). The brain was collected as a lethal sample, and gills, caudal fin and blood as non-lethal tissues for detecting Betanodavirus by quantitative reverse transcription PCR (RT-qPCR). The presence of virus in non-lethal tissues was inconsistent, with lower viral loads than in the brain. For blood, higher viral loads were detected in intramuscular-infected fish at 15 dpi until the end of the challenge. Serum antibodies against Betanodavirus were assessed using an enzyme-linked immunosorbent assay (ELISA). Antibodies were detected as early as 7 dpi, with higher mean antibody titres at 15 and 30 dpi. The presence of Betanodavirus-specific antibodies indicates that this is a suitable evaluation method for detecting early stages of the infection.

Molecular cloning and characterization of grouper Krϋppel-like factor 9 gene: Involvement in the fish immune response to viral infection

Krϋppel-like factor 9 (KLF9) is a member of the SP/KL family, which are transcription factors implicated in several biological processes, including cell proliferation, differentiation, development and apoptosis. Studies have focused on the function of KLF9 in mammalian disease and the immune system, such as its regulatory role in the growth of tumors and its impact on interferon-related genes and inflammatory cytokines. In fish, little is known about the role of KLF9, especially its regulatory function in the innate antiviral immune response. In this study, we characterized the grouper KLF9 gene (EcKLF9) and investigated its role in viral infection. Amino acid alignment analysis showed that EcKLF9 was approximately 228 amino acids long and contained a typical three-tandem Krϋppel-like zinc fingers. Phylogenetic tree analysis revealed that EcKLF9 clustered with three fish species: Amphiprion ocellaris, Acanthochromis pollyacanthus and Stegastes partitus. Comparison analyses showed that the three Kruppel-like zinc finger domains of KLF9 were highly conserved in different fish species. Tissue expression analysis showed that EcKLF9 was constitutively expressed in all 12 tissues tested, in the healthy grouper, the highest expression being detected in the gonads. The relative expression levels of EcKLF9 in the head kidney, spleen and brain was significantly increased during red-spotted grouper nervous necrosis virus (RGNNV) and Singapore grouper iridovirus (SGIV) infections. Using fluorescence microscopy, EcKLF9 was primarily localized to the nucleus and cytoplasm. The in vitro ectopic expression of EcKLF9 significantly increased the severity of vacuoles induced by RGNNV and the cytopathic effect progression evoked by SGIV infection. Real-time PCR results showed that the transcription levels of viral genes, such as the Singapore grouper iridovirus infection genes, MCP (major capsid protein), LITAF (lipopolysaccharide induced TNF-α factor), VP19 (envelop protein) ICP-18 (infected cell protein-18) and the red-spotted grouper nervous necrosis virus genes, CP (coat protein), RdRp (RNA-dependent RNA polymerase), were all significantly increased in EcKLF9 overexpressing cells, when compared to control cells. Furthermore, western blotting analyses showed that protein levels of the RGNNV gene, CP and the SGIV gene, MCP were also increased in EcKLF9 overexpressing cells, suggesting EcKLF9 may promote viral activity against iridovirus and nodavirus, in vitro. Moreover, the overexpression of EcKLF9 significantly inhibited the expression of several interferon related cytokines and several inflammatory cytokines. Accordingly, we speculate that EcKLF9 may exert stimulatory effects on RGNNV and SGIV replication, through the negative regulation of host immune and inflammation responses.

HRI of Epinephelus coioides is a critical factor in the grouper immune response to RGNNV infection

Phosphorylation of eukaryotic initiation factor 2 alpha subunit (eIF2α) occurs under a variety of conditions, including viral infection. Heme-regulated inhibitor (HRI) is an eIF2α kinase that modifies this phosphorylation. In this study, a HRI homologue (EcHRI) from the orange-spotted grouper (Epinephelus coioides) was cloned and its roles during fish viral infection were characterized. EcHRI encodes a 664-amino acid polypeptide that shares a high degree of similarity with HRIs from other species. Quantitative real-time polymerase chain reaction analysis indicated that EcHRI was distributed in all examined tissues. Expression of EcHRI in the spleen of E. coioides was up-regulated when challenged with the synthetic analog of double-stranded RNA (dsRNA) of polyinosine-polycytidylic acid (poly I:C). EcHRI was significantly increased in red-spotted grouper nervous necrosis virus (RGNNV) infected cells. EcHRI was abundantly distributed in the nucleus of grouper spleen (GS) cells. Overexpression of EcHRI inhibited the expression of red-spotted grouper nervous necrosis virus (RGNNV) genes in GS cells. Furthermore, our results showed that EcHRI overexpression significantly increased the expression of interferon (IFN)-related cytokines and enhanced activation of IFN-β, interferon-sensitive response element (ISRE), and nuclear factor κB (NF-κB). Taken together, these results suggest that EcHRI is involved in the fish immune response to virus challenge.

Lack of in vivo cross-protection of two different betanodavirus species RGNNV and SJNNV in European sea bass Dicentrachus labrax

Viral encephalopathy and retinopathy (VER) is a severe infective disease characterized by neuropathological changes in several fish species associated with high mortality. The etiological agent is a virus belonging to the Nodaviridae family, genus Betanodavirus. To date, four different betanodavirus species have been officially recognized by International Committee on Taxonomy of Viruses (ICTV), namely the red-spotted grouper- (RGNNV), the striped jack- (SJNNV), the barfin flounder- (BFNNV) and the tiger puffer nervous necrosis virus (TPNNV). Moreover, two reassortants RGNNV/SJNNV and SJNNV/RGNNV have been described. Betanodaviruses can be classified into three different serotypes (A, B and C) that are antigenically different, so none (between serotype A and C) or partial (between serotype B and C) cross-immunoreactivity has been detected in vitro. In this study we investigated the in vivo cross-protection of the two main betanodavirus species (RGNNV and SJNNV), which belong to distinct serotype, by immunizing intraperitoneally (IP) juvenile sea bass with formalin inactivated RGNNV and SJNNV vaccines, followed by a challenge with RGNNV. Fish IP vaccinated with inactivated RGNNV showed a high protection value (85%). Serological analyses highlighted a great specific anti-NNV immunoglobulin M (IgM) production against the homologous virus, while a good seroconversion with low neutralization property was highlighted against the heterologous virus. In fish IP vaccinated with inactivated SJNNV the protection recorded was equal to 25%, significantly lower respect to the one provided by RGNNV IP vaccine. ELISA test detected good IgM production against the homologous virus, and a lower, but still detectable IgM production against the heterologous one. By contrast, serum neutralization test highlighted a poorly detectable antibody production unable to neutralize either the homologous or the heterologous virus. These results confirm that the two serotypes are not cross-protective in vivo. According to these findings, the production of multivalent formulation, or at least the provision of different types of vaccines based on both fish and virus species requirement, should be recommended in order to broaden the range of protection.

Risk factors of a viral nervous necrosis disease in grouper (Epinephelus spp.) cultured in Bintan district, Indonesia

Aim:

The aim of the study was to determine the prevalence and risk factors of viral nervous necrosis (VNN) disease in tiger grouper cultured in the floating net cage in Bintan district, Indonesia.

Materials and Methods:

Sampling used multiple stages with ponds as interest units. The selection conducted by systematic random sampling from the entire net cage in Bintan district. The fish samples were selected based on the appearance of clinical signs of infected fish. The risk factors investigated in this study included net cage, technical, and sample fish information, culture, water quality, and feed management. A total of 195 fishes pooled to the 39 net cages and tested using the nested polymerase chain reaction technique to determine the VNN status. The brain and eye processed for histopathology.

Results:

The prevalence rate of VNN on the net cage was 38% (15/39). The risk factors affecting VNN using bivariate analysis was cleaning the net (χ²=9.80; p=0.002), replacement of net cage (χ²=5.20; p=0.0226), and floating net cage technicians knowledge (χ²=4.13; p=0.042). The variables of positive risk factors affecting VNN by multivariate analysis were the juvenile source and the level of mid-weather changes (technician experience and dissolved oxygen [DO]). Seven variables associated to the VNN outbreak have detected. The positive multiplier factors were the source of juveniles, mid weather changes, technician experience, and the DO, while the negative factors were salinity, mixed feed, and the low weather changes. Histopathologically, the grouper fish showed the brain, eye, and muscle vacuolization and kidney necrosis.

Conclusion:

It proves that the Bintan waters contaminated by VNN had a prevalence rate of 38% from the total sample based on bivariate method, net cleaning, net replacement, and knowledge on the VNN outbreak. Natural infection of VNN in grouper leads to vacuolization of the brain, the eye, and muscles nearby the eye as well the kidney necrosis.

Diverse RNA viruses of arthropod origin in the blood of fruit bats suggest a link between bat and arthropod viromes

Bats host diverse viruses due to their unique ecology, behavior, and immunology. However, the role of other organisms with which bats interact in nature is understudied as a contributor to bat viral diversity. We discovered five viruses in the blood of fruit bats (Hypsignathus monstrosus) from the Republic of Congo. Of these five viruses, four have phylogenetic and genomic features suggesting an arthropod origin (a dicistrovirus, a nodavirus, and two tombus-like viruses), while the fifth (a hepadnavirus) is clearly of mammalian origin. We also report the parallel discovery of related tombus-like viruses in fig wasps and primitive crane flies from bat habitats, as well as high infection rates of bats with haemosporidian parasites (Hepatocystis sp.). These findings suggest transmission between arthropods and bats, perhaps through ingestion or hyperparasitism (viral infection of bat parasites). Some "bat-associated" viruses may be epidemiologically linked to bats through their ecological associations with invertebrates.

Betanodavirus infection in bath-challenged Solea senegalensis juveniles: A comparative analysis of RGNNV, SJNNV and reassortant strains

Senegalese sole has been shown to be highly susceptible to betanodavirus infection, although virulence differences were observed between strains. To study the mechanisms involved in these differences, we have analysed the replication in brain tissue of three strains with different genotypes during 15 days after bath infection. In addition, possible portals of entry for betanodavirus into sole were investigated. The reassortant RGNNV/SJNNV and the SJNNV strain reached the brain after 1 and 2 days postinfection, respectively. Although no RGNNV replication was detected until day 3-4 postinfection, at the end of the experiment this strain yielded the highest viral load; this is in accordance with previous studies in which sole infected with the reassortant showed more acute signs and earlier mortality than the RGNNV and SJNNV strains. Differences between strains were also observed in the possible portals of entry. Thus, whereas the reassortant strain could infect sole mainly through the skin or the oral route, and, to a minor extent, through the gills, the SJNNV strain seems to enter fish only through the gills and the RGNNV strain could use all tissues indistinctly. Taken together, all these results support the hypothesis that reassortment has improved betanodavirus infectivity for sole.

A new age in AquaMedicine: unconventional approach in studying aquatic diseases

BACKGROUND

Marine and aquaculture industries are important sectors of the food production and global trade. Unfortunately, the fish food industry is challenged with a plethora of infectious pathogens. The freshwater and marine fish communities are rapidly incorporating novel and most up to date techniques for detection, characterization and treatment strategies. Rapid detection of infectious diseases is important in preventing large disease outbreaks.

MAIN TEXT

One hundred forty-six articles including reviews papers were analyzed and their conclusions evaluated in the present paper. This allowed us to describe the most recent development research regarding the control of diseases in the aquatic environment as well as promising avenues that may result in beneficial developments. For the characterization of diseases, traditional sequencing and histological based methods have been augmented with transcriptional and proteomic studies. Recent studies have demonstrated that transcriptional based approaches using qPCR are often synergistic to expression based studies that rely on proteomic-based techniques to better understand pathogen-host interactions. Preventative therapies that rely on prophylactics such as vaccination with protein antigens or attenuated viruses are not always feasible and therefore, the development of therapies based on small nucleotide based medicine is on the horizon. Of those, RNAi or CRISPR/Cas- based therapies show great promise in combating various types of diseases caused by viral and parasitic agents that effect aquatic and fish medicine.

CONCLUSIONS

In our modern times, when the marine industry has become so vital for feed and economic stability, even the most extreme alternative treatment strategies such as the use of small molecules or even the use of disease to control invasive species populations should be considered.

Involvement of eIF2α of Epinephelus coioides in the fish immune response to virus infection

The eukaryotic initiation factor 2 alpha subunit (eIF2α) is a key translation regulator that plays an important role in different cellular pressures and stimuli, including virus infection. In the present study, an eIF2α homolog (EceIF2α) from the orange-spotted grouper (Epinephelus coioides) was cloned and its roles during fish viral infection were characterized. EceIF2α encodes a putative protein of 315 amino acid residues, and shares a high degree of similarity with eIF2αs from other species. Quantitative real-time polymerase chain reaction (qRT-PCR) analysis indicated that EceIF2α was distributed in all examined tissues. Both of the expression levels of EceIF2α in the spleen and head kidney of E. coioides were up-regulated when challenged with polyinosine-polycytidylic acid (poly[I:C]). EceIF2α was abundantly distributed in both the cytoplasm and nucleus in grouper spleen (GS) cells. Over-expression of EceIF2α improved the expression of red-spotted grouper nervous necrosis virus (RGNNV) genes in GS cells. In addition, EceIF2α depressed the activation of NK-κB and IFN-β. Furthermore, dephosphorylation inhibitor treatment led to a significant decrease of RGNNV gene transcription. Taken together, these results suggest that EceIF2α might be involved in the fish immune response to virus challenge.

Near-Complete Genome Sequence of a Fish Nervous Necrosis Virus Isolated from a Clinical Disease Outbreak in Farm-Reared Bream Sparus aurata in Spain

Sparus aurata larvae infected by viral nervous necrosis were collected from an aquaculture fish farm. The isolated viral genome, composed of two segments (RNA1 and RNA2), was sequenced and analyzed comparatively. Phylogenetic tree analyses revealed that the isolated strain is a reassortant, exhibiting a red-spotted grouper nervous necrosis virus (RGNNV)-type RNA1 and a striped jack nervous necrosis virus (SJNNV)-type RNA2.

Stability of virus-like particles of red-spotted grouper nervous necrosis virus in the aqueous state, and the vaccine potential of lyophilized particles

Virus-like particles (VLPs) are multi protein complexes mimicking the structural properties of the native virus. The development of freeze-dried formulations of such complex protein structures remains a challenge. Red-spotted grouper nervous necrosis virus (RGNNV) causes mass mortality in fish culture, and RGNNV VLPs have been suggested to be promising vaccine candidates. In the present study, the stability of RGNNV VLPs in the liquid state was investigated over a 4-week period, along with the influence of freeze-drying on VLP stability. RGNNV VLPs were completely degraded after one week at 37 °C followed by 3 weeks at ambient temperature, and they were partially degraded after 4 weeks at 4 °C. Therefore, the inherent stability of RGNNV VLP in an aqueous milieu is insufficient for long-term storage. When RGNNV VLPs were freeze-dried in the presence or absence of sugar stabilizers, sorbitol was found to improve VLP stability whereas mannitol reduced it. VLP preparations freeze-dried with sorbitol or without stabilizer were as immunogenic as control (non-freeze dried) VLPs, whereas VLPs freeze-dried in mannitol were less immunogenic. These results indicate that freeze-dried RGNNV VLPs have potential as vaccines.

Betanodavirus: Dissection of the viral life cycle

Progressive research has been recently made in dissecting the molecular biology of Betanodavirus life cycle, the causative pathogen of viral encephalopathy and retinopathy in economic important marine fish species. Establishment of betanodavirus infectious clone allows the manipulation of virus genome for functional genomic study, which elucidates the biological event of the viral life cycle at molecular level. The betanodavirus strategizes its replication by expressing anti-apoptosis/antinecrotic proteins to maintain the cell viability during early infection. Subsequently utilizes and controls the biological machinery of the infected cells for viral genome replication. Towards the late phase of infection, mass production of capsid protein for virion assembly induces the activation of host apoptosis pathway. It eventually leads to the cell lysis and death, which the lysis of cell contributes to the accomplishment of viral shedding that completes a viral life cycle. The recent efforts to dissect the entire betanodavirus life cycle are currently reviewed.

Establishment of a cell line with high transfection efficiency from zebrafish Danio rerio embryos and its susceptibility to fish viruses

A cell line ZBE3 isolated from a continuous cell culture derived from zebrafish Danio rerio blastomeres by clonal growth was characterized. ZBE3 cells had been subcultured for >120 passages since the initial primary culture of the blastomeres. The ZBE3 cells grow stably at temperature from 20 to 32° C with an optimum temperature of 28° C in ESM2 or ESM4 medium with 15% foetal bovine serum (FBS). The optimum FBS concentration for ZBE3 cell growth ranged from 15 to 20%. Cytogenetical analysis indicated that the modal chromosome number of ZBE3 cells was 50, the same as the diploid chromosome number of D. rerio. Significant cytopathic effect was observed in ZBE3 cells after infection with redspotted grouper nervous necrosis virus, Singapore grouper iridovirus and grass carp reovirus, and the viral replication in the cells was confirmed by real-time quantitative PCR and transmission electron microscopy, indicating the susceptibility of ZBE3 cells to the three fish viruses. After transfected with pEGFP-N3 plasmid, ZBE3 cells showed a transfection efficiency of about 40% which was indicated by the percentage of cells expressing green fluorescence protein. The stable growth, susceptibility to fish viruses as well as high transfection efficiency make ZBE3 cells be a useful tool in transgenic manipulation, fish virus-host cell interaction and immune response in fish.

Complete Genome Sequence of Nervous Necrosis Virus Isolated from Orange-Spotted Grouper ( Epinephelus coioides ) in Taiwan

The genome sequence of nervous necrosis virus strain KS1 isolated from orange-spotted grouper (Epinephelus coioides) was cloned and analyzed. The viral genome is composed of two single-stranded positive-sense RNA molecules, RNA1 and RNA2. Phylogenetic analysis shows that the virus strain KS1 belongs to the red-spotted grouper nervous necrosis virus genotype.

Regional genetic diversity for NNV grouper viruses across the Indo-Asian region - implications for selecting virus resistance in farmed groupers

Grouper aquaculture around Asia is impacted by the nervous necrosis virus (NNV) and, in response, host resistance to this infection is being considered as a trait for selection. However efficient selection may be confounded if there are different genetic strains of NNV within and between regions and over years. This study uses statistical approaches and assessment of “characteristic attributes” (i.e. nucleotide positions that discriminate among strains) to assess whether published and new NNV RNA2 cds sequences show genetic differentiation over geography, host species and years. Rather clear evidence was found for regional strains of NNV. Interestingly, most of the geographic defining “characteristic attributes” were in codon position three, and not translated into differences for the protein capsid (i.e. they were synonymous variations), suggesting that while NNV strains were geographically isolated and had diverged in different regions for RNA sequences, selection had largely conserved the protein sequences among regions. The apparent selection constraint on the capsid protein may mitigate the risk that despite geographic subdivision, NNV strain variability will confound genetic selection for host resistance. The existence of regional Asian NNV strains may suggest that hatcheries are at risk from NNV not only from imported material but also from endemic reservoirs.

Viral gut metagenomics of sympatric wild and domestic canids, and monitoring of viruses: Insights from an endangered wolf population

Animal host-microbe interactions are a relevant concern for wildlife conservation, particularly regarding generalist pathogens, where domestic host species can play a role in the transmission of infectious agents, such as viruses, to wild animals. Knowledge on viral circulation in wild host species is still scarce and can be improved by the recent advent of modern molecular approaches. We aimed to characterize the fecal virome and identify viruses of potential conservation relevance of diarrheic free-ranging wolves and sympatric domestic dogs from Central Portugal, where a small and threatened wolf population persists in a highly anthropogenically modified landscape. Using viral metagenomics, we screened diarrheic stools collected from wolves (n = 8), feral dogs (n = 4), and pet dogs (n = 6), all collected within wolf range. We detected novel highly divergent viruses as well as known viral pathogens with established effects on population dynamics, including canine distemper virus, a novel bocavirus, and canine minute virus. Furthermore, we performed a 4-year survey for the six wolf packs comprising this endangered wolf population, screening 93 fecal samples from 36 genetically identified wolves for canine distemper virus and the novel bocavirus, previously identified using our metagenomics approach. Our novel approach using metagenomics for viral screening in noninvasive samples of wolves and dogs has profound implications on the knowledge of both virology and wildlife diseases, establishing a complementary tool to traditional screening methods for the conservation of threatened species.

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.

Viral encephalopathy and retinopathy in aquaculture: a review

Viral encephalopathy and retinopathy (VER), otherwise known as viral nervous necrosis (VNN), is a major devastating threat for aquatic animals. Betanodaviruses have been isolated in at least 70 aquatic animal species in marine and in freshwater environments throughout the world, with the notable exception of South America. In this review, the main features of betanodavirus, including its diversity, its distribution and its transmission modes in fish, are firstly presented. Then, the existing diagnosis and detection methods, as well as the different control procedures of this disease, are reviewed. Finally, the potential of selective breeding, including both conventional and genomic selection, as an opportunity to obtain resistant commercial populations, is examined.

Characterization of a novel disease resistance gene rtp3 and its association with VNN disease resistance in Asian seabass

Asian seabass, an important food fish in Southeast Asia, has suffered from nervous necrosis virus (NNV) infection, resulting in massive mortality of Asian seabass larvae and enormous economic losses. Identification and characterization of disease resistance genes is important. Previous transcriptome analysis of Asians seabass epithelial cells after NNV infection revealed a highly inducible gene, receptor-transporting protein 3 (rtp3), indicating it could play an important role in Asian seabass - NNV interaction. To characterize this gene, we determined its expression pattern and subcellular localization. The rtp3 was highly induced in most examined tissues and organs of Asian seabass after NNV infection, and protein Rtp3 was localized in cytoplasm. Further association study in multiple families revealed that a microsatellite marker, (GT)ntt(GT)n, in the 3' UTR of rtp3 was significantly associated with VNN disease resistance in Asian seabass. Our results imply that rtp3 may be a novel disease resistance gene in Asian seabass. This data could improve our understanding of molecular interaction between Asian seabass and NNV, and has the potential to be applied in marker-assisted selection for disease resistance breeding in Asian seabass.