Betanodavirus meningoencephalitis in an Atlantic blue marlin.

Viral nervous necrosis (viral encephalopathy and retinopathy) is caused by piscine nodavirus (Nodaviridae, Betanodavirus). Since 1986, this highly infectious virus has caused mass mortalities of up to 100% in farmed saltwater and freshwater fish around the world (with the exception of South America and Antarctica), affecting >60 species across 10 orders. The Atlantic blue marlin (Makaira nigricans Lacépède, 1802) is a top-level predator found throughout the tropical waters of the Atlantic and Indo-Pacific oceans. Despite their popularity as a sportfish, relatively little is known about the Atlantic blue marlin and other billfish. We describe here chronic betanodavirus infection in a juvenile Atlantic blue marlin, which is, to our knowledge, the first report of disease in M. nigricans.

Reservoirs of Red-Spotted Grouper Nervous Necrosis Virus (RGNNV) in Squid and Shrimp Species of Northern Alboran Sea.

The production of the aquaculture industry has increased to be equal to that of the world fisheries in recent years. However, aquaculture production faces threats such as infectious diseases. Betanodaviruses induce a neurological disease that affects fish species worldwide and is caused by nervous necrosis virus (NNV). NNV has a nude capsid protecting a bipartite RNA genome that consists of molecules RNA1 and RNA2. Four NNV strains distributed worldwide are discriminated according to sequence homology of the capsid protein encoded by RNA2. Since its first description over 30 years ago, the virus has expanded and reassortant strains have appeared. Preventive treatments prioritize the RGNNV (red-spotted grouper nervous necrosis virus) strain that has the highest optimum temperature for replication and the broadest range of susceptible species. There is strong concern about the spreading of NNV in the mariculture industry through contaminated diet. To surveil natural reservoirs of NNV in the western Mediterranean Sea, we collected invertebrate species in 2015 in the Alboran Sea. We report the detection of the RGNNV strain in two species of cephalopod mollusks (Alloteuthis media and Abralia veranyi), and in one decapod crustacean (Plesionika heterocarpus). According to RNA2 sequences obtained from invertebrate species and reported to date in the Mediterranean Sea, the strain RGNNV is predominant in this semienclosed sea. Neither an ecosystem- nor host-driven distribution of RGNNV were observed in the Mediterranean basin.

Effect of rearing density on nervous necrosis virus infection in Senegalese sole (Solea senegalensis).

Intensive fish farming at high densities results in a wide range of adverse consequences on fish welfare, including pathogen spreading, stress and increased mortality rates. In this work, we have assessed whether the survival of Senegalese sole infected with the nervous necrosis virus (NNV), a pathogen responsible for severe disease outbreaks, is affected by rearing density. Based on the different fish ratios per surface area (g cm-2 ) and water volume (g L-1 ), our research showed an earlier mortality onset in the tanks containing NNV-infected fish reared at medium density (MD: 0.071 g cm-2 /5 g L-1 ) and high density (HD: 0.142 g cm-2 /10 g L-1 ), as well as higher cumulative mortality values. However, transcription analysis of hsp70, gr1 and pepck genes, well-known stress biomarkers, seems to indicate that none of the challenged fish were under high stress conditions. NNV load was slightly higher both in dead and in sampled fish from MD and HD groups, and especially in the rearing water from these groups, where peaks in mortality seemed to correlate with increasing NNV load in the water. In conclusion, our results suggest that rearing NNV-infected Senegalese sole at high densities resulted in an earlier mortality onset and higher cumulative values and viral load.

Developing a Virus-Binding Bacterium Expressing Mx Protein on the Bacterial Surface to Prevent Grouper Nervous Necrosis Virus Infection.

Grouper nervous necrosis virus (GNNV) infection causes mass grouper mortality, leading to substantial economic loss in Taiwan. Traditional methods of controlling GNNV infections involve the challenge of controlling disinfectant doses; low doses are ineffective, whereas high doses may cause environmental damage. Identifying potential methods to safely control GNNV infection to prevent viral outbreaks is essential. We engineered a virus-binding bacterium expressing a myxovirus resistance (Mx) protein on its surface for GNNV removal from phosphate-buffered saline (PBS), thus increasing the survival of grouper fin (GF-1) cells. We fused the grouper Mx protein (which recognizes and binds to the coat protein of GNNV) to the C-terminus of outer membrane lipoprotein A (lpp-Mx) and to the N-terminus of a bacterial autotransporter adhesin (Mx-AIDA); these constructs were expressed on the surfaces of Escherichia coli BL21 (BL21/lpp-Mx and BL21/Mx-AIDA). We examined bacterial surface expression capacity and GNNV binding activity through enzyme-linked immunosorbent assay; we also evaluated the GNNV removal efficacy of the bacteria and viral cytotoxicity after bacterial adsorption treatment. Although both constructs were successfully expressed, only BL21/lpp-Mx exhibited GNNV binding activity; BL21/lpp-Mx cells removed GNNV and protected GF-1 cells from GNNV infection more efficiently. Moreover, salinity affected the GNNV removal efficacy of BL21/lpp-Mx. Thus, our GNNV-binding bacterium is an efficient microparticle for removing GNNV from 10‰ brackish water and for preventing GNNV infection in groupers.

Natural infection of covert mortality nodavirus affects Zebrafish (Danio rerio).

Covert mortality nodavirus (CMNV), a novel aquatic pathogen, causes viral covert mortality disease (VCMD) in shrimps and also known to infect farmed marine fish. To date, there has no report regarding the ability of this virus to infect freshwater fish. In this study, we screened and discovered CMNV-positive freshwater zebrafish individuals by reverse transcription-nested PCR (RT-nPCR). The sequence of CMNV amplicons from zebrafish was found to share 99% identity with RNA-dependent RNA polymerase (RdRp) gene of the original CMNV isolate. Histopathological examination of the CMNV-positive zebrafish samples revealed extensive vacuolation and karyopyknosis lesions in the retina of the eye and the midbrain mesencephalon. CMNV-like virus particles were visualized in these tissues under transmission electron microscope. Different degrees of pathological damages were also found in muscle, gills, thymus and ovarian tissues. Strong positive signals of CMNV probe were observed in these infected tissues by in situ hybridization. Overall, all results indicated that zebrafish, an acknowledged model organism, could be infected naturally by CMNV. Thus, it is needed to pay close attention to the possible interference of CMNV whether in assessment of toxic substances, or in studying the developmental characterization and the nerval function, when zebrafish was used as model animal.

First Report on Natural Infection of Nodavirus in an Echinodermata, Sea Cucumber (Apostichopus japonicas).

Cross-species transmission of emerging viruses happens occasionally due to epidemiological, biological, and ecological factors, and it has caused more concern recently. Covert mortality nodavirus (CMNV) was revealed to be a unique shrimp virus that could cross species barrier to infect vertebrate fish. In the present study, CMNV reverse transcription-nested PCR (RT-nPCR)-positive samples were identified from farmed sea cucumber (Apostichopus japonicas) in the CMNV host range investigation. The amplicons of RT-nPCR from sea cucumber were sequenced, and its sequences showed 100% identity with the RNA-dependent RNA polymerase gene of the original CMNV isolate. Histopathological analysis revealed pathologic changes, including karyopyknosis and vacuolation of the epithelial cells, in the sea cucumber intestinal tissue. The extensive positive hybridization signals with CMNV probe were shown in the damaged epithelial cells in the in situ hybridization assay. Meanwhile, transmission electron microscopy analysis revealed CMNV-like virus particles in the intestine epithelium. All the results indicated that the sea cucumber, an Echinodermata, is a new host of CMNV. This study supplied further evidence of the wide host range of CMNV and also reminded us to pay close attention to its potential risk to threaten different aquaculture animal species.

Development and evaluation of colloidal gold immunochromatography test strip for rapid diagnosis of nervous necrosis virus in golden grey mullet (Chelon aurata).

A lateral flow immunochromatography strip test, based on antibody-gold nanoparticles specific for nervous necrosis virus (NNV), was developed for rapid, on-site detection of the virus in fish stocks. A monoclonal antibody against NNV was conjugated with colloidal gold as the detector antibody. A rabbit anti-NNV polyclonal antibody and goat anti-mouse IgG antibody were blotted onto the nitrocellulose membrane as the capture antibodies on the test line and control line, respectively. The reaction could be seen by the eye within 15 min and did not cross-react with the other viruses tested. The detection limit of the strip was approximately 103 TCID50 /ml and had good stability after storage at 4°C for 8 months. When brains of 70 naturally infected golden grey mullet, Chelon aurata, were tested with the strip test, the diagnostic specificity and sensitivity of the test compared to real-time RT-PCR were 100% and 74%, respectively. Therefore, the one-step test strip developed here had high specificity, reproducibility, and stability. This, together with its simplicity to use and rapid detection, without the requirement of sophisticated equipment or specialized skills, makes the strip suitable for pond-side detection of NNV in farmed fish.

Case Report and Genomic Characterization of a Novel Porcine Nodavirus in the United States.

Nodaviruses are small bisegmented RNA viruses belonging to the family Nodaviridae. Nodaviruses have been identified in different hosts, including insects, fishes, shrimps, prawns, dogs, and bats. A novel porcine nodavirus was first identified in the United States by applying next-generation sequencing on brain tissues of pigs with neurological signs, including uncontrollable shaking. RNA1 of the porcine nodavirus had the highest nucleotide identity (51.1%) to the Flock House virus, whereas its RNA2 shared the highest nucleotide identity (48%) with the RNA2 segment of caninovirus (Canine nodavirus). Genetic characterization classified porcine nodavirus as a new species under the genus Alphanodavirus. Further studies are needed to understand the pathogenicity and clinical impacts of this virus.

A Novel RNA Virus, Macrobrachium rosenbergii Golda Virus (MrGV), Linked to Mass Mortalities of the Larval Giant Freshwater Prawn in Bangladesh.

Mass mortalities of the larval stage of the giant freshwater prawn, Macrobrachium rosenbergii, have been occurring in Bangladesh since 2011. Mortalities can reach 100% and have resulted in an 80% decline in the number of hatcheries actively producing M. rosenbergii. To investigate a causative agent for the mortalities, a disease challenge was carried out using infected material from a hatchery experiencing mortalities. Moribund larvae from the challenge were prepared for metatranscriptomic sequencing. De novo virus assembly revealed a 29 kb single­stranded positive-sense RNA virus with similarities in key protein motif sequences to yellow head virus (YHV), an RNA virus that causes mass mortalities in marine shrimp aquaculture, and other viruses in the Nidovirales order. Primers were designed against the novel virus and used to screen cDNA from larvae sampled from hatcheries in the South of Bangladesh from two consecutive years. Larvae from all hatcheries screened from both years were positive by PCR for the novel virus, including larvae from a hatchery that at the point of sampling appeared healthy, but later experienced mortalities. These screens suggest that the virus is widespread in M. rosenbergii hatchery culture in southern Bangladesh, and that early detection of the virus can be achieved by PCR. The hypothesised protein motifs of Macrobrachium rosenbergii golda virus (MrGV) suggest that it is likely to be a new species within the Nidovirales order. Biosecurity measures should be taken in order to mitigate global spread through the movement of post-larvae within and between countries, which has previously been linked to other virus outbreaks in crustacean aquaculture.

Genomic characterization of covert mortality nodavirus from farming shrimp: Evidence for a new species within the family Nodaviridae.

The prevalence of covert mortality nodavirus (CMNV) has become one of the major threats to the shrimp farming industry in Asia and South America recently. Here, the genomic RNA1 and RNA2 of CMNV were characterized by using transcriptome sequencing and RT-PCR. Our study revealed that RNA1 is 3228 bp in length, and contains two putative Open Reading Frames (ORFs), one encoding the RNA dependent RNA polymerase (RdRp) of length 1043 amino acids and another encoding the protein B2 with a length of 132 amino acids. RNA2 is 1448 bp in length and encodes a capsid protein of 437 amino acids. CMNV shared the highest similarity of 51.78 % for RdRp with the other known nodaviruses. Phylogenetic analyses on the basis of RdRp, B2 and capsid proteins indicated that CMNV might represent a novel viral species in the family Nodaviridae. This study reported the first genome sequence of CMNV and it would be helpful for further studies of CMNV in relation to its evolution, diagnostic technique and control strategy.

Development of a Nanoparticle-based Lateral Flow Strip Biosensor for Visual Detection of Whole Nervous Necrosis Virus Particles.

Effective analysis of pathogens causing human and veterinary diseases demands rapid, specific and sensitive detection methods which can be applied in research laboratory setups and in field for routine diagnosis. Paper lateral flow biosensors (LFBs) have been established as attractive tools for such analytical applications. In the present study a prototype LFB was designed for whole particles (virions) detection of nodavirus or fish nervous necrosis virus. Nodavirus is an important threat in the aquaculture industry, causing severe economic losses and environmental problems. The LFB was based on polyclonal antibodies conjugated on gold nanoparticles for signal visualization. Brain and retinas from fish samples were homogenized, centrifuged and the supernatant was directly applied on the LFB. Formation of a red test line was indicative of nodavirus virions presence. Nodavirus visual detection was completed in short time (30 min). Key factors of the LFB development influencing the assays' detection limit were characterized and the optimum parameters were determined, enabling increased efficiency, excluding non-specific interactions. Therefore, the proposed LFB assay consists a robust, simple, low cost and accurate method for detection of nodavirus virions in fish samples. The proposed biosensor is ideal for development of a commercial kit to be used on aquaculture facilities by fish farmers. It is anticipated that disease monitoring and environmental safety will benefit from the simplification of time consuming and costly procedures.

Reproducibility of antigen-immobilized enzyme-linked immunosorbent assay (ELISA) and sandwich ELISA for quantitative detection of NNV particles.

Nervous necrosis virus (NNV) is a fish virus belonging to family Nodaviridae. In this study, we prepared partially aggregated and monometric NNV particles to determine reproducibility of two different enzyme-linked immunosorbent assays (ELISAs): antigen-immobilized ELISA and sandwich ELISA. Passing ratios of purified NNV particles through ultrafilters with molecular weight cut off (MWCO) of 105, 3 × 105 and 106 were 0%, 35.2% and 80.3%, respectively, suggesting that purified NNV particles were partially aggregated whereas those in filtrates with MWCO of 3 × 105 could be monometric. Both NNV particles were subjected to ELISAs. Reduction ratios of ELISA values by 2-fold dilution of antigens were 50% in sandwich ELISA regardless of aggregation state of NNV particles. In contrast, those in antigen-immobilized ELISA were 42% (partially aggregated NNV) to 43% (monometric NNV), which were lower than the theoretical value (50%). This could be due to changes in aggregation state of NNV particles during dry-immobilization. Sandwich ELISA has excellent reproducibility from five times of experiments, in comparison with antigen-immobilized ELISA. Furthermore, available range of regression lines (R2 > 0.99) in sandwich ELISA was wider than that in antigen-immobilized ELISA. These results revealed that sandwich ELISA had better quantitativeness, reproducibility and available range of ELISA values than antigen-immobilized ELISA.

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.

Complete genome sequence and pathogenic analysis of a new betanodavirus isolated from shellfish.

We determined the complete genomic RNA sequence of a new type of betanodavirus Korea shellfish nervous necrosis virus (KSNNV) isolated from shellfish. Compared with other isolates representing four genotypes of betanodaviruses, the identity of the whole nucleotide sequence of the virus was in the range of 76%-83% with the presence of specific genetic motifs and formed a separate new branch in the phylogenetic analysis. In pathogenic analysis by immersion method, KSNNV-KOR1 shows 100% cumulative mortality like SFRG10/2012BGGa1 (RGNNV) in newly hatched sevenband grouper and mandarin fish, which is clearly different from those found in negative control groups. There were no significant differences in increasing rates of mortality and viral intra-tissue concentration of larval fishes infected with KSNNV-KOR1 at both 20 and 25°C water temperature. Histopathological examination of each fish species in the moribund stage revealed the presence of clear vacuoles in both brain and retinal tissues similar to typical histopathology features of RGNNV. In the present study, we first report a new betanodavirus from shellfish as the aetiological agent of viral nervous necrosis disease in fish with complete genomic nucleotide sequence and pathogenic analysis.

Infection of covert mortality nodavirus in Japanese flounder reveals host jump of the emerging alphanodavirus.

Interspecies transmission of viruses, where a pathogen crosses species barriers and jumps from its original host into a novel species, has been receiving increasing attention. Viral covert mortality disease, caused by covert mortality nodavirus (CMNV), is an emerging disease that has recently had a substantial impact on shrimp aquaculture in Southeast Asia and Latin America. While investigating the host range of CMNV, we found that this virus is also capable of infecting populations of the farmed Japanese flounder Paralichthys olivaceus, a vertebrate host. The infected fish were being raised in aquaculture facilities that were also producing marine shrimp. Through RT-nPCR, targeting the RNA-dependent RNA polymerase (RdRp) gene of CMNV, we found that 29 % of the fish sampled were positive. The amplicons were sequenced and aligned to the RdRp gene of shrimp CMNV and were found to have 98 % identity. Histopathological examination indicated that CMNV-positive fish showed vacuolation of nervous tissue in the eye and brain, as well as extensive necrosis of cardiac muscle. In situ hybridization showed positive reactions in tissues of the eye, brain, heart, liver, spleen and kidney of infected fish. Transmission electron microscopy showed the presence of CMNV-like particles in all of the above-mentioned tissues, except for brain. The novel finding of a shrimp alphanodavirus that can also infect farmed P. olivaceus indicates that this virus is capable of naturally crossing the species barrier and infecting another vertebrate. This finding will contribute to the development of efficient strategies for disease management in aquaculture.

ICTV Virus Taxonomy Profile: Nodaviridae.

The family Nodaviridae includes two genera, Alphanodavirus and Betanodavirus. The family name derives from the Japanese village of Nodamura where Nodamura virus was first isolated from Culex tritaeniorhynchus mosquitoes. Virions are non-enveloped and spherical in shape with icosahedral symmetry (T=3) and diameters ranging from 25 to 33 nm. The genome consists of two molecules of single-stranded positive-sense RNA: RNA1 and RNA2. The virion capsid consists of 180 protein subunits arranged on a T=3 surface lattice. Alphanodaviruses infect insects, whereas betanodaviruses are pathogens of fish. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the Nodaviridae, which is available at www.ictv.global/report/nodaviridae.

Resistance of pearlspot larvae, Etroplus suratensis, to redspotted grouper nervous necrosis virus by immersion challenge.

Viral nervous necrosis (VNN) affects more than 120 species mostly belonging to the order Perciformes. However, none of the brackishwater species belonging to the family Cichlidae under the order Perciformes are reported to be susceptible. Hence, the present experiment was undertaken to study the susceptibility of the brackishwater cichlid, pearlspot, Etroplus suratensis to NNV. Thirty-day-old pearlspot larvae were infected with NNV by immersion. Mortality was recorded till 14 days post-infection, and the infected larvae were subjected to nested RT-PCR and histology. The virus was isolated from infected larvae using SSN-1 cells. To study the replication of the virus in vitro, primary cultured brain cells of E. suratensis and IEK cells were infected with NNV. No mortality was observed in any of the control or experimentally infected larvae. However, the experimentally infected larvae were positive for NNV by nested RT-PCR and the virus was isolated using SSN-1 cells. Further, the infected pearlspot brain cells and IEK cells showed cytopathic effect at second and third passage of the virus and they were positive for NNV by nested RT-PCR. Pearlspot is relatively resistant to VNN although the virus could replicate in the larvae and in cell culture.

Development and validation of a TaqMan RT-qPCR for the detection of convert mortality nodavirus (CMNV).

Covert mortality nodavirus (CMNV), an emerging RNA virus, is the pathogen of viral covert mortality disease (VCMD), which has emerged as a cause of serious losses in shrimp aquaculture in China. To improve VCMD diagnosis, a one-step, real-time TaqMan probe-based reverse transcription quantitative PCR (RT-qPCR) was developed in this study. The TaqMan RT-qPCR was optimized firstly, whereby the best results were obtained with 0.2 µM of each primer, 0.2 µM probe, and 0.5 µL Enzyme Mix II. The optimal reaction program was determined as 15 min at 51ºC for reverse transcription and 5 min at 95 ºC, followed by 40 cycles of denaturation at 94 ºC for 10 s, and annealing and extension at 52.7 ºC for 30 s. The optimized assay detected as little as 9.6 pg total RNA from CMNV-infected shrimp and 5.7 copies of the target plasmid. The RT-qPCR assay for CMNV with a high correlation coefficient (r2 = 0.996) was developed basing on the standard curve generated by plotting the threshold cycle values (y) against the common logarithmic copies (log10nc as x; nc is copy number) of pMD20-CMNV. The diagnostic sensitivity and specificity of this assay versus the previously reported RT-qPCR was 96.2% and 98.0%, respectively. This method is highly specific to CMNV, as it showed no cross-reactivity with other common shrimp viruses. It is anticipated that the newly developed and optimized RT-qPCR assay will be instrumental for the rapid diagnosis and quantitation of CMNV.

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.

Isolation and initial characterization of new betanodaviruses in shellfish.

Betanodaviruses cause the disease viral nervous necrosis (VNN) in finfish. Using a novel approach with two consecutive PCRs, detection semi-nested two-step RT-PCR (DSN-2 RT-PCR) and discriminative multiplex two-step RT-PCR (DMT-2 RT-PCR), we have identified the presence of a new type of betanodavirus in shellfish and called it Korean shellfish nervous necrosis virus (KSNNV). Partial nucleotide sequences of the T4 region in RNA2 fragment of KSNNVs were 73%-75% homologous to those of other reported genotypes and formed a new cluster of betanodavirus in phylogenetic tree analysis. Successful isolation of KSNNV was achieved in two of six shellfish samples containing high concentrations of virus using the blind passage method, and the typical shapes of betanodavirus were confirmed in KSNNV-KOR1 by electron microscopy. In the experimental infection test, seven of 14 fish species showed susceptibility to KSNNV-KOR1 isolate but without clinical signs or death. Although the range of susceptible host species was not significantly different from the RGNNV type, the concentration of KSNNV in the brain of infected fish (102 -105 copies/mg brain) was much lower compared to that found in sevenband grouper (Epinephelus septemfasciatus Thunberg) sampled in the moribund stage with RGNNV infection (106 -107 copies/mg brain). However, histopathological analyses showed the presence of multiple vacuoles in brains of all KSNNV-infected fish at 14 days postinjection. In detection test, as a single or multiple type with the other genotype(s) (RGNNV or BFNNV), the prevalence of KSNNV was 8.4% and 8.7% in domestic (62 of 741 samples) and Chinese samples (12 of 138 samples), respectively, but not in finfish. We propose that KSNNVs obtained from shellfish be classified into a separate and new genotype of betanodavirus.