Determination and Characterization of Novel Papillomavirus and Parvovirus Associated with Mass Mortality of Chinese Tongue Sole (Cynoglossus semilaevis) in China

A massive mortality event concerning farmed Chinese tongue soles occurred in Tianjin, China, and the causative agent remains unknown. Here, a novel Cynoglossus semilaevis papillomavirus (CsPaV) and parvovirus (CsPV) were simultaneously isolated and identified from diseased fish via electron microscopy, virus isolation, genome sequencing, experimental challenges, and fluorescence in situ hybridization (FISH). Electron microscopy showed large numbers of virus particles present in the tissues of diseased fish. Viruses that were isolated and propagated in flounder gill cells (FG) induced typical cytopathic effects (CPE). The cumulative mortality of fish given intraperitoneal injections reached 100% at 7 dpi. The complete genomes of CsPaV and CsPV comprised 5939 bp and 3663 bp, respectively, and the genomes shared no nucleotide sequence similarities with other viruses. Phylogenetic analysis based on the L1 and NS1 protein sequences revealed that CsPaV and CsPV were novel members of the Papillomaviridae and Parvoviridae families. The FISH results showed positive signals in the spleen tissues of infected fish, and both viruses could co-infect single cells. This study represents the first report where novel papillomavirus and parvovirus are identified in farmed marine cultured fish, and it provides a basis for further studies on the prevention and treatment of emerging viral diseases.

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.

Efficacy of algal Ecklonia cava extract against viral hemorrhagic septicemia virus (VHSV)

The inhibition efficacy of an extract from Ecklonia cava (E. cava) was studied to determine whether the extract and compounds exhibited inhibitory activity against VHSV in the fathead minnow (FHM) cell line and following oral administration to the olive flounder. Based on its low toxicity and effective concentration, the E. cava extract (Ext) and compounds (eckol and phlorofucofuroeckol A) were selected for further analysis. In the plaque reduction assay, simultaneous co-exposure of VHSV to Ext, eckol and phlorofucofuroeckol A showed a higher level of inhibition than the pre- and post-exposure groups. The antiviral activity in the FHM cell line was time-dependent and increased with the exposure time with the virus and Ext or the compounds. In the in vivo experiments, different Ext concentrations were orally administered to the olive flounder. In trial I, the relative percent survival (RPS) following oral administration of 500 and 50 μg/g/day of Ext was 31.25% and 12.50%, respectively. In trial II, the RPS for 1000, 500 and 50 μg/g/day of Ext was 31.57%, 0% and 0%, respectively. In trial III, the RPS after 1 and 2 weeks (1000 μg/g/day) of exposure to Ext was 26.31% and 31.57%, respectively. Oral administration of Ext (1000 μg/g/day) significantly induced inflammatory cytokine responses (IL-1β, IL-6 and IFN-γ) at 1 and 2 days post-oral administration (dpa). Additionally, IFN-α/β (7-12 dpa), ISG15 (2, 7 and 10 dpa) and Mx (7-12 dpa) were significantly activated in the olive flounder. In conclusion, we demonstrated an inhibitory ability of the E. cava extract and compounds against VHSV in the FHM cell line. Moreover, oral administration of the E. cava extract to the olive flounder enhanced antiviral immune responses and the efficacy of protection against VHSV, resulting in an anti-viral status in the olive flounder.

Expression Profile Analysis of miR-221 and miR-222 in Different Tissues and Head Kidney Cells of Cynoglossus semilaevis, Following Pathogen Infection

Half-smooth tongue sole (Cynoglossus semilaevis) is an important marine commercial fish species in China, which suffers from widespread disease outbreaks. Recently, in this regard, our group identified immune-related microRNAs (miRNAs) of C. semilaevis following Vibrio anguillarum infection. Furthermore, miRNA microarray was utilized to characterize the immune roles of important miRNA candidates in response to bacterial infection. Therefore, in the present study, we characterized miR-221 and miR-222 and profiled their expression after challenge. Here, miR-221 and miR-222 precursors were predicted to have a typical hairpin structure. Both miRNAs were expressed in a broad range of tissues in C. semilaevis, while miR-221 and miR-222 were significantly differentially expressed in the immune tissues of C. semilaevis among three small RNA libraries [control group (CG), bacteria-challenged fish without obvious symptoms of infection (NOSG), and bacteria-challenged fish with obvious symptoms of infection (HOSG)]. In order to further characterize and understand the immune response of miR-221 and miR-222, therefore, we profiled miR-221 and miR-222 expression in selected immune tissues after challenge with V. anguillarum. Both miR-221 and miR-222 were upregulated in the liver and spleen, while different expression patterns were observed in the head kidney. In addition, in half-smooth tongue sole head kidney cell line after challenge with lipopolysaccharide (LPS), polyinosinic:polycytidylic acid (poly I:C), peptidoglycan (PGN), and red-spotted grouper nervous necrosis virus (RGNNV), both miR-221 and miR-222 showed significant difference in expression response to pathogen. Meanwhile, the target gene of miR-221 and miR-222 was predicted, which indicated that tumor necrosis factor receptor-associated factor 6 (TRAF6) and interleukin-1 beta (IL-1β) were the target genes of miR-221 and miR-222, respectively. Collectively, these findings indicated that miR-221 and miR-222 have putative roles in innate immune response during C. semilaevis exposure to pathogens. Our findings could expand the knowledge of immune function of C. semilaevis miRNA and guide future studies on C. semilaevis immunity.

Tongue sole (Cynoglossus semilaevis) prothymosin alpha: Cytokine-like activities associated with the intact protein and the C-terminal region that lead to antiviral immunity via Myd88-dependent and -independent pathways respectively

Prothymosin alpha (ProTα) is a small protein that in mammals is known to participate in diverse biological processes including immunomodulation. In teleost, the immunological function of ProTα is unknown. In the current study, we investigated the expression and function of the ProTα (named CsProTα) from the teleost fish tongue sole (Cynoglossus semilaevis). We found that CsProTα expression was abundant in immune relevant tissues and upregulated by megalocytivirus infection. Immunoblot detected secretion of CsProTα by peripheral blood leukocytes. Recombinant CsProTα (rCsProTα) as well as the C-terminal 11-residue (Ct11) were able to bind head kidney monocytes (HKM) and induce immune gene expression; however, the induction patterns caused by rCsProTα and Ct11 differed considerably. When introduced in vivo, rCsProTα and Ct11 significantly reduced megalocytivirus infection in fish tissues, whereas rCsProTα antibody significantly promoted viral replication. Blocking of Myd88 activity abolished the virus-inhibitory effect of rCsProTα but not Ct11. Taken together, these results demonstrate for the first time that both the intact protein and the C-terminal segment of a teleost ProTα can act like cytokines and induce antiviral immunity via, however, distinct signaling pathways that differ in the requirement of Myd88.

ORF75 of megalocytivirus RBIV-C1: A global transcription regulator and an effective vaccine candidate

Megalocytivirus, a DNA virus belonging to the Iridoviridae family, is a severe pathogen to a wide range of marine and freshwater fish. In this study, using turbot (Scophthalmus maximus) as a host model, we examined the immunoprotective property of one megalocytivirus gene, ORF75, in the form of DNA vaccine (named pORF75). Immunofluorescence microscopy and RT-PCR analysis showed that P444, the protein encoded by ORF75, was naturally produced in the tissues of turbot during megalocytivirus infection, and that the vaccine gene in pORF75 was expressed in fish cells transfected with pORF75 and in the tissues of turbot immunized with pORF75. Following vaccination of turbot with pORF75, a high level of survival (73%) was observed against a lethal megalocytivirus challenge. Consistently, viral replication in the vaccinated fish was significantly inhibited. Immune response analysis showed that pORF75-vaccinated fish (i) exhibited upregulated expression of the genes involved in innate and adaptive immunity, (ii) possessed specific memory immune cells that showed significant response to secondary antigen stimulation, and (iii) produced specific serum antibodies which, when co-introduced into turbot with megalocytivirus, blocked viral replication. Furthermore, whole-genome transcriptome analysis revealed that ORF75 knockdown altered the transcription of 43 viral genes. Taken together, these results indicate that ORF75 encoded a highly protective immunogen that is also a global transcription regulator of megalocytivirus.

Influence of temperature on Betanodavirus infection in Senegalese sole (Solea senegalensis)

In this study Senegalese sole juveniles were experimentally infected with a reassortant Betanodavirus strain at three different temperatures: 22 °C, 18 °C and 16 °C by bath challenge and cohabitation. The results obtained showed that virus virulence decreased by reducing the water temperature. At 22 °C mortalities reached 100%, at 18 °C they ranged from 75 to 80% and at 16 °C only 8% of the fish died. In addition, horizontal transmission was demonstrated regardless of the rearing temperature. At 16 °C active viral replication was detected up to 66 days post-infection, but no signs of the disease were observed and only a very low level of mortality was recorded. The increase in water temperature from 16 to 22 °C caused a quick rise in the viral load and a subsequent outbreak of mortalities. These findings demonstrate that this reassortant Betanodavirus strain can cause a persistent infection in Senegalese sole at low temperatures (16 °C) for long periods of time, and when temperature increases the virus is able to trigger an acute infection and provoke high mortalities.

Expression profiling analysis of the microRNA response of Cynoglossus semilaevis to Vibrio anguillarum and other stimuli

To investigate the roles of microRNAs (miRNA) of Cynoglossus semilaevis in response to Vibrio anguillarum that were previously identified using high-throughput sequencing, microarray analyses was performed on three small RNA libraries (CG, NOSG, and HOSG) prepared from C. semilaevis immune tissues. In total, of 1279 designed probes, 739 (57.78 %) were detectable. The expression levels of these miRNAs were analyzed using pairwise comparisons among the three libraries, and a total of 99 miRNAs were observed to be significantly differentially expressed. The expression patterns of 10 differentially expressed miRNAs were validated by real-time quantitative PCR (RT-qPCR). In addition, expression of miR-142-5p, miR-223, and miR-181a in response to V. anguillarum at numerous time-points in four tissues, as well as the responses to lipopolysaccharide (LPS), polyinosinic:polycytidylic acid (poly I:C), peptidoglycan (PGN), and red-spotted grouper nervous necrosis virus (RGNNV) in head kidney cells, were studied by qRT-PCR. Taken together, all of the expression profiles showed significant differences compared to the control group; both similarities and differences in the expression responses to the same pathogen were observed. Collectively, these findings highlighted the putative roles for miRNAs in the context of the innate immune response of C. semilaevis exposing to pathogens and that further studies are needed to understand the molecular mechanisms of miRNA regulation in C. semilaevis host-pathogen interactions.

P247 and p523: two in vivo-expressed megalocytivirus proteins that induce protective immunity and are essential to viral infection

Megalocytivirus is a DNA virus with a broad host range among teleost fish. Although the complete genome sequences of a number of megalocytivirus isolates have been reported, the functions of most of the genes of this virus are unknown. In this study, we selected two megalocytivirus immunogens, P247 and P523, which were expressed during host infection and, when in the form of DNA vaccines (pCN247 and pCN523 respectively), elicited strong protectivity against lethal megalocytivirus challenge in a turbot (Scophthalmus maximus) model. Compared to control fish, fish vaccinated with pCN247 and pCN523 exhibited drastically reduced viral loads in tissues and high levels of survival rates. Immune response analysis showed that pCN247 and pCN523 (i) induced production of specific serum antibodies, (ii) caused generation of cytotoxic immune cells and specific memory immune cells that responded to secondary antigen stimulation, and (iii) upregulated the expression of genes involved in innate and adaptive immunity. To examine the potential role of P247 and P523 in viral infection, the expression of P247 and P523 was knocked down by siRNA. Subsequent in vivo infection study showed that P247 and P523 knockdown significantly impaired viral replication. Furthermore, whole-genome transcriptome analysis revealed that P247 and P523 knockdown altered the expression profiles of 26 and 41 viral genes, respectively, putatively participating in diverse aspects of viral infection. Taken together, these results indicate that P247 and P523 induce protective immunity in teleost and play fundamental roles essential to viral replication. These observations provide the first evidence that suggests a likely link between the protectivity of viral immunogens and their biological significance in viral replication.

Detection of salmonid alphavirus RNA in Celtic and Irish Sea flatfish

Pancreas disease (PD) caused by the salmonid alphavirus (SAV) has been the most significant cause of mortalities in Irish farmed salmon Salmo salar L. over the past decade. SAV is a single-strand positive-sense RNA virus, originally thought to be unique to salmonids, but has recently been detected using real-time RT-PCR in a number of wild non-salmonid fish. In the present report, 610 wild flatfish (common dab Limanda limanda, plaice Pleuronectes platessa and megrim Lepidorhombus whiffiagonis) were caught from the Irish and Celtic Seas and screened for SAV using real-time RT-PCR and sequencing. In general, a very low prevalence was recorded in common dab and plaice, except for 1 haul in Dublin Bay where 25% of common dab were SAV-positive. SAV sequence analysis supported the fact that real-time RT-PCR detections were specific and further characterised the detected viruses within SAV Subtype I, the predominant subtype found in farmed salmon in Ireland.

SmCCL19, a CC chemokine of turbot Scophthalmus maximus, induces leukocyte trafficking and promotes anti-viral and anti-bacterial defense

Chemokines are classified into several different subfamilies, of which CC chemokines constitute the largest subfamily in teleost. The prominent structural characteristic of CC chemokines is the presence of an Asp-Cys-Cys-Leu (DCCL) motif. To date, cDNA sequences of several CC chemokines have been identified in turbot (Scophthalmus maximus), however, the activity and function of these putative chemokines remain unknown. In this study, we examined the biological effect of the turbot CC chemokine SmCCL19, which has been previously reported as KC70 and shown to be regulated in expression by bacterial infection. To facilitate functional analysis, recombinant SmCCL19 (rSmCCL19) and a mutant form of SmCCL19, SmCCL19M, that bears serine substitutions at the two cysteine residues of the DCCL motif were purified from Escherichia coli. Chemotactic analysis showed that rSmCCL19 induced migration of head kidney leukocytes in a dose-dependent manner, whereas rSmCCL19M caused no apparent cellular migration. To examine the in vivo effect of rSmCCL19, turbot were administered with rSmCCL19 or rSmCCL19M before being inoculated with viral and bacterial pathogens. Subsequent tissue infection analysis showed that the viral and bacterial loads in rSmCCL19-adminsitered fish were significantly reduced, whereas the pathogen loads in rSmCCL19M-adminsitered fish were largely comparable to those in the control fish. Consistent with these observations, significant inductions of immune relevant genes were observed in rSmCCL19-adminsitered fish but not in rSmCCL19M-adminsitered fish. Taken together, these results indicate that SmCCL19 recruits leukocytes and augments host immune defense in a manner that depends on the conserved DCCL motif.

A NK-lysin from Cynoglossus semilaevis enhances antimicrobial defense against bacterial and viral pathogens

NK-lysin is an effector protein of cytotoxic T lymphocytes and natural killer cells. Mammalian NK-lysin is known to possess antibacterial property and antitumor activity. Homologues of NK-lysin have been identified in several teleost species, but the natural function of fish NK-lysin remains essentially unknown. In this study, we identified a NK-lysin, CsNKL1, from half-smooth tongue sole (Cynoglossus semilaevis) and analyzed its expression, genetic organization, and biological effect on pathogen infection. CsNKL1 is composed of 135 residues and shares 33.1-56.5% overall sequence identities with other teleost NK-lysin. CsNKL1 possesses a Saposin B domain and six conserved cysteine residues that in mammals are known to form three intrachain disulfide bonds essential to antimicrobial activity. The genomic sequence of the ORF region of CsNKL1 is 1240bp in length and, like human NK-lysin, contains five exons and four introns. Expression of CsNKL1 occurred in multiple tissues and was upregulated by bacterial and viral infection in a time dependent manner. When CsNKL1 was overexpressed in tongue sole, significant upregulation of interleukin-1 and chemokines was observed in spleen and head kidney. Following bacterial and viral infection, the pathogen loads in the tissues of CsNKL1-overexpressing fish were significantly lower than those in control fish. These results indicate that CsNKL1 possesses the novel capacities of immunomodulation and enhancing antimicrobial defense against pathogens of both bacterial and viral nature.

Identification and expressional analysis of two cathepsins from half-smooth tongue sole (Cynoglossus semilaevis)

Cathepsins are a family of lysosomal proteases that play an important role in protein degradation, antigen presentation, apoptosis, and inflammation. Cathepsins are divided into three groups, i.e., cysteine protease, serine protease, and aspartic protease. Cathepsin D and cathepsin L, which are aspartic protease and cysteine protease respectively, have been identified in a number of teleosts; however, the immunological relevance of fish cathepsins is largely unknown. In this study, we cloned and analyzed the expression profiles of a cathepsin D (CsCatD) and a cathepsin L (CsCatL) homologs from half-smooth tongue sole (Cynoglossus semilaevis). CsCatD is composed of 396 amino acid residues and shares 67.6-88.4% overall sequence identities with fish and human cathepsin D. Structurally CsCatD possesses an aspartic endopeptidase domain, which contains two conserved aspartic acid residues that form the catalytic site. CsCatL is 336 residues in length and shares 64.7-90.2% overall sequence identities with fish and human cathepsin L. CsCatL has an N-terminal cathepsin propeptide inhibitor domain followed by a Papain family cysteine protease domain, the latter containing four conserved catalytic residues: Gln-133, Cys-139, His-279, and Asn-303. Recombinant CsCatL purified from Escherichia coli exhibited apparent protease activity. Quantitative real time RT-PCR analysis detected constitutive expression of CsCatD and CsCatL in multiple tissues, with the lowest level found in heart and the highest level found in liver. Experimental challenge of tongue sole with the bacterial pathogen Vibrio anguillarum and megalocytivirus caused significant inductions of both CsCatD and CsCatL expression in kidney and spleen in time-dependent manners. Immunization of the fish with a subunit vaccine also enhanced CsCatD and CsCatL expression in the first week post-vaccination. These results suggest involvement of CsCatD and CsCatL in host immune reactions to bacterial and viral infections and in the process of antigen-induced immune response.

Rapid diagnosis of turbot reddish body iridovirus in turbot using the loop-mediated isothermal amplification method

Turbot reddish body iridovirus (TRBIV) is a new piscine iridovirus that infects the turbot, Scophthalmus maximus, cultured in northern China and can cause high mortality. In this study, a loop-mediated isothermal amplification (LAMP) method was developed for the specific detection of this virus using primers designed from an Msp I restriction DNA fragment of the TRBIV genome. Mg(2+) concentrations, the reaction temperature, and the reaction time of LAMP were optimized to 6mM, 65 degrees C, and 60 min, respectively. The detection limit of the LAMP method was as low as seven copies and was 100 times more sensitive than the conventional PCR technique. Visual inspection of LAMP amplifications demonstrated that the positive and negative reactions exhibit distinct and different colors in daylight, which means that gel electrophoresis is not necessary to judge the presence or absence of the virus. LAMP can be conducted in 1h and requires only a simple heating device for incubation. Thus, the LAMP-TRBIV detection protocol has great potential for use in the detection of TRBIV in both the laboratory and the farm.

Immunological responses of turbot (Psetta maxima) to nodavirus infection or polyriboinosinic polyribocytidylic acid (pIC) stimulation, using expressed sequence tags (ESTs) analysis and cDNA microarrays

To investigate the immunological responses of turbot to nodavirus infection or pIC stimulation, we constructed cDNA libraries from liver, kidney and gill tissues of nodavirus-infected fish and examined the differential gene expression within turbot kidney in response to nodavirus infection or pIC stimulation using a turbot cDNA microarray. Turbot were experimentally infected with nodavirus and samples of each tissue were collected at selected time points post-infection. Using equal amount of total RNA at each sampling time, we made three tissue-specific cDNA libraries. After sequencing 3230 clones we obtained 3173 (98.2%) high quality sequences from our liver, kidney and gill libraries. Of these 2568 (80.9%) were identified as known genes and 605 (19.1%) as unknown genes. A total of 768 unique genes were identified. The two largest groups resulting from the classification of ESTs according to function were the cell/organism defense genes (71 uni-genes) and apoptosis-related process (23 uni-genes). Using these clones, a 1920 element cDNA microarray was constructed and used to investigate the differential gene expression within turbot in response to experimental nodavirus infection or pIC stimulation. Kidney tissue was collected at selected times post-infection (HPI) or stimulation (HPS), and total RNA was isolated for microarray analysis. Of the 1920 genes studied on the microarray, we identified a total of 121 differentially expressed genes in the kidney: 94 genes from nodavirus-infected animals and 79 genes from those stimulated with pIC. Within the nodavirus-infected fish we observed the highest number of differentially expressed genes at 24 HPI. Our results indicate that certain genes in turbot have important roles in immune responses to nodavirus infection and dsRNA stimulation.

Distribution of marine birnavirus in cultured olive flounder Paralichthys olivaceus in Korea

Surveys of marine birnavirus (MABV) were undertaken in cultured olive flounder Paralichthys olivaceus from the south and west coastal areas and Jeju in Korea during the period January 1999 to April 2007. MABV was detected in all seasons from the fry, juveniles and adult fish from the areas examined. Evident cytopathic effects of the virus including rounding and cell lysis were observed in chinook salmon embryo (CHSE-214) and rainbow trout gonad (RTG-2) cells, but not in fathead minnow (FHM) and epithelial papilloma of carp (EPC) cells. Nucleotide sequences of the VP2/NS junction region of the Korean isolates showed 97.8% ~ 100% similarity, and they belonged to the same genogroup. Cross neutralization tests with serotype-specific rabbit antisera against MABV strains exhibited a close antigenic relationships between strains, and were distinct from infectious pancreatic necrosis virus (IPNV) strains. Coinfection of MABV with bacteria (Streptococcus iniae, Vibrio spp.) and viruses (nervous necrosis virus, lymphocystis disease virus, viral hemorrhagic septicemia virus) was observed.

Poly I:C induces Mx transcription and promotes an antiviral state against sole aquabirnavirus in the flatfish Senegalese sole (Solea senegalensis Kaup)

Mx is an interferon-induced protein that protects against viral infections. In this study the absolute number of Mx transcripts after poly I:C injection (a synthetic dsRNA) or sole aquabirnavirus (solevirus) inoculation in Senegalese sole (Solea senegalensis Kaup) has been quantified. Mx expression profiles differed clearly in both experimental conditions; the induction response was faster and more intense after poly I:C injection than after solevirus inoculation. Moreover, pre-injection of soles with poly I:C prior to solevirus infection eliminated the induction of Mx expression associated with this virus. To evaluate the possible interference of poly I:C treatments on solevirus replication, the mRNA levels of the virus capsid protein (VP2) were determined by RT-PCR. VP2 transcripts were hardly detected in poly I:C pre-injected animals from 12 to 72 h after solevirus inoculation. All these data suggest that poly I:C is able to induce an antiviral state that interferes with solevirus replication, and support the suitability of Mx expression analysis as a marker to study the defensive response against solevirus.

Phylogenetic analysis of betanodaviruses isolated from cultured fish in Korea

Since the publication of the first report on fish nodaviruses in Korea in 1998, fish nodaviruses have caused widespread epizootic events among various fish species in Korea. However, the genotypes of fish nodaviruses in Korea have not yet been determined due to a lack of information about their nucleotide sequences. In this study, we isolated 5 fish nodaviruses from 4 fish species cultured in 4 different regions in Korea: rock bream Oplegnathus fasciatus, Japanese flounder Paralichthys olivaceus, sevenband grouper Epinephelus septemfasciatus, and grey mullet Mugil cophalus. The full open-reading frame (ORF) encoding the coat protein (1017 nt) was sequenced from each of the 5 fish nodaviruses and the nucleotide sequences were phylogenetically analyzed. Results showed that even though their sequences were not identical, all 5 Korean isolates were clustered in the RGNNV genotype. This is the first report on the phylogenetic analysis of fish nodaviruses from cultured fish in Korea.

Isolation and characterization of Scophthalmus maximus rhabdovirus

A rhabdovirus associated with a lethal hemorrhagic disease in cultured turbot Scophthalmus maximus Linnaeus was isolated. The virus induced typical cytopathogenic effects (CPE) in 9 of 15 fish cell lines examined and was then propagated and isolated from infected carp leucocyte cells (CLC). Electron microscopy observations revealed that the negatively stained virions had a typical bullet-shaped morphology with one rounded end and one flat base end. The bullet-shaped morphology was more obvious and clear in ultrathin sections of infected cells. Experimental infections also indicated that the S. maximus rhabdovirus (SMRV) was not only a viral pathogen for cultured turbot, but also had the ability to infect other fish species, such as freshwater grass carp. A partial nucleotide sequence of the SMRV polymerase gene was determined by RT-PCR using 2 pairs of degenerate primers designed according to the conserved sequences of rhabdovirus polymerase genes. Homology analysis, amino acid sequence alignment, and phylogenetic relationship analysis of the partial SMRV polymerase sequence indicated that SMRV was genetically distinct from other rhabdoviruses. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the purified SMRV revealed 5 major structural proteins, and their molecular masses were estimated to be about 250, 58, 47, 42, and 28 kDa. Significant serological reactivity differences were also observed between SMRV and its nearest neighbor, spring viremia of carp virus (SVCV). The data suggest that SMRV is likely a novel fish rhabdovirus, although it is closely related to rhabdoviruses in the genus Vesiculovirus.

Characterization and Application of Monoclonal Antibodies against Turbot (Scophthalmus maximus) Rhabdovirus

Five monoclonal antibodies (mAbs), 1G8, 1H9, 2D2, 2D3, and 2F5, against Scophthalmus maximus rhabdovirus (SMRV) were prepared. Characterization of the mAbs included indirect enzyme-linked immunosorbent assay, isotyping, viral inhibition assay, immunofluorescence staining of virus-infected cell cultures, and Western blot analysis. Isotyping revealed that 1G8 and 1H9 were of the IgG2b subclass and that the other three were IgM. 2D2, 2D3, and 2F5 partially inhibited SMRV infection in epithelioma papulosum cyprinid (EPC) cell culture. Western blotting showed that all five mAbs could react with two SMRV proteins with molecular masses of approximately 30 kDa (P) and 26 kDa (M). These two proteins were localized within the cytoplasm of SMRV-infected EPC cells by immunofluorescence assay. Also, progressive foci of viral replication in cell cultures were monitored from 6 to 24 h, using mAb 2D3 as the primary antibody. A flow cytometry procedure was used to detect and quantify SMRV-infected (0.01 PFU/cell) EPC cells with mAb 2D3, and 10.8% of cells could be distinguished as infected 36 h postinfection. Moreover, mAb 2D3 was successfully applied for the detection of viral antigen in cryosections from flounder tissues by immunohistochemistry tests.

Susceptibility of marine fish species to a megalocytivirus, turbot iridovirus, isolated from turbot, Psetta maximus (L.)

Turbot iridovirus (TBIV), a member of the genus Megalocytivirus in the family Iridoviridae, was isolated from diseased turbot, Psetta maximus (L.), in Korea in 2003. In this study, experimental infection of turbot, Japanese flounder, Paralichthys olivaceus (Temminck & Schlegel), and rock bream, Oplegnathus fasciatus (Temminck & Schlegel), with TBIV was performed to evaluate the viral susceptibility of these fish species. After virus exposure, the mortalities of turbot reared at 22 and 25 degrees C were 60% and 100%, respectively, suggesting that TBIV is the causative agent of the mass mortality of turbot that occurred in Korea in 2003. Moreover, TBIV was detected in Japanese flounder and rock bream by polymerase chain reaction after experimental infection (26 days post-inoculation) despite no viral pathogenicity in these fish, suggesting that these two fish species are also susceptible to the virus. It is possible that horizontal transmission of TBIV occurs among these three fish species because turbot is routinely cultured with Japanese flounder and rock bream in Korea.

Genotyping and pathogenicity of viral hemorrhagic septicemia virus from free-living turbot (Psetta maxima) in a Turkish coastal area of the Black Sea

Viral hemorrhagic septicemia (VHS) is one of the most serious fish viral diseases for cultured rainbow trout (Oncorhynchus mykiss), although VHS virus (VHSV) seems to be ubiquitous among marine fishes. In the present study, VHSV isolation was performed with free-living and cultured turbot (Psetta maxima) in the Trabzon coastal area of the Black Sea to evaluate participation of VHSV in mass mortalities of seed-produced turbot larvae. VHSV was detected in 14 of 66 free-living spawners (positive ratio, 21.2%), 1 of 65 free-living immature fish (1.5%) and 7 of 40 cultured brood stock (17.5%), respectively. Based on a partial glycoprotein gene nucleotide sequence, Turkish VHSV isolates were classified into the class I-e of genotype I and were the most closely related to the GE-1.2 isolate (>98% identity), which was found >20 years ago in Georgia. Thus, it was revealed that Turkish VHSV isolates were not introduced from European countries, it could be an indigenous type of VHSV distributing in the Black Sea environment. In pathogenicity tests, the Turkish isolates did not induce mortality in turbot larvae and rainbow trout fingerlings. Mass mortalities at a rate of approximately 90% occurred in turbot larvae produced by experimental seeding, although VHSV was not detected in any dead fish. Thus, it was concluded that mass mortality in the seed-produced turbot larvae was not caused by VHSV infection.

Induction of apoptosis in a carp leucocyte cell line infected with turbot (Scophthalmus maximus L.) rhabdovirus

A rhabdovirus was observed from the diseased turbot (Scophthalmus maximus L.) with lethal syndrome. In this study, a carp leucocyte (CLC) cell line was used to investigate the infection process and cell death mechanism occurring during the virus infection. Strong cytopathogenic effect (CPE) and the morphological changes, such as extreme chromatin condensation, nucleus fragmentation, and apoptotic body formation, were observed under fluorescence microscopy after DAPI staining in the infected CLC cells. Transmission electron microscopy analysis showed cell shrinkage, plasma membrane blebbing, cytoplasm vacuolization, chromatin condensation, nuclear breakdown and formation of discrete apoptotic bodies. The bullet-shaped nucleocapsids were measured and ranged in size from 110 to 150 nm in length and 40 to 60 nm in diameter. And therefore the virus is called Scophthalmus maximus rhabdovirus (SMRV). Agarose gel electrophoresis analysis of the DNA extracted from infected cells showed typical DNA ladder in the course of SMRV infection. Flow cytometry analysis of SMRV infected CLC cells detected apoptotic peak in the virus infected CLC cells. Virus titre analysis and electron microscopic observation revealed that the virus replication fastigium was earlier than that of the apoptosis occurrence. No apoptosis was observed in the CLC infected with UV-inactivated SMRV. All these supported that SMRV infected CLC cells undergo apoptosis and the virus replication is necessary for apoptosis induction of CLC cells.

Restriction fragment length polymorphisms and sequence analysis: an approach for genotyping infectious pancreatic necrosis virus reference strains and other aquabirnaviruses isolated from northwestern Spain

Reference strains of infectious pancreatic necrosis virus resembling the 10 recognized serotypes and local isolates of aquabirnaviruses isolated in northwestern Spain from reservoirs (mollusks) and from asymptomatic and carrier cultured fish were genotyped by restriction fragment length polymorphism (RFLP) and nucleic acid sequence analyses. The RFLP analysis yielded seven genogroups, each of which was clearly correlated with a serotype. Sequence analysis of the three open reading frames provided quite similar results in terms of genogrouping. Based on the results of this study and in order to unify the two types of assays, we propose placing aquabirnaviruses into six genogroups, four of which can be subdivided into two genotypes based on a two-step restriction analysis. The genotyping corresponds with serotyping as follows: genogroup I includes two genotypes corresponding to serotypes A9 (genotype I.1) and A1 (genotype I.2); genogroup II corresponds to serotype A3; genogroup III includes genotypes III.1 (serotype A2) and III.2 (serotype B1); genogroups IV and V include two genotypes, each corresponding to serotypes A5, A6, A7, and A8 (genotypes IV.1, IV.2, V.1, and V.2, respectively);and genogroup VI corresponds to serotype A4. As expected, most local isolates belonged to genotype III.1 and genogroup II. However, a few local isolates corresponded to the American types of genogroup I. Finally, based on the results of this study and due to its simplicity, the two-step restriction analysis assay is proposed as a method for typing new isolates of aquabirnaviruses, and the results correspond to the results of conventional serotyping.

Protein and glycoprotein content of lymphocystis disease virus (LCDV)

The polypeptide and glycoprotein composition of eight strains of the fish-pathogenic lymphocystis disease virus (LCDV) isolated from gilt-head seabream (Sparus aurata), blackspot seabream (Pagellus bogaraveo), and sole (Solea senegalensis) were determined. The protein electrophoretic patterns of all LCDV isolates were quite similar regardless of the host fish, showing two major proteins (79.9 and 55.6 kDa) and a variable number of minor proteins. Three groups of LCDV isolates were distinguished according to the number and molecular masses of the minor proteins. Eight glycoproteins were detected inside viral particles of LCDV 2, LCDV 3 and LCDV 5 isolates, but only seven glycoproteins were found inside viral particles of LCDV 1, LCDV 4, LCDV 6, LCDV 7, and LCDV 11 isolates and the reference virus ATCC VR 342 by using five lectins. LCDV glycoproteins were mainly composed of mannose and sialic acid. These glycoproteins could be part of an external viral envelope probably derived from the host cell membrane.

Development of an in situ hybridisation procedure for the detection of sole aquabirnavirus in infected fish cell cultures

An in situ hybridisation (ISH) technique has been developed to detect sole aquabirnavirus in infected fish cell lines bluegill fibroblast (BF-2), EPC, and chinook salmon embryo cells (CHSE-214). A 613 bp cDNA probe for viral RNA coding for a fragment of VP2 protein was generated by reverse transcription polymerase chain reaction (RT-PCR) using infectious pancreatic necrosis virus (IPNV) specific DNA primers. Infected cells were strongly labelled, and no non-specific reaction was observed in non-infected cells used as negative controls. The specificity of the probe was examined by testing it against a range of IPNV serotypes such as Ab, Sp and VR-299. The ISH technique was compared with the immunofluorescence procedure to determine the sensitivity of detection of sole aquabirnavirus in BF-2 cells. The probe used in the ISH technique detected weak positivity at 8h post-inoculation (p.i.) in the cytoplasm of infected BF-2 cells inoculated with 10(3) TCID50/ml, whilst the labelling appears at 24h p.i. when the immunofluorescence technique was applied. At all other time intervals the results were equivalent.

A DNA vaccine directed against a rainbow trout rhabdovirus induces early protection against a nodavirus challenge in turbot

A DNA vaccine encoding the envelope glycoprotein from a fish rhabdovirus, viral hemorrhagic septicemia virus (VHSV), has previously been shown to induce both early and long time protection against the virus in rainbow trout. Challenge experiments have revealed that the immunity established shortly after vaccination is cross-protective against heterologous fish rhabdoviruses. In this study, we show that the DNA vaccine encoding the VHSV glycoprotein also induces early protection against a non-enveloped, positive-sense RNA virus belonging to the Nodavirus family, the Atlantic halibut nodavirus (AHNV). In a vaccine efficacy test using juvenile turbot as model fish, the fish injected with the VHSV vaccine were completely protected against a nodavirus challenge performed 8 days post vaccination, while the cumulative mortality in the control group reached 54%. A DNA vaccine carrying the gene encoding the capsid protein of AHNV revealed no protective properties against the nodavirus challenge. Histological examination of muscle tissue sections from the vaccine injection site showed that the DNA vaccine against VHSV triggered a pronounced inflammatory response in turbot similar to what has earlier been observed in rainbow trout.

Viral hemorrhagic septicemia virus alters turbot Scophthalmus maximus macrophage nitric oxide production

The effect of viral hemorrhagic septicemia virus (VHSV) in vitro infection on the nitric oxide (NO) production by turbot Scophthalmus maximus kidney macrophages has been addressed in the past. Previously, we had determined that only a small fraction of turbot possess head kidney macrophages that respond to a single exposure of lipopolysaccharide (LPS) with NO production (LPS-responsive macrophages), whereas macrophage cultures from other individuals were not activated by LPS alone and needed a combination of stimuli to respond (LPS-non-responsive macrophages). In the current work, we examined the effect of VHSV on NO production by macrophages characterized as LPS-responsive macrophages or LPS-non-responsive macrophages. Combinations of LPS and tumor necrosis factor alpha (TNF-alpha) and macrophage-activating factor (MAF) were also used to stimulate the cells for NO production. The effect of VHSV on NO production depends on the response to LPS alone. When a low multiplicity of infection was used (1.78 x 10(-3)), the NO production in response to LPS in LPS-responsive macrophages was significantly decreased. However, LPS-non-responsive macrophage cultures produced NO when a combination of LPS and VHSV was used. In the case of a higher VHSV multiplicity of infection (1.78), no significant change was observed in LPS-non-responsive animals. Combinations of LPS with TNF-alpha, LPS with MAF, and TNF-alpha with MAF were used to induce NO production in LPS-non-responsive macrophages. In all these cases, VHSV suppressed NO production, although at a significant level only when a combination of TNF-alpha and MAF was used for the induction of NO.

Experimental susceptibility of Atlantic salmon Salmo salar and turbot Scophthalmus maxim us to European freshwater and marine isolates of viral haemorrhagic septicaemia virus

A number of viral haemorrhagic septicaemia (VHS) virus isolates of European marine origin were shown to be of low pathogenicity or non-pathogenic to Atlantic salmon parr by water-borne infection. A reference freshwater VHS virus isolate known to be highly pathogenic to rainbow trout was also of low pathogenicity to Atlantic salmon. Virus was detected in some mortalities, however, demonstrating viral entry and replication. European marine VHS virus isolates do not appear to pose an imminent threat to the Atlantic salmon culture industry. Turbot were found to be refractive or of low susceptibility to marine VHS virus isolates of sprat origin and to a reference freshwater isolate, with mortalities of 0 to 13.5%. Conversely, turbot were susceptible by varying degrees to a number of VHS virus isolates taken from herring, with mortalities ranging from 16 to 68%. These results emphasise the vulnerability of turbot culture to the VHS virus isolates that are enzootic to the European marine environment.

Immune response to a recombinant capsid protein of striped jack nervous necrosis virus (SJNNV) in turbot Scophthalmus maximus and Atlantic halibut Hippoglossus hippoglossus, and evaluation of a vaccine against SJNNV

Immunisation by intraperitoneal injection of an oil-emulgated recombinant partial capsid protein (rT2) from striped jack nervous necrosis virus (SJNNV) was performed on adult turbot Scophthalmus maximus and Atlantic halibut Hippoglossus hippoglossus. A specific humoral immune response was recorded in both species, and the levels of rT2-specific antibodies increased markedly in all groups during the 20 wk experiment. A challenge model for SJNNV was established by intramuscular injection of juvenile turbot. The turbot developed viral encephalopathy and retinopathy (VER), also known as viral nervous necrosis (VNN), with cumulative mortality in the range of 25 to 66%, after intramuscular inoculation with SJNNV propagated in the striped snake head cell line (SSN-1). Although neither clinical signs nor mortality were registered, SJNNV was neuroinvasive after bath exposure. The infection after both modes of challenge was verified by means of immunohistochemistry and RT-PCR, and SJNNV was reisolated in cell culture. The results indicate that SJNNV may have entered the central nervous system (CNS) by axonal transport through motor nerves after intramuscular inoculation. A vaccine efficacy test was performed on juvenile turbot, employing oil emulsified rT2 as a test vaccine and intramuscular inoculation of SJNNV. Significant protection was observed when the challenge was performed 10 wk post-vaccination.

Pathology associated with an aquareovirus in captive juvenile Atlantic halibut Hippoglossus hippoglossus and an experimental treatment strategy for a concurrent bacterial infection

A large-scale mortality of larval and juvenile halibut Hippoglossus hippoglossus occurred at a semi-commercial halibut farm in Atlantic Canada. Investigation of the cause revealed aquareovirus particles in necrotic liver tissue of affected fish. Cytopathic effect on CHSE-214 cell lines occurred from all fish cultured for viruses, and the viral morphology of the particles in culture was consistent with that observed in necrotic host tissue. The virus was placed in the family of Reoviridae, genus Aquareovirus based on morphology and RT-PCR results. Multifocal hepatocellular necrosis was a consistent finding in all fish as well as acute necrosis of proximal renal tubules. Concurrent bacterial infections were present in some specimens. Fish experimentally treated with oxytetracycline or a combination of oxytetracycline and chloramine-T had a significantly lower mortality rate than untreated fish. Fish treated with chloramine-T alone had a significantly elevated mortality rate compared to controls. Despite supportive medical therapy, mortality levels in treated and untreated groups remained elevated, supporting the hypothesis that the primary pathogen was of viral origin. This is the first report of elevated mortalities in Atlantic halibut associated with an aquareovirus.

Characterization of the capsid protein gene from a nodavirus strain affecting the Atlantic halibut Hippoglossus hippoglossus and design of an optimal reverse-transcriptase polymerase chain reaction (RT-PCR) detection assay

A 1349 nucleotide fragment of the RNA2 from a nodavirus affecting Atlantic halibut Hippoglossus hippoglossus was characterised and the nuclotide sequence (accession no. AJ245641) was employed to develop an optimal reverse-transcriptase polymerase chain reaction (RT-PCR) detection assay. The sequenced part of the RNA2 of Atlantic halibut nodavirus (strain AH95NorA) was highly similar in organisation to that of the RNA2 of striped jack nervous necrosis virus (SJNNV), and comprised features common to all nodaviruses. These characteristics confirmed that the virus that causes viral encephalopathy and retinopathy (VER) in Atlantic halibut is a nodavirus. The nucleotide sequence of the 1349 nucleotide fragment of Atlantic halibut nodavirus RNA2 was 80% identical to the RNA2 of SJNNV. The T2 region (830 nucleotides) of the RNA2 of Atlantic halibut nodavirus shared 98% of the nucleotide sequence when compared with the homologous region of barfin flounder nervous necrosis virus (BFNNV), while the nucleotide sequence identity to SJNNV in this region was 76%. Phylogenetic analysis based on the nucleotide sequences of the T4 region (421 nucleotides) of Atlantic halibut nodavirus and of other fish nodaviruses revealed a close relationship to the nodaviruses of the barfin flounder clad that have been found in other cold-water species (Pacific cod Gadus macrocephalus and barfin founder Verasper moseri). The nucleotide sequence of the RNA2 of Atlantic halibut nodavirus included some features that differ from that of SJNNV. The ORF of the RNA2 of Atlantic halibut nodavirus lacked 6 nucleotides through a single deletion and a 5-nucleotide deletion, separated by 4 nucleotides. The 3'-non-encoding region contained a 21 nucleotide insert and a 3 nucleotide deletion when compared with SJNNV. In comparison with the RNA2 of SJNNV, the 3'-non-encoding region showed a nucleotide sequence identity of 84.5%. A primer set based on the Atlantic halibut nodavirus nucleotide sequence was employed in order to design an optimal RT-PCR. The detection limit of the PCR was 10 to 100 copies of plasmid, while the detection limit of the RT-PCR assay was 100 to 1000 copies of in vitro transcribed viral RNA.

Surface disinfection of Atlantic halibut Hippoglossus hippoglossus eggs with ozonated sea-water inactivates nodavirus and increases survival of the larvae

Disinfection by ozonation of sea-water may reduce the risk of transmission of nodavirus, a major fish pathogen, via Atlantic halibut Hippoglossus hippoglossus eggs. In the present study, eggs at 4 d prior to hatching were exposed to nodavirus and then to ozonated sea-water using different concentrations (0.3 to 10 mg l-1) and exposure times (0.5 to 10 min). None of the larvae from virus-exposed eggs washed with ozonated sea-water developed viral encephalopathy and retinopathy (VER), which was detected in all dead larvae from eggs exposed to nodavirus but not washed with ozonated sea-water. In the non-treated control group about 20% of the dead larvae developed the disease. This suggests that the halibut eggs taken from a large-scale production facility were already contaminated with nodavirus. The egg groups which had been treated with 4 mg O3 l-1 for 0.5 min or with lower total ozone exposures had a higher survival and no adverse effects on the development of the larvae after hatching were observed. Although a slight delay in hatching was found, after 2 d the cumulative hatching had normalised. In the egg groups with high total exposure (4 mg O3 l-1 for 1 min or higher total ozone exposures) a pronounced negative effect on hatching was observed. Our results indicate that the egg surface may be important in the transfer of nodavirus and that nodavirus associated with the surface of the egg may be inactivated by ozonated sea-water.

In vitro interaction of viral haemorrhagic septicaemia virus and leukocytes from trout (Oncorhynchus mykiss) and turbot (Scophthalmus maximus)

Viral haemorrhagic septicaemia virus (VHSV), a well known salmonids pathogen, has also been reported to be pathogenic for turbot (Scophthalmus maximus). In the present work, the replication of VHSV was studied in vitro in turbot head kidney macrophages and blood leukocytes. VHSV was able to infect both primary cultures and viral titer increased with time, either inside the cells or in the supernatant. However, no cytopathic effect was observed during the experiments and the titers were always lower than those obtained in the fish cell lines. The number of trout and turbot macrophages after several days of in vitro infection with VHSV was compared with uninfected controls by viable cell count but no significant differences were observed. The number of cells supporting viral replication evaluated by immunofluorescence in trout and turbot was low (8 and 1.7%, respectively). Respiratory burst activity of head kidney macrophages was assayed at different days post-infection, but no significant differences were found between the control and the infected cultures neither in trout nor turbot.