Risk assessment of wild fish as environmental sources of red sea bream iridovirus (RSIV) outbreaks in aquaculture

Red sea bream iridovirus (RSIV) causes substantial economic damage to aquaculture. In the present study, RSIV in wild fish near aquaculture installations was surveyed to evaluate the risk of wild fish being an infection source for RSIV outbreaks in cultured fish. In total, 1102 wild fish, consisting of 44 species, were captured from 2 aquaculture areas in western Japan using fishing, gill nets, and fishing baskets between 2019 and 2022. Eleven fish from 7 species were confirmed to harbor the RSIV genome using a probe-based real-time PCR assay. The mean viral load of the RSIV-positive wild fish was 101.1 ± 0.4 copies mg-1 DNA, which was significantly lower than that of seemingly healthy red sea bream Pagrus major in a net pen during an RSIV outbreak (103.3 ± 1.5 copies mg-1 DNA) that occurred in 2021. Sequencing analysis of a partial region of the major capsid protein gene demonstrated that the RSIV genome detected in the wild fish was identical to that of the diseased fish in a fish farm located in the same area in which the wild fish were captured. Based on the diagnostic records of RSIV in the sampled area, the RSIV-infected wild fish appeared during or after the RSIV outbreak in cultured fish, suggesting that RSIV detected in wild fish was derived from the RSIV outbreak in cultured fish. Therefore, wild fish populations near aquaculture installations may not be a significant risk factor for RSIV outbreaks in cultured fish.

Scale Drop Disease Virus Associated Yellowfin Seabream (Acanthopagrus latus) Ascites Diseases, Zhuhai, Guangdong, Southern China: The First Description

Scale drop disease virus (SDDV), an emerging piscine iridovirus prevalent in farmed Asian seabass Lates calcarifer in Southeast Asia, was firstly scientifically descripted in Singapore in 2015. Here, an SDDV isolate ZH-06/20 was isolated by inoculating filtered ascites from diseased juvenile yellowfin seabream into MFF-1 cell. Advanced cytopathic effects were observed 6 days post-inoculation. A transmission electron microscopy examination confirmed that numerous virion particles, about 140 nm in diameter, were observed in infected MFF-1 cell. ZH-06/20 was further purified and both whole genome and virion proteome were determined. The results showed that ZH-06/20 was composed of 131,122 bp with 135 putative viral proteins and 113 of them were further detected by virion proteome. Western blot analysis showed that no (or weak) cross-reaction was observed among several major viral proteins between ZH-06/20 and ISKNV-like megalocytivirus. An artificial challenge showed that ZH-06/20 could cause 100% death to juvenile yellowfin seabream. A typical sign was characterized by severe ascites, but not scale drop, which was considerably different from SDD syndrome in Asian seabass. Collectively, SDDV was confirmed, for the first time, as the causative agent of ascites diseases in farmed yellowfin seabream. Our study offers useful information to better understanding SDDV-associated diseases in farmed fish.

Transcriptome analysis reveals a complex response to the RGNNV/SJNNV reassortant Nervous Necrosis Virus strain in sea bream larvae

The gilthead sea bream (Sparus aurata) is a marine fish of great importance for Mediterranean aquaculture. This species has long been considered resistant to Nervous Necrosis Virus (NNV), an RNA virus that causes massive mortalities in several farmed fish animals. However, the recent appearance of RGNNV/SJNNV reassortant strains started to pose a serious threat to sea bream hatcheries, as it is able to infect larvae and juveniles of this species. While host response to NNV has been extensively studied in adult fish, little attention has been devoted to early life history stages, which are generally the most sensitive ones. Here we report for the first time a time-course RNA-seq analysis on 21-day old fish gilthead sea bream larvae experimentally infected with a RGNNV/SJNNV strain. NNV-infected and mock-infected samples were collected at four time points (6 h, 12 h, 24 h, and 48 h post infection). Four biological replicates, each consisting of five pooled larvae, were analysed for each time point and group. A large set of genes were found to be significantly regulated, especially at early time points (6 h and 12 h), with several heat shock protein encoding transcripts being up-regulated (e.g. hspa5, dnaj4, hspa9, hsc70), while many immune genes were down-regulated (e.g. myd88 and irf5 at T06, pik3r1, stat3, jak1, il12b and il6st at T12). A gene set enrichment analysis (GSEA) identified several altered pathways/processes. For instance, the formation of peroxisomes, which are important anti-viral components as well as essential for nervous system homeostasis, and the autophagy pathway were down-regulated at 6 h and 24 h post infection (hpi). Finally, two custom "reactomes" (i.e. significant gene sets observed in other studies) were defined and used. The first reactome integrated the transcriptomic response to NNV in different fish species, while the second one included all genes found to be stimulated either by interferon (IFN) or by IFN and Chikungunya virus in zebrafish. Genes in both reactomes showed predominant up-regulation at 6hpi and 12hpi and a general down-regulation at 24hpi. Such evidence suggest a certain degree of similarity between the response of sea bream and that of other fish species to NNV, while the observed down-regulation of IFN- and viral-stimulated pathways argues for a possible interference of NNV against the host response.

Red seabream iridovirus associated with cultured Florida pompano Trachinotus carolinus mortality in Central America

Mariculture of Florida pompano Trachinotus carolinus in Central America has increased over the last few decades and it is now a highly valued food fish. High feed costs and infectious diseases are significant impediments to the expansion of mariculture. Members of the genus Megalocytivirus (MCV), subfamily Alphairidovirinae, within the family Iridoviridae, are emerging pathogens that negatively impact Asian mariculture. A significant mortality event in Florida pompano fingerlings cultured in Central America occurred in October 2014. Affected fish presented with abdominal distension, darkening of the skin, and periocular hemorrhages. Microscopic lesions included cytomegalic 'inclusion body-bearing cells' characterized by basophilic granular cytoplasmic inclusions in multiple organs. Transmission electron microscopy revealed arrays of hexagonal virions (155-180 nm in diameter) with electron-dense cores within the cytoplasm of cytomegalic cells. Pathological findings were suggestive of an MCV infection, and the diagnosis was later confirmed by partial PCR amplification and sequencing of the viral gene encoding the myristylated membrane protein. The viral sequence revealed that the fingerlings were infected with an MCV genotype, red seabream iridovirus (RSIV), previously reported only from epizootics in Asian mariculture. This case underscores the threat RSIV poses to global mariculture, including the production of Florida pompano in Central America.

Identification of Quantitative Trait Loci for Resistance to RSIVD in Red Sea Bream (Pagrus major)

Red sea bream iridoviral disease (RSIVD) is a major viral disease in red sea bream farming in Japan. Previously, we identified one candidate male individual of red sea bream that was significantly associated with convalescent individuals after RSIVD. The purpose of this study is to identify the quantitative trait loci (QTL) linked to the RSIVD-resistant trait for future marker-assisted selection (MAS). Two test families were developed using the candidate male in 2014 (Fam-2014) and 2015 (Fam-2015). These test families were challenged with RSIV, and phenotypes were evaluated. Then, de novo genome sequences of red sea bream were obtained through next-generation sequencing, and microsatellite markers were searched and selected for linkage map construction. One immune-related gene, MHC class IIβ, was also used for linkage map construction. Of the microsatellite markers searched, 148 and 197 were mapped on 23 and 27 linkage groups in the female and male linkage maps, respectively, covering approximately 65% of genomes in both sexes. One QTL linked to an RSIVD-resistant trait was found in linkage group 2 of the candidate male in Fam-2014, and the phenotypic variance of the QTL was 31.1%. The QTL was closely linked to MHC class IIβ. Moreover, the QTL observed in Fam-2014 was also significantly linked to an RSIVD-resistant trait in the candidate male of Fam-2015. Our results suggest that the RSIVD-resistant trait in the candidate male was controlled by one major QTL closely linked to the MHC class IIβ gene and could be useful for MAS of red sea bream.

Innate Cell-Mediated Cytotoxic Activity of European Sea Bass Leucocytes Against Nodavirus-Infected Cells: A Functional and RNA-seq Study

Nervous necrosis virus (NNV) causes high mortalities in several marine species. We aimed to evaluate the innate cell-mediated cytotoxic (CMC) activity of head-kidney leucocytes (HKLs) isolated from naïve European sea bass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata), a very susceptible and resistant fish species to NNV, respectively, against fish cell lines infected with NNV. Seabream HKLs showed significantly increased innate CMC activity against NNV-infected cells, compared to those uninfected, while sea bass HKLs failed to do so. Thus, we performed a RNA-seq study to identify genes related to the CMC activity of sea bass leucocytes. Thus, we found that sea bass HKLs incubated with DLB-1 cells alone (CMC_DLB1) or with NNV-infected DLB-1 cells (CMC_DLB1-NNV) showed very similar transcriptomic profiles and the GO analysis revealed that most of the up-regulated genes were related to immunity. Strikingly, when the CMC samples with and without NNV were compared, GO analysis revealed that most of the up-regulated genes in CMC_DLB1-NNV samples were related to metabolism and very few to immunity. This is also in agreement with the functional data. These data point to the escape of CMC activity by NNV infection as an important factor involved in the high susceptibility to nodavirus infections of European sea bass.

Rapid and Sensitive Detection of Lymphocystis Disease Virus Genotype VII by Loop-Mediated Isothermal Amplification

Lymphocystis disease virus (LCDV) infections have been described in gilthead seabream (Sparus aurata L.) and Senegalese sole (Solea senegalensis, Kaup), two of the most important marine fish species in the Mediterranean aquaculture. In this study, a rapid, specific, and sensitive detection method for LCDV genotype VII based on loop-mediated isothermal amplification (LAMP) was developed. The LAMP assay, performed using an apparatus with real-time amplification monitoring, was able to specifically detect LCDV genotype VII from clinically positive samples in less than 12 min. In addition, the assay allowed the detection of LCDV in all asymptomatic carrier fish analysed, identified by qPCR, showing an analytical sensitivity of ten copies of viral DNA per reaction. The LCDV LAMP assay has proven to be a promising diagnostic method that can be used easily in fish farms to detect the presence and spread of this iridovirus.

Mapping QTL for Resistance Against Viral Nervous Necrosis Disease in Asian Seabass

Viral nervous necrosis disease (VNN), caused by nervous necrosis virus (NNV), leads to mass mortality in mariculture. However, phenotypic selection for resistance against VNN is very difficult. To facilitate marker-assisted selection (MAS) for resistance against VNN and understanding of the genetic architecture underlying the resistance against this disease, we mapped quantitative trait loci (QTL) for resistance against VNN in Asian seabass. We challenged fingerlings at 37 days post-hatching (dph), from a single back-cross family, with NNV at a concentration of 9 × 10(6) TCID50/ml for 2 h. Daily mortalities were recorded and collected. A panel of 330 mortalities and 190 surviving fingerlings was genotyped using 149 microsatellites with 145 successfully mapped markers covering 24 linkage groups (LGs). Analysis of QTL for both resistance against VNN and survival time was conducted using interval mapping. Five significant QTL located in four LGs and eight suggestive QTL in seven LGs were identified for resistance. Another five significant QTL in three LGs and five suggestive QTL in three LGs were detected for survival time. One significant QTL, spanning 3 cM in LG20, was identified for both resistance and survival time. These QTL explained 2.2-4.1% of the phenotypic variance for resistance and 2.2-3.3% of the phenotypic variance for survival time, respectively. Our results suggest that VNN resistance in Asian seabass is controlled by many loci with small effects. Our data provide information for fine mapping of QTL and identification of candidate genes for a better understanding of the mechanism of disease resistance.

Nodavirus Colonizes and Replicates in the Testis of Gilthead Seabream and European Sea Bass Modulating Its Immune and Reproductive Functions

Viruses are threatening pathogens for fish aquaculture. Some of them are transmitted through gonad fluids or gametes as occurs with nervous necrosis virus (NNV). In order to be transmitted through the gonad, the virus should colonize and replicate inside some cell types of this tissue and avoid the subsequent immune response locally. However, whether NNV colonizes the gonad, the cell types that are infected, and how the immune response in the gonad is regulated has never been studied. We have demonstrated for the first time the presence and localization of NNV into the testis after an experimental infection in the European sea bass (Dicentrarchus labrax), and in the gilthead seabream (Sparus aurata), a very susceptible and an asymptomatic host fish species, respectively. Thus, we localized in the testis viral RNA in both species using in situ PCR and viral proteins in gilthead seabream by immunohistochemistry, suggesting that males might also transmit the virus. In addition, we were able to isolate infective particles from the testis of both species demonstrating that NNV colonizes and replicates into the testis of both species. Blood contamination of the tissues sampled was discarded by completely fish bleeding, furthermore the in situ PCR and immunocytochemistry techniques never showed staining in blood vessels or cells. Moreover, we also determined how the immune and reproductive functions are affected comparing the effects in the testis with those found in the brain, the main target tissue of the virus. Interestingly, NNV triggered the immune response in the European sea bass but not in the gilthead seabream testis. Regarding reproductive functions, NNV infection alters 17β-estradiol and 11-ketotestosterone production and the potential sensitivity of brain and testis to these hormones, whereas there is no disruption of testicular functions according to several reproductive parameters. Moreover, we have also studied the NNV infection of the testis in vitro to assess local responses. Our in vitro results show that the changes observed on the expression of immune and reproductive genes in the testis of both species are different to those observed upon in vivo infections in most of the cases.

Transmission of lymphocystis disease virus to cultured gilthead seabream, Sparus aurata L., larvae

The transmission of lymphocystis disease virus (LCDV) to gilthead seabream, Sparus aurata L., larvae was investigated using fertilized eggs from a farm with previous reports of lymphocystis disease. LCDV genome was detected by PCR-hybridization in blood samples from 17.5% of the asymptomatic gilthead seabream broodstock analysed. Using the same methodology, eggs spawned from these animals were LCDV positive, as well as larvae hatched from them. The presence of infective viral particles was confirmed by cytopathic effects development on SAF-1 cells. Whole-mount in situ hybridization (ISH) and immunohistochemistry (IHC) showed the presence of LCDV in the epidermis of larvae hatched from LCDV-positive eggs. When fertilized eggs were disinfected with iodine, no viral DNA was detected either in eggs (analysed by PCR-hybridization) or in larvae (PCR-hybridization and ISH). These results suggest the vertical transmission of LCDV, the virus being transmitted on the egg surface. Larvae hatched from disinfected eggs remain LCDV negative during the endotrophic phase, as showed by PCR-hybridization, ISH and IHC. After feeding on LCDV-positive rotifers, viral antigens were observed in the digestive tract, which suggests that viral entry could be achieved via the alimentary canal, and that rotifers can act as a vector in LCDV transmission to gilthead seabream larvae.

Effect of lipopolysaccharides from Vibrio alginolyticus on the Mx gene expression and virus recovery from gilthead sea bream (Sparus aurata L.) experimentally infected with Nodavirus

Infections with nodavirus affect a wild and farmed fish species throughout the world, mostly from the marine environment. The aim of this work was to determine the immune status of gilthead sea bream that comes as a result of a Nodavirus infection, induced by activation of the interferon response pathway by lipopolysaccharides from Vibrio alginolyticus and the expression of interferoninduced Mx protein in liver samples. The enhancement of Mx protein gene expression was detected in liver samples of experimentally nodavirus infected fish and, furthermore, the immunostimulant LPS of V. alginolyticus decreased almost three times the virus titration with respect to no-immunized or infected with nodavirus group of fish.

Application of in situ detection techniques to determine the systemic condition of lymphocystis disease virus infection in cultured gilt-head seabream, Sparus aurata L

Immunohistochemistry (IHC) and in situ hybridization (ISH) techniques have been used for the detection of lymphocystis disease virus (LCDV) in formalin-fixed, paraffin-embedded tissues from gilt-head seabream, Sparus aurata L. Diseased and recovered fish from the same population were analysed. IHC was performed with a polyclonal antibody against a 60-kDa viral protein. A specific digoxigenin-labelled probe, obtained by PCR amplification of a 270-bp fragment of the gene coding the LCDV major capsid protein, was used for ISH. LCDV was detected in skin dermis and gill lamellae, as well as in several internal organs such as the intestine, liver, spleen and kidney using both techniques. Fibroblasts, hepatocytes and macrophages seem to be target cells for virus replication. The presence of lymphocystis cells in the dermis of the skin and caudal fin, and necrotic changes in the epithelium of proximal renal tubules were the only histological alterations observed in fish showing signs of the disease.

Nodavirus increases the expression of Mx and inflammatory cytokines in fish brain

Nodavirus has become a serious pathogen for a wide range of cultured marine fish species. In the present work, the expression of genes related to immune and inflammatory responses of sea bream (Sparus aurata L.), considered as non susceptible species, was studied both in vitro and in vivo. No replication of the virus was observed in head kidney macrophages and blood leukocytes. Moreover, the enhancement of expression of several immune genes (tumor necrosis factor alpha (TNFalpha), interleukin-1-beta (IL-1beta), interferon-induced Mx protein) was not detected in both head kidney macrophages and blood leucocytes in response to an in vitro infection with nodavirus. However, in vivo, nodavirus was detected 1 day post-infection (p.i.) by a reverse transcription-polymerase chain reaction (RT-PCR) in blood, liver, head kidney and brain of experimentally infected sea bream, while its presence clearly decreased in blood after 3 days p.i. Also, a transitory increment of the expression of TNFalpha and IL-1beta was detected in the brain of intramuscular (i.m.) infected sea bream 3 days p.i. In head kidney, the over expression of TNFalpha was only observed 1 day p.i. The expression of Mx, an interferon induced gene, was increased in brain and head kidney of infected sea bream, reaching values of 1300-fold compared to controls in brain three days post-infection. For comparative purposes, we analyzed the expression of the same genes on a susceptible species, such as sea bass (Dicentrarchus labrax) and, although the same pattern of expression was observed both in brain and kidney, the magnitude was different mainly in the case of brain, the key organ of the infection, where higher expression of TNFalpha and lower expression of Mx compared with control was observed.

Application of the arming system for the expression of the 380R antigen from red sea bream iridovirus (RSIV) on the surface of yeast cells: a first step for the development of an oral vaccine

The cell surface is a functional interface between the inside and the outside of the cell. Moreover, cells have systems for anchoring surface specific proteins and for confining surface proteins to particular domains on the cell surface. For use in bioindustrial processes applied to oral vaccination, we consider that cell-surface display systems must be useful and that the yeast Saccharomyces cerevisiae, the most suitable microorganism for practical purposes, is available as a host for genetic engineering because it can be subjected to many genetic manipulations. In particular, the rigid structure of the cell makes the yeast suitable for several of the applications. In this study, we describe the expression of one of the target antigens, 380R, from the red sea bream iridovirus (RSIV), which is one of the most common viral diseases in the cultured marine fish Pagrus major in Japan, using the arming yeast system and aiming at its application for oral vaccination. We first performed the molecular cloning and expression of the 380R antigen from RSIV in Escherichia coli. The nucleotide sequence of the 380R antigen was composed of an open reading frame (ORF) of 1360 bp encoding a protein of 453 residues. To prepare a specific antibody against the 380R antigen, the recombinant protein was overexpressed and purified in E. coli. As a result of indirect immunofluorescence with the specific antibody, we could observe the expression of the 380R antigen on the surface of the yeast cells. Thus, we have successfully prepared the source of an oral vaccine using cell-surface display technology in yeast.

Transcriptional profile of red seabream iridovirus in a fish model as revealed by viral DNA microarrays

Red seabream iridovirus (RSIV) disease is a serious disease of many marine fish species in Japan and elsewhere. For a better understanding of the molecular pathogenic mechanism, we examined the transcriptional profile of RSIV in infected fish using a DNA microarray. Expression of RSIV open reading frames (ORFs) was first detected at about 5 days post-infection (d.p.i.), and accounted for about 45% of total ORFs. Almost all the ORFs (97-99%) were expressed at their maximum levels during 7-9 d.p.i. The expression levels and the number of expressed ORFs started to decrease at 10 d.p.i. These results suggest that pathogenesis of RSIV infection began at around day 5, and continued with high levels of viral multiplication until viral clearance by host antiviral defenses starting from around 10 d.p.i. A comparison of viral gene expressions in the spleen and kidney over the course of the infection suggests that RSIV preferentially targets the spleen. The spleen may thus be the most susceptible organ for diagnosis of iridoviral disease.

Co-occurrence of viral and bacterial pathogens in disease outbreaks affecting newly cultured sparid fish

Several microbial disease outbreaks in farm stocks of newly cultured sparid fish species, such as common seabream, redbanded seabream, and white seabream, were recorded from 2004 to 2006. This study describes the isolation and characterization of the potential causative agents, either bacteria or viruses, of these outbreaks. The isolated bacterial strains were characterized according to traditional taxonomical analyses and sequencing of a 16S rDNA fragment. Most bacteria were identified as Vibrio spp. and Photobacterium damselae subsp. damselae. The development of cytopathic effects (CPE) on different fish cell lines, the application of specific nested-PCR tests for infectious pancreatic necrosis virus (IPNV), viral nervous necrosis virus (VNNV) and viral hemorrhagic septicemia virus (VHSV), and subsequent sequence analyses were used for virus detection and identification. VNNV, related to the striped jack neural necrosis virus (SJNNV) genotype, and VHSV, related to the genotype Ia, were the only viruses detected. VNNV was isolated from the three fish species under study in five different outbreaks, whereas VHSV was isolated from common seabream and white seabream during two of these outbreaks. IPNV was not detected in any case.

Suppression subtraction hybridization (SSH) and macroarray techniques reveal differential gene expression profiles in brain of sea bream infected with nodavirus

Despite of the impact that viruses have on aquatic organisms, relatively little is known on how fish fight against these infections. In this work, the brain gene expression pattern of sea bream (Sparus aurata) in response to nodavirus infection was investigated. We used the suppression subtractive hybridization (SSH) method to generate a subtracted cDNA library enriched with gene transcripts differentially expressed after 1 day post-infection. Some of the ESTs from the infected tissues fell in gene categories related to stress and immune responses. For the reverse library (ESTs expressed in controls compared with infected tissues) the most abundant transcripts were of ribosomal and mitochondrial nature. Several ESTs potentially induced by virus exposure were selected for in vivo expression studies. We observed a clear difference in expression between infected and control samples for two candidate genes, ubiquitin conjugating enzyme 7 interacting protein, which seems to play an important role in apoptosis and the interferon induced protein with helicase C domain 1 (mda-5) that contributes to apoptosis and regulates the type I IFN production, a key molecule of the antiviral innate response in most organisms.

Characterization of a new fibroblast cell line from a tail fin of red sea bream, Pagrus major, and phylogenetic relationships of a recent RSIV isolate in Japan

Red sea bream iridovirus (RSIV) is a causative agent of red sea bream iridoviral disease (RSIVD) in marine fish species in Japan. Fibroblast cells were developed from a tail fin of red sea bream, Pagrus major, and then underwent single cell cloning. The successful cloned cells were named CRF-1 cells. Most CRF-1 cells had a normal diploid karyotype with 2n=48 by chromosomal analysis. RSIV-infected CRF-1 cells showed typical morphological changes that were associated with apoptosis by EGFP-annexin V staining. The serial viral passages were successful in CRF-1 cells but failed in BF-2 cells as judged by MTT assay. The expression of three genes obviously decreased in BF-2 cells compared with CRF-1 cells and finally was below detectable level. Because the expression of 591R gene showed the fastest decrease among three transcripts, the suppression of IE transcript may be responsible for the restricted replication in BF-2 cells. MCP and ATPase phylogenetic trees showed that RSIV strain U-1 belongs to a distinct group from RSIV strain ehime-1. Therefore, possibly recent epizootics of RSIVD in Japan do not originate directly from RSIV strain ehime-1. Taken together, this study confirmed that RSIV strain U-1 is more closely related to Korean RSIV isolates.

The molecular epidemiology of iridovirus in Murray cod (Maccullochella peelii peelii) and dwarf gourami (Colisa lalia) from distant biogeographical regions suggests a link between trade in ornamental fish and emerging iridoviral diseases

Iridoviruses have emerged over 20 years to cause epizootics in finfish and amphibians in many countries. They may have originated in tropical Asia and spread through trade in farmed food fish or ornamental fish, but this has been difficult to prove. Consequently, MCP, ATPase and other viral genes were sequenced from archival formalin-fixed, paraffin-embedded tissues from farmed Murray cod (Maccullochella peelii peelii) that died during an epizootic in 2003 and from diseased gouramis that had been imported from Asia. There was almost complete homology (99.95%) over 4,527 bp between Murray cod iridovirus (MCIV) and an iridovirus (DGIV) present in dwarf gourami (Colisa lalia) that had died in aquarium shops in Australia in 2004, and very high homology with infectious spleen and kidney necrosis virus (ISKNV) (99.9%). These viruses are most likely to be a single species within the genus Megalocytivirus and probably have a common geographic origin. Primers for genus-specific PCR and for rapid discrimination of MCIV/DGIV/ISKNV and red sea bream iridovirus (RSIV), a notifiable pathogen, were developed. These were used in a survey to determine that the prevalence of DGIV infection in diseased gourami in retail aquarium shops in Sydney was 22% (95% confidence limits 15-31%). The global trade in ornamental fish may facilitate the spread of Megalocytivirus and enable emergence of disease in new host species in distant biogeographic regions.

Transcription program of red sea bream iridovirus as revealed by DNA microarrays

Red sea bream iridovirus (RSIV) has been identified as the causative agent of a serious disease in red sea bream and at least 30 other marine fish species. We developed a viral DNA microarray containing 92 putative open reading frames of RSIV to monitor the viral gene transcription program over the time course of an in vitro infection and to classify RSIV transcripts into temporal kinetic expression classes. The microarray analysis showed that viral genes commenced expression as early as 3 h postinfection (p.i.) and this was followed by a rapid escalation of gene expression from 8 h p.i. onwards. Based on the expression of some enzymes associated with viral DNA replication, the DNA replication of RSIV appeared to begin at around 8 h p.i. in infected cells in vitro. Using a de novo protein synthesis inhibitor (cycloheximide) and a viral DNA replication inhibitor (phosphonoacetic acid), 87 RSIV transcripts could be classified into three temporal kinetic classes: nine immediate-early (IE), 40 early (E), and 38 late (L) transcripts. The gene expression of RSIV occurred in a temporal kinetic cascade with three stages (IE, E, and L). Although the three classes of transcripts were distributed throughout the RSIV genome, E transcripts appeared to cluster in at least six discrete regions and L transcripts appeared to originate from seven discrete regions. The microarray data were statistically confirmed by using a t test, and were also clustered into groups based on similarity in the gene expression patterns by using a cluster program.

Detection of lymphocystis disease virus (LCDV) in asymptomatic cultured gilt-head seabream (Sparus aurata, L.) using an immunoblot technique

An immunoblot technique for the detection of lymphocystis disease virus (LCDV) in naturally infected gilt-head seabream (Sparus aurata, L.) has been developed. A specific antiserum against a 60 kDa viral protein has been proven to be an appropriate tool for LCDV diagnosis either from inoculated cell cultures or from fish tissues using the immunoblot assay. The sensitivity of this technique varied between 10(-1) and 10(2) TCID50. LCDV has also been detected in fish tissues from both, diseased and asymptomatic gilt-head seabream. For the asymptomatic fish detection, a viral amplification step in cell culture and a subsequent viral concentration using polyethylene glycol (PEG) (600 wt) are required. On the contrary, immunoblot allowed the detection of LCDV antigens directly from tissue homogenates of diseased fish. The method described in this study shows higher sensitivity than classical detection techniques based on cell culture inoculation.

Rapid detection of a fish iridovirus using loop-mediated isothermal amplification (LAMP)

A loop-mediated isothermal amplification (LAMP) procedure is described to detect the genomic DNA molecule of red seabream iridovirus (RSIV), a fish iridovirus belonging to the Iridoviridae family. The RSIV DNA was amplified using DNA extracts obtained from spleen of infected red seabream, Pagrus major and from various RSIV isolates. The method was at least 10 times more sensitive than conventional PCR in detecting for the presence of RSIV. A striking feature of the LAMP reaction is its ability to synthesize a large amount of DNA leading to the production of a white precipitate, magnesium pyrophosphate, as a by-product. The presence or absence of this white precipitate facilitates easy detection of the RSIV genomic DNA without the use of gel electrophoresis. A strong correlation exists between the amount of input viral DNA copy and the corresponding turbidity reading at the end of the reaction; hence, the LAMP reaction may be used potentially to quantify RSIV particles in the infected fish.

Morphological changes contribute to apoptotic cell death and are affected by caspase-3 and caspase-6 inhibitors during red sea bream iridovirus permissive replication

Red sea bream iridovirus (RSIV) of the Iridoviridae family is a causative agent of lethal infections in many cultured marine fish species in southwestern Japan. RSIV-induced apoptosis was divided as follows: (1). cell shrinkage and rounding at the early apoptotic stage, (2). cell enlargement at the middle apoptotic stage, (3). formation of apoptotic body-like vesicles at the late apoptotic stage and phagocytosis by neighboring cells, and (4). loss of membrane integrity in apoptotic body-like vesicles without phagocytosis by neighboring cells. By affinity labeling, RSIV-induced apoptosis included caspase-dependent apoptosis. RSIV infection caused cell rounding but not cell enlargement or formation of apoptotic body-like vesicles and further restricted part of the structural protein synthesis in the presence of caspase-3 and -6 inhibitors. These findings showed the involvement of caspase-3 and -6 in the morphological changes at the middle and late apoptotic stages and viral protein synthesis in the late stage of RSIV infection.

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.

Early interactions of marine birnavirus infection in several fish cell lines

Marine birnavirus (MABV), a member of the genus Aquabirnavirus, family Birnaviridae, is an unenveloped icosahedral virus with two genomes of double-stranded RNA. The mechanisms of MABV adsorption and penetration are still undetermined. This work examined MABV infection in susceptible and resistant fish cell lines. MABV adsorbed not only onto the cell surfaces of susceptible (CHSE-214 and RSBK-2) cells but also onto resistant (FHM and EPC) cells. Furthermore, the virus entered the cytoplasm through the endocytotic pathway in CHSE-214, RSBK-2 and FHM cells but did not penetrate EPC cells. Thus, restriction of the MABV replication cycle is different between resistant FHM and EPC cells. The virus was found to bind to an around 250 kDa protein on CHSE-214, RSBK-2, FHM and EPC cells. Thus, this 250 kDa protein may be a major MABV receptor that exists in the plasma membranes of all four cell lines examined. This result suggests further that another receptor for virus penetration may exist in CHSE-214, RSBK-2 and FHM cells but not in EPC cells.

Isolation and characterisation of rhabdovirus from wild common bream Abramis brama, roach Rutilus rutilus, farmed brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss in Northern Ireland

Rhabdovirus was isolated from wild common bream Abramis brama during a disease outbreak with high mortality in Northern Ireland during May 1998. Rhabdovirus was also isolated at the same time from healthy farmed rainbow Oncorhynchus mykiss and brown trout Salmo trutta on the same stretch of river and 11 mo later from healthy wild bream and roach Rutilus rutilus in the same river system. Experimental intra-peritoneal infection of bream and mirror carp Cyprinus carpio var specularis with 2 of these isolates produced low mortality rates of < or = 12%. Serological testing of these isolates by virus neutralisation indicated that they were antigenically closely related to pike fry rhabdovirus (PFRV) but not to spring viraemia of carp virus (SVCV), while testing by enzyme-linked immunosorbent assay indicated them to be antigenically different from both. Comparison of nucleotide sequence data of a 550 base pair segment of the viral glycoprotein generated by reverse transcription-polymerase chain reaction indicated a high (> or = 96.6%) degree of similarity between these isolates and a previous Northern Ireland isolate made in 1984, a 1997 isolate from bream in the Republic of Ireland and an earlier Dutch isolate from roach. In contrast, similarity between these isolates and PFRV was < 82.4%, indicating that these viruses belong to 2 distinct genogroups, while similarity to SVCV was even lower (< 67.4%).

Sea bream Sparus aurata, an asymptomatic contagious fish host for nodavirus

During an epidemiological survey of viral encephalopathy and retinopathy (VER) in diseased sea bass Dicentrarchus labrax, a nodavirus isolate was recovered from net pen-reared sea bream Sparus aurata harboured in the same farming premises. After the virus was isolated and identified by immunofluorescence on SSN-1 cells, sequence analysis with a PCR product from the T4 region of the capsid protein gene indicated that the virus shared 100% identity with a pathogenic virus strain isolated from sea bass. Infection trials demonstrated the pathogenicity of the sea bream virus isolate for juvenile sea bass whereas sea bream infected with the same virus isolate remained asymptomatic even following intramuscular injection of virus. Nevertheless, the sea bream appeared to be a potential carrier of nodavirus, as juvenile sea bass became infected when maintained in a tank containing experimentally contaminated sea bream.