Megalocytiviruses

Complete genome analysis of a red seabream iridovirus (RSIV) isolated from Asian seabass (Lates calcarifer) in India

Red sea bream iridovirus (RSIV) is the causative agent of the iridoviral disease with high mortality rates in cultured fish. Our laboratory reported the first case of RSIV infection in India which resulted in mass mortalities of Asian seabass, Lates calcarifer. The RSIV-LC strain isolated from infected fish was subjected to complete genome sequencing and analysis. The complete genome of RSIV-LC was found to be of 111,557 bp in size having a G + C content of 53 %. The complete genome has 114 open reading frames (ORFs) of which 38 ORFs were predicted as functional proteins while the rest were hypothetical proteins. Among the ORFs 26 were found to be core genes reported earlier to be homologous in iridovirus complete genomes. Phylogenetic tree constructed based on the 26 core gene sequences, major capsid protein and ATPase genes revealed RSIV-LC in this study to belong to the genus Megalocytivirus of the RSIV-Genotype II. The present study provides the first report of the complete genome sequence and annotation of the RSIV strain isolated from India.

Transcriptional programs of infectious spleen and kidney necrosis virus (ISKNV) in vitro and in vivo

Infectious spleen and kidney necrosis virus (ISKNV), causing serious infectious diseases to marine and freshwater fishes, is the type species of the genus Megalocytivirus, family Iridoviridae. In this study, the transcriptional programs of ISKNV in vitro (MFF-1 cells) and in vivo (spleens from mandarin fish) were investigated using real-time PCR. Transcription of all the putative open reading frames (ORFs) of ISKNV was verified. The temporal expression patterns of ISKNV ORFs in vitro and in vivo, including peak expression times (PETs) and relative maximal expression levels, were determined and compared. The K-means clustering with Spearman rank correlation was generated in heat maps constructed based on ISKNV ORF expression profiles in vivo and in vitro. The current study may provide a global picture of ISKNV infection at the transcription level and help better understand the molecular pathogenic mechanism of megalocytiviruses.

First detection of infectious spleen and kidney necrosis virus (ISKNV) associated with massive mortalities in farmed tilapia in Africa

In late 2018, unusual patterns of very high mortality (>50% production) were reported in intensive tilapia cage culture systems across Lake Volta in Ghana. Samples of fish and fry were collected and analysed from two affected farms between October 2018 and February 2019. Affected fish showed darkening, erratic swimming and abdominal distension with associated ascites. Histopathological observations of tissues taken from moribund fish at different farms revealed lesions indicative of viral infection. These included haematopoietic cell nuclear and cytoplasmic pleomorphism with marginalization of chromatin and fine granulation. Transmission electron microscopy showed cells containing conspicuous virions with typical iridovirus morphology, that is enveloped, with icosahedral and/or polyhedral geometries and with a diameter c.160 nm. PCR confirmation and DNA sequencing identified the virions as infectious spleen and kidney necrosis virus (ISKNV). Samples of fry and older animals were all strongly positive for the presence of the virus by qPCR. All samples tested negative for TiLV and nodavirus by qPCR. All samples collected from farms prior to the mortality event were negative for ISKNV. Follow‐up testing of fish and fry sampled from 5 additional sites in July 2019 showed all farms had fish that were PCR‐positive for ISKNV, whether there was active disease on the farm or not, demonstrating the disease was endemic to farms all over Lake Volta by that point. The results suggest that ISKNV was the cause of disease on the investigated farms and likely had a primary role in the mortality events. A common observation of coinfections with Streptococcus agalactiae and other tilapia bacterial pathogens further suggests that these may interact to cause severe pathology, particularly in larger fish. Results demonstrate that there are a range of potential threats to the sustainability of tilapia aquaculture that need to be guarded against.

First report of the infectious spleen and kidney necrosis virus (ISKNV) infection in ornamental fishes in India

Infectious spleen and kidney necrosis virus (ISKNV), a member of family iridoviridae, reported for the first time in a wide range of ornamental fish species in India. Significant mortalities during the year 2018-19 were reported from a number of retailers in the region with various clinical signs. The samples of moribund, dead and apparently healthy ornamental fishes were collected from retailers, located in three districts of Karnataka, India. Out of 140 fish samples, 16 samples (11.42%) representing 10 different fish species were found positive to ISKNV by OIE listed primers and same samples were reported to amplify the major capsid protein (MCP) gene of ISKNV. Further, sequence analysis of MCP gene showed that all strains detected in this study were closely related to other documented isolates from different countries with an identity ranging from 98.76% to 100%. Further, they clustered in the clade of ISKNV, during the phylogenetic analysis. The sequence similarity was high (99.94%) to ISKNV strains from Japan, Australia and Malaysia. This is the first report of an ISKNV infection in India. Moreover, out of 10 ISKNV-positive fish species, three species were reported positive to ISKNV for the first time in the world. Further, the in vitro experiment showed the growth of virus in Asian sea bass cell line, which is a natural host of ISKNV. Therefore, considering the lethal nature of megalocytiviruses to infect a vast range of species, proper biosecurity measures need to be taken to control these emerging pathogens.

Isolation and characterisation of an ISKNV-genotype megalocytivirus from imported angelfish Pterophyllum scalare

Using cultures of the SKF-9 cell line, megalocytivirus AFIV-16 was isolated from imported angelfish Pterophyllum scalare held in quarantine at the Australian border. The cytopathic effect caused by isolate AFIV-16 presented as cell rounding and enlargement, but complete destruction of the infected cell cultures did not occur. The infected cells demonstrated immunocytochemical reactivity with monoclonal antibody M10, which is used for diagnosis of OIE-listed red sea bream iridoviral disease. Using electron microscopy, the virus particles, consisting of hexagonal nucleocapsids, were observed in the cytoplasm of SKF-9 cells. The replication of AFIV-16 in cultured SKF-9 cells was significantly greater at 28°C incubation than at 22 and 25°C incubation, whereas no difference in growth characteristics was observed for red sea bream iridovirus (RSIV) isolate KagYT-96 across this temperature range. Whole genome sequencing demonstrated that AFIV-16 has a 99.96% similarity to infectious spleen and kidney necrosis virus (ISKNV), the type species in the genus Megalocytivirus. AFIV-16 was classified into ISKNV genotype Clade 1 by phylogenetic analysis of the major capsid protein gene nucleotide sequence. This is the first report of whole genome sequencing of an ISKNV genotype megalocytivirus isolated from ornamental fish.

Simultaneous detection of scale drop disease virus and Flavobacterium columnare from diseased freshwater-reared barramundi Lates calcarifer

Freshwater farming of barramundi Lates calcarifer in Thailand is a growing sector in aquaculture, but mortalities due to infectious diseases are still a major threat to this industry. In 2018, an episode of severe mortality in juvenile barramundi was noted in a freshwater earth pond site. Fish presented with severe gill necrosis, as well as severe cutaneous hemorrhages, scale loss, and discoloration at the base of dorsal fin (saddleback lesions). Histopathology revealed extensive necrosis of skeletal muscle and gill filaments, as well as basophilic inclusion bodies and megalocytosis in muscle, gill, liver, and kidney. Scale drop disease virus (SDDV) infection was subsequently confirmed by virus-specific semi-nested PCR. Further, DNA sequences of the viral major capsid protein (MCP) and ATPase genes had a respective homology of 99.85 and 99.92% with sequences of SDDV infecting barramundi in saltwater culture. Gill necrosis and saddleback lesions are not typical lesions associated with scale drop syndrome. Their presence was explained by Flavobacterium columnare isolation from the gill, followed by positive F. columnare-specific PCR. To our knowledge, this is the first report of SDDV-associated mortality in freshwater-farmed barramundi. Furthermore, this mortality presented as a concurrent infection with SDDV and F. columnare, with typical lesions of both infections.

Development of primary cell culture from spleen of giant gourami Osphronemus goramy for propagation of giant gourami iridovirus (GGIV)

The severe mortality of fish due to the infection of megalocytivirus caused significant economic losses. Since 2011, megalocytivirus (giant gourami iridovirus (GGIV)) has become the main pathogen in giant gourami (Osphronemus goramy), particularly in West Java, Central Java and Bali. This study aimed to develop primary cell culture from spleen as the target organ for propagating megalocytivirus in vitro, which was developed by explant method with enzymatic dissociation. Optimization was carried out at incubation temperature, medium and serum concentrations. The origin of the primary cell, cell susceptibility and GGIV pathogenicity were observed. The results showed that the primary cell (GP cells) can grow well in 10% foetal bovine serum L-15 medium at 27°C, which was sufficient for cell growth. PCR and BLAST analyses showed the primary cell was originated from giant gourami. In infected GP cells, cell enlargement and cell rounding were observed. Virus propagated in GP cells was highly virulent when injecting giant gourami in an artificial infection experiment. Intraperitoneal injection of diluted virus supernatant showed 100% mortality in 7-11 days post-injection and 97% mortality in 21 days post-cohabitation, with abnormalities observed in spleen and kidney. In conclusion, GP cell was successfully subcultured for more than 30 passages and susceptible to GGIV.

Prevalence of Infectious Spleen and Kidney Necrosis Virus (ISKNV), Nervous Necrosis Virus (NNV) and Ectoparasites in Juvenile Epinephelus spp. Farmed in Aceh, Indonesia

A cross-sectional survey was used to estimate the prevalence of infections with the Infectious spleen and kidney necrosis virus (ISKNV, Megalocytivirus), nervous necrosis virus (NNV, Betanodavirus), and infestations with ectoparasites during the rainy season in juvenile grouper (Epinephelus spp.) farmed in Aceh, Indonesia. The survey was intended to detect aquatic pathogens present at 10% prevalence with 95% confidence, assuming 100% sensitivity and specificity using a sample size of 30 for each diagnostic test. Eight populations of grouper from seven farms were sampled. Additional targeted sampling was conducted for populations experiencing high mortality. Infection with NNV was detected at all farms with seven of the eight populations being positive. The apparent prevalence for NNV ranged from 0% (95% CI: 0–12) to 73% (95% CI: 54–88). All of the fish tested from the targeted samples (Populations 9 and 10) were positive for NNV and all had vacuolation of the brain and retina consistent with viral nervous necrosis (VNN). Coinfections with ISKNV were detected in five populations, with the highest apparent prevalence being 13% (95% CI: 4–31%). Trichodina sp., Cryptocaryonirritans and Gyrodactylus sp. were detected at three farms, with 66% to 100% of fish being infested. Hybrid grouper sourced from a hatchery were 5.4 and 24.9 times more likely to have a NNV infection and a higher parasite load compared to orange-spotted grouper collected from the wild (p < 0.001). This study found that VNN remains a high-impact disease in grouper nurseries in Aceh, Indonesia.

PCR Detection and Phylogenetic Analysis of Megalocytivirus Isolates in Farmed Giant Sea Perch Lates calcarifer in Southern Taiwan

The Megalocytivirus genus includes three genotypes, red sea bream iridovirus (RSIV), infectious spleen and kidney necrosis virus (ISKNV), and turbot reddish body iridovirus (TRBIV), and has caused mass mortalities in various marine and freshwater fish species in East and Southeast Asia. Of the three genotypes, TRBIV-like megalocytivirus is not included in the World Organization for Animal Health (OIE)-reportable virus list because of its geographic restriction and narrow host range. In 2017, 39 cases of suspected iridovirus infection were isolated from fingerlings of giant sea perch (Lates calcarifer) cultured in southern Taiwan during megalocytivirus epizootics. Polymerase chain reaction (PCR) with different specific primer sets was undertaken to identify the causative agent. Our results revealed that 35 out of the 39 giant sea perch iridovirus (GSPIV) isolates were TRBIV-like megalocytiviruses. To further evaluate the genetic variation, the nucleotide sequences of major capsid protein (MCP) gene (1348 bp) from 12 of the 35 TRBIV-like megalocytivirus isolates were compared to those of other known. High nucleotide sequence identity showed that these 12 TRBIV-like GSPIV isolates are the same species. Phylogenetic analysis based on the MCP gene demonstrated that these 12 isolates belong to the clade II of TRBIV megalocytiviruses, and are distinct from RSIV and ISKNV. In conclusion, the GSPIV isolates belonging to TRBIV clade II megalocytiviruses have been introduced into Taiwan and caused a severe impact on the giant sea perch aquaculture industry.

Characterization and Genomic Analysis of ValSw3-3, a New Siphoviridae Bacteriophage Infecting Vibrio alginolyticus

A novel lytic bacteriophage, ValSw3-3, which efficiently infects pathogenic strains of Vibrio alginolyticus, was isolated from sewage water and characterized by microbiological and in silico genomic analyses. Transmission electron microscopy indicated that ValSw3-3 has the morphology of siphoviruses. This phage can infect four species in the Vibrio genus and has a latent period of 15 min and a burst size of 95 ± 2 PFU/infected bacterium. Genome sequencing results show that ValSw3-3 has a 39,846-bp double-stranded DNA genome with a GC content of 43.1%. The similarity between the genome sequences of ValSw3-3 and those of other phages recorded in the GenBank database was below 50% (42%), suggesting that ValSw3-3 significantly differs from previously reported phages at the DNA level. Multiple genome comparisons and phylogenetic analysis based on the major capsid protein revealed that phage ValSw3-3 is grouped in a clade with five other phages, including Listonella phage phiHSIC (GenBank accession no. NC_006953.1), Vibrio phage P23 (MK097141.1), Vibrio phage pYD8-B (NC_021561.1), Vibrio phage 2E1 (KX507045.1), and Vibrio phage 12G5 (HQ632860.1), and is distinct from all known genera within the Siphoviridae family that have been ratified by the International Committee on Taxonomy of Viruses (ICTV). An in silico proteomic comparison of diverse phages from the Siphoviridae family supported this clustering result and suggested that ValSw3-3, phiHSIC, P23, pYD8-B, 2E1, and 12G5 should be classified as a novel genus cluster of Siphoviridae A subsequent analysis of core genes also revealed the common genes shared within this new cluster. Overall, these results provide a characterization of Vibrio phage ValSw3-3 and support our proposal of a new viral genus within the family Siphoviridae IMPORTANCE Phage therapy has been considered a potential alternative to antibiotic therapy in treating bacterial infections. For controlling the vibriosis-causing pathogen Vibrio alginolyticus, well-documented phage candidates are still lacking. Here, we characterize a novel lytic Vibrio phage, ValSw3-3, based on its morphology, host range and infectivity, growth characteristics, stability under various conditions, and genomic features. Our results show that ValSw3-3 could be a potent candidate for phage therapy to treat V. alginolyticus infections due to its stronger infectivity and better pH and thermal stability than those of previously reported Vibrio phages. Moreover, genome sequence alignments, phylogenetic analysis, in silico proteomic comparison, and core gene analysis all support that this novel phage, ValSw3-3, and five unclassified phages form a clade distant from those of other known genera ratified by the ICTV. Thus, we propose a new viral genus within the Siphoviridae family to accommodate this clade, with ValSw3-3 as a representative member.

Co-Expression Network Analysis of Spleen Transcriptome in Rock Bream (Oplegnathus fasciatus) Naturally Infected with Rock Bream Iridovirus (RBIV)

Rock bream iridovirus (RBIV) is a notorious agent that causes high mortality in aquaculture of rock bream (Oplegnathus fasciatus). Despite severity of this virus, no transcriptomic studies on RBIV-infected rock bream that can provide fundamental information on protective mechanism against the virus have been reported so far. This study aimed to investigate physiological mechanisms between host and RBIV through transcriptomic changes in the spleen based on RNA-seq. Depending on infection intensity and sampling time point, fish were divided into five groups: uninfected healthy fish at week 0 as control (0C), heavy infected fish at week 0 (0H), heavy mixed RBIV and bacterial infected fish at week 0 (0MH), uninfected healthy fish at week 3 (3C), and light infected fish at week 3 (3L). We explored clusters from 35,861 genes with Fragments Per Kilo-base of exon per Million mapped fragments (FPKM) values of 0.01 or more through signed co-expression network analysis using WGCNA package. Nine of 22 modules were highly correlated with viral infection (|gene significance (GS) vs. module membership (MM) |> 0.5, p-value < 0.05). Expression patterns in selected modules were divided into two: heavy infected (0H and 0MH) and control and light-infected groups (0C, 3C, and 3L). In functional analysis, genes in two positive modules (5448 unigenes) were enriched in cell cycle, DNA replication, transcription, and translation, and increased glycolysis activity. Seven negative modules (3517 unigenes) built in this study showed significant decreases in the expression of genes in lymphocyte-mediated immune system, antigen presentation, and platelet activation, whereas there was significant increased expression of endogenous apoptosis-related genes. These changes lead to RBIV proliferation and failure of host defense, and suggests the importance of blood cells such as thrombocytes and B cells in rock bream in RBIV infection. Interestingly, a hub gene, pre-mRNA processing factor 19 (PRPF19) showing high connectivity (kME), and expression of this gene using qRT-PCR was increased in rock bream blood cells shortly after RBIV was added. It might be a potential biomarker for diagnosis and vaccine studies in rock bream against RBIV. This transcriptome approach and our findings provide new insight into the understanding of global rock bream-RBIV interactions including immune and pathogenesis mechanisms.

EPA and DHA can modulate cell death via inhibition of the Fas/tBid-mediated signaling pathway with ISKNV infection in grouper fin cell line (GF-1) cells

Polyunsaturated fatty acids (PUFAs) play important roles in organisms, including the structure and liquidity of cell membranes, anti-oxidation and anti-inflammation. Very little has been done in terms of the effect of PUFAs on cell death, especially on DNA virus. In this study, we demonstrated that the infectious spleen and kidney necrosis virus (ISKNV) can induce host cell death via the apoptotic cell death pathway, which correlated to modulation by PUFAs in grouper fin cell line (GF-1) cells. We screened the PUFAs, including docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), for the ability of different dosages to prevent cell death in GF-1 cells with ISKNV infection. In the results, each 10 μM of DHA and EPA treatment enhanced host cell viability up to 80% at day 5 post-infection. Then, in Terminal deoxynucleotidyl transferase (TdT) dUTP Nick-End Labeling (TUNEL) assay, DHA- and EPA-treated groups reduced TUNEL positive signals 50% in GF-1 cells with ISKNV infection. Then, through studies of the mechanism of cell death, we found that ISKNV can induce both the Bax/caspase-3 and Fas/caspase-8/tBid death signaling pathways in GF-1 cells, especially at day 5 post-infection. Furthermore, we found that DHA and EPA treatment can either prevent caspase-3 activation on 17-kDa form cleavage or Bid cleaved (15-kDa form) for activation by caspase-8, apparently. On the other hand, the anti-apoptotic gene Bcl-2 was upregulated 0.3-fold and 0.15-fold at day 3 and day 5, respectively, compared to ISKNV-infected and DHA-treated cells; that this did not happen in the EPA-treated group showed that different PUFAs trigger different signals. Finally, ISKNV-infected GF-1 cells treated with either DHA or EPA showed a 5-fold difference in viral titer at day 5. Taken together, these results suggest that optimal PUFA treatment can affect cell death signaling through both the intrinsic and extrinsic death pathways, reducing viral expression and viral titer in GF-1 cells. This finding may provide insight in DNA virus infection and control.

Current use and development of fish vaccines in China

In the past decades, the aquaculture industry made great progress in China, which contributes more than 70% yield of the world's farmed fish. Along with the rapid growth of fish production, increased emergence and outbreak of numbers of diseases pose harm to the aquaculture industry and food safety. From the efficient, safe, environmental and ethical aspects, vaccines is definitely the most appropriate and focused method to control different kinds of fish diseases. In China, researchers have done huge works on the fish vaccines, and so far six domestic aquatic vaccine products along with one imported aquatic vaccine have obtained the national veterinary medicine certificate. More critically, some new vaccines have also entered the field experiment stage and showed broad market prospects. In the present review, authors summarize seven aquatic vaccines, including the live vaccine against grass carp hemorrhagic disease, the inactivated vaccine against Aeromonas hydrophila sepsis in fish, the live vaccine against Edwardsiella tarda in turbot, the anti-idiotypic antibody vaccine against Vibrio alginolyticus, V. parahaemolyticus, and E. tarda in Japanese flounder, the cell-cultured inactivated vaccine against grass carp hemorrhagic disease, the inactivated vaccine against fish infectious spleen and kidney necrosis virus (ISKNV), and the genetically engineered live vaccine against V. anguillarum in turbot. Moreover, different delivery routes of fish vaccines are also compared in this review, along with differential fish immune response after vaccination. All these efforts will ultimately benefit the healthy and sustainable development of aquaculture industry in China.

Experimental transmission of infectious spleen and kidney necrosis virus (ISKNV) from freshwater ornamental fish to silver sweep Scorpis lineolata, an Australian marine fish

The Australian native marine fish species, silver sweep Scorpis lineolata, is susceptible to the megalocytivirus Infectious spleen and kidney necrosis virus (strain DGIV-10) obtained from a freshwater ornamental fish, dwarf gourami Trichogaster lalius. This was demonstrated by direct inoculation and through cohabitation. Transmission by cohabitation was also demonstrated from inoculated freshwater Murray cod Maccullochella peelii to euryhaline Australian bass Macquaria novemaculeata and to marine silver sweep. The virus was also transmitted from infected marine silver sweep to euryhaline Australian bass and then to freshwater Murray cod. This study is the first to demonstrate the virulence of a megalocytivirus derived from ornamental fish in an Australian marine species and the first to show a feasible pathway for the exchange of megalocytiviruses between freshwater and marine finfish hosts. These results demonstrate that megalocytiviruses from freshwater ornamental fish have the potential to spread to diverse aquatic environments.

The impact of pooling samples on surveillance sensitivity for the megalocytivirus Infectious spleen and kidney necrosis virus

Movements of large volumes and species varieties make the ornamental fish industry a high-risk pathway for the transfer of aquatic pathogens to new geographical regions and naïve hosts, potentially resulting in emergency disease events. Infectious spleen and kidney necrosis virus (genus Megalocytivirus) is considered exotic to Australia despite documented incursions since 2003. There are current import controls requiring freedom from infection for entry to Australia. The objective was to evaluate the effect of tissue pooling strategies for qPCR testing using a SYBR® assay for freedom from ISKNV at 2% expected prevalence with 95% confidence. Tissue homogenates from apparently healthy imported ornamental fish were tested as individuals and in pools of 5 and 10. Analytical sensitivity of the qPCR assay was reduced by two orders of magnitude when the nucleic acid extraction process was accounted for by spiking the plasmid in fish tissues and compared with molecular grade water. Diagnostic sensitivity of the assay was substantially reduced when testing tissues in pools compared with individual testing. For Population 1 (66% positive for ISKNV with moderate viral loads), surveillance sensitivity was only achieved using individual testing. For Population 2 (100% positive ISKNV with high viral loads), surveillance sensitivity was achieved using 260 fish in pools of 10 for a total of 26 tests or 200 fish in pools of 5 for 40 tests. Surveillance sensitivity could be maximized even when there was a reduction in pooled diagnostic sensitivity compared with diagnostic sensitivity for individual fish by increasing the sample size. Pooled sensitivity was influenced by the prevalence and variable virus load among fish with subclinical infections. Pooled testing is highly effective when the prevalence is >10% which should be informed by prior knowledge or pooling can be used for a screening test to rapidly identify populations with high prevalence.

Susceptibility of Exopalaemon carinicauda to the Infection with Shrimp Hemocyte Iridescent Virus (SHIV 20141215), a Strain of Decapod Iridescent Virus 1 (DIV1)

In this study, ridgetail white prawns-Exopalaemon carinicauda-were infected per os (PO) with debris of Penaeus vannamei infected with shrimp hemocyte iridescent virus (SHIV 20141215), a strain of decapod iridescent virus 1 (DIV1), and via intramuscular injection (IM with raw extracts of SHIV 20141215. The infected E. carinicauda showed obvious clinical symptoms, including weakness, empty gut and stomach, pale hepatopancreas, and partial death with mean cumulative mortalities of 42.5% and 70.8% by nonlinear regression, respectively. Results of TaqMan probe-based real-time quantitative PCR showed that the moribund and surviving individuals with clinical signs of infected E. carinicauda were DIV1-positive. Histological examination showed that there were darkly eosinophilic and cytoplasmic inclusions, of which some were surrounded with or contained tiny basophilic staining, and pyknosis in hemocytes in hepatopancreatic sinus, hematopoietic cells, cuticular epithelium, etc. On the slides of in situ DIG-labeling-loop-mediated DNA amplification (ISDL), positive signals were observed in hematopoietic tissue, stomach, cuticular epithelium, and hepatopancreatic sinus of infected prawns from both PO and IM groups. Transmission electron microscopy (TEM) of ultrathin sections showed that icosahedral DIV1 particles existed in hepatopancreatic sinus and gills of the infected E. carinicauda from the PO group. The viral particles were also observed in hepatopancreatic sinus, gills, pereiopods, muscles, and uropods of the infected E. carinicauda from the IM group. The assembled virions, which mostly distributed along the edge of the cytoplasmic virogenic stromata near cellular membrane of infected cells, were enveloped and approximately 150 nm in diameter. The results of molecular tests, histopathological examination, ISDL, and TEM confirmed that E. carinicauda is a susceptible host of DIV1. This study also indicated that E. carinicauda showed some degree of tolerance to the infection with DIV1 per os challenge mimicking natural pathway.

Australian bass Macquaria novemaculeata susceptibility to experimental megalocytivirus infection and utility as a model disease vector

Megalocytiviruses, particularly red seabream iridovirus, infect a broad range of fish including both freshwater and marine species. Although a limited number of infectious spleen and kidney necrosis virus (ISKNV) strains have been reported in association with mortality events in marine aquaculture species, the potential host range for ISKNV strains, particularly of those that have been detected in ornamental fish, has not been well characterised. There have also been few reports on the susceptibility of euryhaline fish species that could potentially transmit megalocytiviruses between freshwater and marine environments. We found that the euryhaline Australian native percichthyid fish, Australian bass Macquaria novemaculeata, is susceptible experimentally to ISKNV (strain DGIV-10), obtained from a freshwater ornamental fish, dwarf gourami Trichogaster lalius. Australian bass developed clinical disease following direct inoculation and also following cohabitation with infected fish, and were able to transmit DGIV-10 to naïve Murray cod Maccullochella peelii. This study demonstrated the potential for a euryhaline species to become infected with, and transmit, the megalocytivirus ISKNV between fish populations.

The Megalocytivirus RBIV Induces Apoptosis and MHC Class I Presentation in Rock Bream (Oplegnathus fasciatus) Red Blood Cells

Rock bream iridovirus (RBIV) causes severe mass mortality in Korean rock bream (Oplegnathus fasciatus) populations. To date, immune defense mechanisms of rock bream against RBIV are unclear. While red blood cells (RBCs) are known to be involved in the immune response against viral infections, the participation of rock bream RBCs in the immune response against RBIV has not been studied yet. In this study, we examined induction of the immune response in rock bream RBCs after RBIV infection. Each fish was injected with RBIV, and virus copy number in RBCs gradually increased from 4 days post-infection (dpi), peaking at 10 dpi. A total of 318 proteins were significantly regulated in RBCs from RBIV-infected individuals, 183 proteins were upregulated and 135 proteins were downregulated. Differentially upregulated proteins included those involved in cellular amino acid metabolic processes, cellular detoxification, snRNP assembly, and the spliceosome. Remarkably, the MHC class I-related protein pathway was upregulated during RBIV infection. Simultaneously, the regulation of apoptosis-related proteins, including caspase-6 (CASP6), caspase-9 (CASP9), Fas cell surface death receptor (FAS), desmoplakin (DSP), and p21 (RAC1)-activated kinase 2 (PAK2) changed with RBIV infection. Interestingly, the expression of genes within the ISG15 antiviral mechanism-related pathway, including filamin B (FLNB), interferon regulatory factor 3 (IRF3), nucleoporin 35 (NUP35), tripartite motif-containing 25 (TRIM25), and karyopherin subunit alpha 3 (KPNA3) were downregulated in RBCs from RBIV-infected individuals. Overall, these findings contribute to the understanding of RBIV pathogenesis and host interaction.

Global profiling of megalocytivirus-induced proteins in tongue sole (Cynoglossus semilaevis) spleen identifies cellular processes essential to viral infection

Megalocytivirus is a DNA virus with a broad host range among farmed fish including tongue sole (Cynoglossus semilaevis). In this study, label-free proteomics was performed to examine protein expression in tongue sole spleen induced by megalocytivirus at 8 and 12 days post infection (dpi). Compared to uninfected control fish, virus-infected fish displayed 315 up-regulated proteins and 111 down-regulated proteins at 8 dpi, and 48 up-regulated proteins and 43 down-regulated proteins at 12 dpi. The expressions of five differentially expressed proteins were confirmed by Western blot. The differentially expressed proteins were enriched in the pathways and processes associated with innate immune response and viral infection. Interference with the expression of two up-regulated proteins of the ubiquitin proteasome system (UPS), i.e. proteasome assembly chaperone 2 and proteasome maturation protein, significantly reduced viral propagation in fish, whereas overexpression of these two proteins significantly enhanced viral propagation. Consistently, inhibition of the functioning of proteasome significantly impaired viral replication in vivo. This study provided the first global protein profile responsive to megalocytivirus in tongue sole, and revealed an essential role of UPS in viral infection.

Mortality from scale drop disease in farmed Lates calcarifer in Southeast Asia

In Southeast Asia, a new disease called scale drop disease (SDD) caused by a novel Megalocytivirus (SDDV) has emerged in farmed Asian sea bass (Lates calcarifer) in Singapore, Malaysia and Indonesia. We received samples from an Eastern Thai province that also showed gross signs of SDD (loss of scales). Clinical samples of 0.2-1.1 kg L. calcarifer collected between 2016 and 2018 were examined for evidence of SDDV infection. Histopathology was similar to that in the first report of SDDV from Singapore including necrosis, inflammation and nuclear pyknosis and karyorrhexis in the multiple organs. Intracytoplasmic inclusion bodies were also observed in the muscle tissue. In a density-gradient fraction from muscle extracts, TEM revealed enveloped, hexagonal megalocytiviral-like particles (~100-180 nm). By PCR using primers derived from the Singaporean SDDV genome sequence, four different genes were amplified and sequenced from the Thai isolate revealing 98.7%-99.9% identity between the two isolates. Since viral inclusions were rarely observed, clinical signs and histopathology could not be used to easily distinguish between SDD caused by bacteria or SDDV. We therefore recommend that PCR screening be used to monitor broodstock, fry and grow-out fish to estimate the current impact of SDDV in Southeast Asia and to prevent its spread.

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.

Development, characterization and virus susceptibility of a continuous cell line from the caudal fin of marbled eel (Anguilla marmorata)

A continuous cell line consisting mostly of epithelioid cells was established from the caudal fin of marbled eels (Anguilla marmorata) and designated as marbled eel caudal fin (MECF)-1. The cells multiplied well in Leibovitz's L-15 medium containing 2% to 15% foetal bovine serum at temperatures of 20°C to 35°C and were subcultured for >90 passages during a 5-year period from 2012 to 2017. Transcripts of ictacalcin, keratin 13, cd146, nestin, ncam1 and myod1 were demonstrated in the cells using reverse transcription polymerase chain reaction. The results indicated that MECF-1 was composed of epidermal and mesenchyme stem and progenitor cells including myoblasts. MECF-1 was susceptible to Japanese eel herpesvirus HVA980811, marbled eel polyoma-like virus (MEPyV), aquabirnavirus MEIPNV1310 and aquareovirus CSV. By contrast, MECF-1 was noted refractory to megalocytiviruses RSIV-Ku and GSIV-K1 infection. Moreover, the cells were resistant to betanodavirus infection. In conclusion, MECF-1 derived from marbled eel is suitable for studies on anguillid viruses and interaction with host cells.

Exploitation of receptor tyrosine kinases by viral-encoded growth factors

Receptor tyrosine kinases (RTKs) are essential components of cell communication pathways utilized from the embryonic to adult stages of life. These transmembrane receptors bind polypeptide ligands, such as growth factors, inducing signalling cascades that control cellular processes such as proliferation, survival, differentiation, motility and inflammation. Many viruses have acquired homologs of growth factors encoded by the hosts that they infect. Production of growth factors during infection allows viruses to exploit RTKs for entry and replication in cells, as well as for host and environmental dissemination. This review describes the genetic diversity amongst virus-derived growth factors and the mechanisms by which RTK exploitation enhances virus survival, then highlights how viral ligands can be used to further understanding of RTK signalling and function during embryogenesis, homeostasis and disease scenarios.

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

BACKGROUND

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

MAIN TEXT

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

CONCLUSIONS

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

First description of a natural infection with spleen and kidney necrosis virus in zebrafish

Zebrafish has become a popular research model in the last years, and several diseases affecting zebrafish research facilities have been reported. However, only one case of naturally occurring viral infections was described for this species. In 2015, infectious spleen and kidney necrosis virus (ISKNV) was detected in zebrafish from a research facility in Spain. Affected fish showed lethargy, loss of appetite, abnormal swimming, distention of the coelomic cavity and, in the most severe cases, respiratory distress, pale gills and petechial haemorrhages at the base of fins. Cytomegaly was the most relevant histopathological finding in organs and tissues, sometimes associated to degenerative and necrotic changes. ISKNV belongs to the relatively newly defined genus Megalocytivirus, family Iridoviridae, comprising large, icosahedral cytoplasmic DNA viruses. This is the first case of naturally occurring Megalocytivirus infection in zebrafish research facilities, associated with morbidity. The virus has been identified based on both pathologic and genetic evidence, to better understand the pathogenesis of the infection in zebrafish and the phylogenetic relationship with other iridoviruses. Given the ability of megalocytiviruses to cross-species boundaries, it seems necessary to implement stringent biosecurity practices as these infections may invalidate experimental data and have major impact on laboratory and cultured fish.

A natural infection by the red sea bream iridovirus-type Megalocytivirus in the golden mandarin fish Siniperca scherzeri

An outbreak of a Megalocytivirus infection was found in the golden mandarin fish Siniperca scherzeri during September and October 2016, in Korea. Phylogeny and genetic diversity based on the major capsid protein (MCP) and adenosine triphosphatase (ATPase) genes showed a new strain. Designated as GMIV, this strain derived from the golden mandarin fish was suggested to belong to the red sea bream iridovirus (RSIV)-subgroup I. Additionally, this train clustered with the ehime-1 strain from red sea bream Pagrus major in Japan and was distinguished from circulating isolates (RSIV-type subgroup II and turbot reddish body iridovirus [TRBIV] type) in Korea. The infection level, evaluated by qPCR, ranged from 8.18 × 102 to 7.95 × 106  copies/mg of tissue individually, suggesting that the infected fish were in the disease-transmitting stage. The diseased fish showed degenerative changes associated with cytomegaly in the spleen as general sign of Megalocytivirus infection. The results confirm that the RSIV-type Megalocytivirus might have crossed the environmental and species barriers to cause widespread infection in freshwater fish.

Characterization and virus susceptibility of a continuous cell line derived from the brain of Aequidens rivulatus (Günther)

Cell cultures derived from the brain tissues of Aequidens rivulatus (Günther) have been characterized previously. In this study, a continuous cell line ARB8 was further established, and its growth characteristics, transcription and susceptibility to fish viruses-including chum salmon reovirus (CSV), marbled eel infectious pancreative necrosis virus (MEIPNV), grouper nervous necrosis virus (GNNV), giant seaperch iridovirus (GSIV), red seabream iridovirus (RSIV), koi herpesvirus (KHV), herpesvirus anguilla (HVA) and marbled eel polyoma-like virus (MEPyV)-were examined. ARB8 cells that showed epithelioid morphology and were passaged >80 times grew well at temperatures ranging from 25°C to 30°C in L-15 medium containing 5%-15% foetal bovine serum. The cells constitutively transcribed connexion 43, glutamine synthetase, nestin and nkx6-2, which are markers for neural progenitor cells. The cells were highly susceptible to CSV, MEIPNV, GSIV and RSIV and showed the typical cytopathic effect (CPE). However, the cells were resistant to GNNV, KHV, HVA and MEPyV because no significant CPE was noted after infection. Optimal temperatures for virus production ranged from 25°C to 30°C. The results revealed that the neural progenitor cell line ARB8 can potentially serve as a useful tool for investigating fish viruses and isolating new viruses in ornamental cichlid fishes.

Composition and transcription of all interferon regulatory factors (IRFs), IRF1‒11 in a perciform fish, the mandarin fish, Siniperca chuatsi

Interferon regulatory factors (IRFs) are a family of mediators in various biological processes including immune modulation of interferon (IFN) and proinflammatory cytokine expression. However, the data on the complete composition of IRFs is rather limited in teleost fish. In the present study, all IRF members, i.e. IRF1‒11 with two IRF4, IRF4a and IRF4b have been characterised in an aquaculture species of fish, the mandarin fish, Siniperca chuatsi, in addition to the previous report of IRF1, IRF2, IRF3 and IRF7 from the fish. These IRFs are constitutively expressed in various organs/tissues of the fish, and their expression can be induced following the stimulation of polyinosinic:polycytidylic acid (poly(I:C)) and the infection of infectious spleen and kidney necrosis virus (ISKNV), a viral pathogen of mandarin fish in aquaculture. The ISKNV infection induced the significant increase in the expression of some IRF genes, i.e. IRF2, IRF4a, IRF7, IRF9, IRF10 at 24 or 36 h post-infection (hpi) in spleen and head-kidney, and the significant increase of some other IRF genes, e.g. IRF1, IRF3, IRF4b, IRF5, IRF6, IRF8 at later stage of infection from 72, or 96, or even 120 hpi, which may imply the inhibitory effect of ISKNV on fish immune response. It is considered that the present study provides the first detailed analysis on all IRF members in an aquaculture species of fish, and can be served as the base for further investigation on the role of IRFs in teleost fish.

Invertebrate Iridoviruses: A Glance over the Last Decade

Members of the family Iridoviridae (iridovirids) are large dsDNA viruses that infect both invertebrate and vertebrate ectotherms and whose symptoms range in severity from minor reductions in host fitness to systemic disease and large-scale mortality. Several characteristics have been useful for classifying iridoviruses; however, novel strains are continuously being discovered and, in many cases, reliable classification has been challenging. Further impeding classification, invertebrate iridoviruses (IIVs) can occasionally infect vertebrates; thus, host range is often not a useful criterion for classification. In this review, we discuss the current classification of iridovirids, focusing on genomic and structural features that distinguish vertebrate and invertebrate iridovirids and viral factors linked to host interactions in IIV6 (Invertebrate iridescent virus 6). In addition, we show for the first time how complete genome sequences of viral isolates can be leveraged to improve classification of new iridovirid isolates and resolve ambiguous relations. Improved classification of the iridoviruses may facilitate the identification of genus-specific virulence factors linked with diverse host phenotypes and host interactions.

Comparative evaluation of MCP gene in worldwide strains of Megalocytivirus (Iridoviridae family) for early diagnostic marker

Members of the Iridoviridae family have been considered as aetiological agents of iridovirus diseases, causing fish mortalities and economic losses all over the world. Virus identification based on candidate gene sequencing is faster, more accurate and more reliable than other traditional phenotype methodologies. Iridoviridae viruses are covered by a protein shell (capsid) encoded by the important candidate gene, major capsid protein (MCP). In this study, we investigated the potential of the MCP gene for use in the diagnosis and identification of infections caused Megalocytivirus of the Iridoviridae family. We selected data of 66 Iridoviridae family isolates (53 strains of Megalocytivirus, eight strains of iridoviruses and five strains of Ranavirus) infecting various species of fish distributed all over the world. A total of 53 strains of Megalocytivirus were used for designing the complete primer sets for identifying the most hypervariable region of the MCP gene. Further, our in silico analysis of 102 sequences of related and unrelated viruses reconfirms that primer sets could identify strains more specifically and offers a useful and fast alternative for routine clinical laboratory testing. Our findings suggest that phenotype observation along with diagnosis using universal primer sets can help detect infection or carriers at an early stage.

Complete genome sequence and phylogenetic analysis of megalocytivirus RSIV-Ku: A natural recombination infectious spleen and kidney necrosis virus

Megalocytiviruses are classified into three genotypes, infectious spleen and kidney necrosis virus (ISKNV), red seabream virus (RSIV), and turbo reddish body iridovirus (TRBIV), based on the major capsid protein and ATPase genes. However, only a few complete genome sequences have been obtained. This paper reports the complete genome sequence and phylogenetic analysis of an RSIV-Ku strain megalocytivirus. The genome sequence comprises 111,154 bp, has 132 putative open reading frames, and is homologous mostly to ISKNV, except for the sequence in the region 58981-66830, which is more closely related to that of the RSIV genotype. The results imply that RSIV-Ku is actually a natural recombinant virus.

Innate immune responses against rock bream iridovirus (RBIV) infection in rock bream (Oplegnathus fasciatus) following poly (I:C) administration

Poly (I:C) showed promise as an immunoprotective agents in rock bream against rock bream iridovirus (RBIV) infection. In this study, we evaluated the time-dependent virus replication pattern and antiviral immune responses in RBIV-infected rock bream with and without poly (I:C) administration. In the poly (I:C)+virus-injected group, virus copy numbers were more than 18.9-, 24.0- and 479.2-fold lower than in the virus only injected group at 4 (4.73 × 10⁴ and 8.95 × 10⁵/μl, respectively), 7 (3.67 × 10⁵ and 8.81 × 10⁶/μl, respectively) and 10 days post infection (dpi) (1.26 × 10⁵ and 6.02 × 10⁷/μl, respectively). Moreover, significantly high expression levels of TLR3 (8.6- and 7.7-fold, at 4 and 7 dpi, respectively) and IL1β (3.6-fold at 2 dpi) were observed in the poly (I:C)+virus-injected group, but the expression levels were not significantly in the virus-injected group. However, IL8 and TNFα expression levels showed no statistical significance in both groups. Mx, ISG15 and PKR were significantly highly expressed from 4 to 10 dpi in the virus-injected group. Nevertheless, in the poly (I:C)+virus-injected group, Mx and ISG15 expression were significantly expressed from 2 dpi. In summary, poly (I:C) administration in rock bream induces TLR3, IL1β, Mx and ISG15-mediated immune responses, which could be a critical factor for inhibition of virus replication.

Gene expression regulation of the TLR9 and MyD88-dependent pathways in rock bream against rock bream iridovirus (RBIV) infection

Rock bream iridovirus (RBIV), which is a member of the Megalocytivirus genus, causes severe mass mortalities in rock bream in Korea. To date, the innate immune defense mechanisms of rock bream against RBIV is unclear. In this study, we assessed the expression levels of genes related to TLR9 and MyD88-dependent pathways in RBIV-infected rock bream in high, low or no mortality conditions. In the high mortality group (100% mortality at 15 days post infection (dpi)), high levels of TLR9 and MyD88 expressions (6.4- and 2.4-fold, respectively) were observed at 8 d and then reduced (0.6- and 0.1-fold, respectively) with heavy viral loads at 10 dpi (2.21 × 10⁷/μl). Moreover, TRAF6, IRF5, IL1β, IL8, IL12 and TNFα expression levels showed no statistical significance until 10 dpi. Conversely, in the low mortality group (28% expected mortality at 35 dpi), TLR9, MyD88 and TRAF6 expression levels were significantly higher than those in the control group at several sampling points until 30 dpi. Higher levels of IRF5, IL1β, IL8, IL12 and TNFα expression were also observed, however, these were not significantly different from those of the control group. In the no mortality group (0% mortality at 40 dpi), significantly higher levels of MyD88 (2 d, 4 d and 40 dpi), TRAF6 (2 dpi), IL1β (4 dpi) and IL8 (2 d and 4 dpi) expression were observed. In summary, RBIV-infected rock bream induces innate immune response, which could be a major contributing factor to effective fish control over viral transcription. MyD88, TRAF6, IL1β and IL8-related immune responses were activated in fish survivor condition (low or no mortality group). This is a critical factor for RBIV disease recovery; however, these immune responses did not efficiently respond in fish dead condition (high mortality group).

Rock bream iridovirus (RBIV) replication in rock bream (Oplegnathus fasciatus) exposed for different time periods to susceptible water temperatures

Rock bream iridovirus (RBIV) is a member of the Megalocytivirus genus that causes severe mortality to rock bream. Water temperature is known to affect the immune system and susceptibility of fish to RBIV infection. In this study, we evaluated the time dependent virus replication pattern and time required to completely eliminate virus from the rock bream body against RBIV infection at different water temperature conditions. The rock bream was exposed to the virus and held at 7 (group A1), 4 (group A2) and 2 days (group A3) at 23 °C before the water temperature was reduced to 17 °C. A total of 28% mortality was observed 24-35 days post infection (dpi) in only the 7 day exposure group at 23 °C. In all 23 °C exposure groups, virus replication peaked at 20 to 22 dpi (10⁶-10⁷/μl). In recovery stages (30-100 dpi), the virus copy number was gradually reduced, from 10⁶ to 10¹ with faster decreases in the shorter exposure period group at 23 °C. When the water temperature was increased in surviving fish from 17 to 26 °C at 70 dpi, they did not show any mortality or signs of disease and had low virus copy numbers (below 10²/μl). Thus, fish need at least 50 days from peaked RBIV levels (approximately 20-25 dpi) to inhibit the virus. This indicates that maintaining the fish at low water temperature (17 °C) for 70 days is sufficient to eradicate RBIV from fish body. Thus, RBIV could be eliminated slowly from the fish body and the virus may be completely eliminated under the threshold of causing mortality.

Outbreaks of ulcerative disease associated with ranavirus infection in barcoo grunter, Scortum barcoo (McCulloch & Waite)

In 2013, an outbreak of ulcerative disease associated with ranavirus infection occurred in barcoo grunter, Scortum barcoo (McCulloch & Waite), farms in Thailand. Affected fish exhibited extensive haemorrhage and ulceration on skin and muscle. Microscopically, the widespread haemorrhagic ulceration and necrosis were noted in gill, spleen and kidney with the presence of intracytoplasmic eosinophilic inclusion bodies. When healthy barcoo grunter were experimentally challenged via intraperitoneal and oral modes with homogenized tissue of naturally infected fish, gross and microscopic lesions occurred with a cumulative mortality of 70-90%. Both naturally and experimentally infected fish yielded positive results to the ranavirus-specific PCR. The full-length nucleotide sequences of major capsid protein gene of ranaviral isolates were similar to largemouth bass virus (LMBV) and identical to largemouth bass ulcerative syndrome virus (LBUSV), previously reported in farmed largemouth bass (Micropterus salmoides L.), which also produced lethal ulcerative skin lesions. To the best of our knowledge, this is the first report of a LMBV-like infection associated with skin lesions in barcoo grunter, adding to the known examples of ranavirus infection associated with skin ulceration in fish.

CpG ODN 1668 induce innate and adaptive immune responses in rock bream (Oplegnathus fasciatus) against rock bream iridovirus (RBIV) infection

Rock bream iridovirus (RBIV) causes severe mass mortalities in rock bream in Korea. CpG ODN 1668 showed promise as immunoprotective agents against RBIV infection in rock bream. In this study, we assessed innate/adaptive-related gene expression patterns in RBIV-infected rock bream with and without CpG ODN 1668 administration to determine important immune defense related factors that may affect fish survival. In the CpG ODN 1668+virus-injected group, virus copies were more than 7.4- to 790591-fold lower than in the virus-injected group at 4 d (8.79 × 10⁴ and 6.58 × 10⁵/μl, respectively), 7 d (5.30 × 10² and 2.29 × 10⁷/μl, respectively) and 10 dpi (7.79 × 10¹ and 6.16 × 10⁷/μl, respectively). Furthermore, in the CpG ODN 1668+virus-injected group, significantly higher levels of MyD88 (6 h, 1 d, 4 d and 7 dpi), IL1β (1 d, 2 d and 7 dpi) and perforin/granzyme (1 dpi) expression were observed, whereas these genes were not significantly expressed in the virus-injected group at that time points. Mx, ISG15 and PKR were significantly highly expressed at 4 d and 7 dpi and reduced when low viral loads at 10 dpi in the CpG ODN 1668+virus-injected group. Conversely, in the virus-injected group, Mx, ISG15 and PKR expression were significantly higher than the control group until 10 dpi. However, MHC class I, CD8, Fas, Fas ligand and caspases (3, 8 and 9) expression levels showed no statistically significant differences between virus- and CpG ODN 1668+virus-injected group. In summary, CpG ODN 1668 administration in fish induces innate immune response or cell death pathway, which could be a major contributing factor to effective fish control over viral transcription on 4 d to 10 dpi. Expression of MyD88, IL1β, perforin and granzyme-related immune gene response is critical factor for inhibition of RBIV replication.

Characterization of a new member of Iridoviridae, Shrimp hemocyte iridescent virus (SHIV), found in white leg shrimp (Litopenaeus vannamei)

A newly discovered iridescent virus that causes severe disease and high mortality in farmed Litopenaeus vannamei in Zhejiang, China, has been verified and temporarily specified as shrimp hemocyte iridescent virus (SHIV). Histopathological examination revealed basophilic inclusions and pyknosis in hematopoietic tissue and hemocytes in gills, hepatopancreas, periopods and muscle. Using viral metagenomics sequencing, we obtained partial sequences annotated as potential iridoviridae. Phylogenetic analyses using amino acid sequences of major capsid protein (MCP) and ATPase revealed that it is a new iridescent virus but does not belong to the five known genera of Iridoviridae. Transmission electron microscopy showed that the virus exhibited a typical icosahedral structure with a mean diameter of 158.6 ± 12.5 nm (n = 30)(v-v) and 143.6 ± 10.8 nm (n = 30)(f-f), and an 85.8 ± 6.0 nm (n = 30) nucleoid. Challenge tests of L. vannamei via intermuscular injection, per os and reverse gavage all exhibited 100% cumulative mortality rates. The in situ hybridization showed that hemopoietic tissue, gills, and hepatopancreatic sinus were the positively reacting tissues. Additionally, a specific nested PCR assay was developed. PCR results revealed that L. vannamei, Fenneropenaeus chinensis, and Macrobrachium rosenbergii were SHIV-positive, indicating a new threat existing in the shrimp farming industry in China.

Infectious spleen and kidney necrosis disease (ISKND) outbreaks in farmed barramundi (Lates calcarifer) in Vietnam

Emergence of a disease with clinical signs resembling megalocytivirus infection seriously affected large-scale barramundi farms in Vietnam in 2012-2014 with estimated losses reaching $435,810 per year. An oil-based, inactivated vaccine against red sea bream iridovirus (RSIV) was applied in one farm for disease prevention without analysis of the causative agent, and the farmer reported inadequate protection. Here we describe histological and molecular analysis of the diseased fish. PCR targeting the major capsid protein (MCP) of megalocytiviruses yielded an amplicon with high sequence identity to infectious spleen and kidney necrosis virus (ISKNV) genotype II previously reported from other marine fish but not barramundi. Detection of the virus was confirmed by positive in situ hybridization results with fish tissue lesions of the kidney, liver, pancreas, and brain of the PCR-positive samples. Based on the complete sequence of the MCP gene, the isolate showed 95.2% nucleotide sequence identity and 98.7% amino acid sequence identity (6 residue differences) with the MCP of RSIV. Prediction of antigenic determinants for MCP antigens indicated that the 6 residue differences would result in a significant difference in antigenicity of the two proteins. This was confirmed by automated homology modeling in which structure superimpositioning revealed several unique epitopes in the barramundi isolate. This probably accounted for the low efficiency of the RSIV vaccine when tested by the farmer.

Transcriptome analysis reveals temperature-regulated antiviral response in turbot Scophthalmus maximus

Megalocytivirus is a severe pathogen to turbot (Scophthalmus maximus), a popular aquaculture species in many countries. In this study, we investigated the effect of temperature on the antiviral response of turbot at transcriptome level. We found that when turbot were infected with megalocytivirus RBIV-C1 at low temperatures (14 °C, 16 °C, and 18 °C), viral replication was undetectable or moderate and no fish mortality occurred; in contrast, when turbot were infected with RBIV-C1 at high temperatures (20 °C, 22 °C, and 24 °C), viral replication was robust and 100% host mortality was observed. During the course of viral infection, downward temperature shift curbed viral replication and augmented host survival, whereas upward temperature shift promoted viral replication and reduced host survival. Comparative transcriptome analyses were conducted to examine the whole-genome transcription of turbot infected with RBIV-C1 at 16 °C and 22 °C for 4 days (samples S16-4d and S22-4d, respectively) and 8 days (samples S16-8d and S22-8d, respectively). The results showed that compared to S22-4d and S22-8d, 1600 and 5927 upregulated unigenes of various functional categories were identified in S16-4d and S16-8d, respectively. Of these genes, 22 were immune-related, most of which were detected in S16-8d and exhibited more genetic subtypes in S16-8d than in S16-4d. In addition, upregulated genes associated with cell junctions and cell membrane were also identified. These results indicate that temperature had a profound effect on the global transcription of turbot, which consequently affects the immune as well as physical resistance of the fish against viral infection.

Protective immunity against rock bream iridovirus (RBIV) infection and TLR3-mediated type I interferon signaling pathway in rock bream (Oplegnathus fasciatus) following poly (I:C) administration

In this study, we evaluated the potential of poly (I:C) to induce antiviral status for protecting rock bream from RBIV infection. Rock bream injected with poly (I:C) at 2 days before infection (1.1 × 10⁴) at 20 °C had significantly higher protection with RPS 13.4% and 33.4% at 100 and 200 μg/fish, respectively, through 100 days post infection (dpi). The addition of boost immunization with poly (I:C) at before/post infection at 20 °C clearly enhanced the level of protection showing 33.4% and 60.0% at 100 and 200 μg/fish, respectively. To investigate the development of a protective immune response, rock bream were re-infected with RBIV (1.1 × 10⁷) at 200 dpi. While 100% of the previously unexposed fish died, 100% of the previously infected fish survived. Poly (I:C) induced TLR3 and Mx responses were observed at several sampling time points in the spleen, kidney and blood. Moreover, significantly high expression levels of IRF3 (2.9- and 3.1-fold at 1 d and 2 days post administration (dpa), respectively), ISG15 and PKR expression (5.4- and 10.2-fold at 2 dpa, respectively) were observed in the blood, but the expression levels were low in the spleen and kidney after poly (I:C) administration. Our results showed the induction of antiviral immune responses and indicate the possibility of developing long term preventive measures against RBIV using poly (I:C).

Protective immunity against Megalocytivirus infection in rock bream (Oplegnathus fasciatus) following CpG ODN administration

Rock bream iridovirus (RBIV) disease in rock bream (Oplegnathus fasciatus) remains an unsolved problem in Korea aquaculture farms. CpG ODNs are known as immunostimulant, can improve the innate immune system of fish providing resistance to diseases. In this study, we evaluated the potential of CpG ODNs to induce anti-viral status protecting rock bream from different RBIV infection conditions. We found that, when administered into rock bream, CpG ODN 1668 induces better antiviral immune responses compared to other 5 CpG ODNs (2216, 1826, 2133, 2395 and 1720). All CpG ODN 1668 administered fish (1/5µg) at 2days before infection (1.1×10⁷) held at 26°C died even though mortality was delayed from 8days (1µg) and 4days (5µg). Similarly, CpG ODN 1668 administered (5µg) at 2days before infection (1.2×10⁶) held at 23/20°C had 100% mortality; the mortality was delayed from 9days (23°C) and 11days (20°C). Moreover, when CpG ODN 1668 administered (1/5/10µg) at 2/4/7days before infection or virus concentration was decreased to 1.1×10⁴ and held at 20°C had mortality rates of 20/60/30% (2days), 30/40/60% (4days) and 60/60/20% (7days), respectively, for the respective administration dose, through 100 dpi. To investigate the development of a protective immune response, survivors were re-infected with RBIV (1.1×10⁷) at 100 and 400 dpi, respectively. While 100% of the previously unexposed fish died, 100% of the previously infected fish survived. The high survival rate of fish following re-challenge with RBIV indicates that protective immunity was established in the surviving rock bream. Our results showed the possibility of developing preventive measures against RBIV using CpG ODN 1668 by reducing RBIV replication speed (i.e. water temperature of 20°C and infection dose of 1.1×10⁴).

Susceptibility of a number of Australian freshwater fishes to dwarf gourami iridovirus (Infectious spleen and kidney necrosis virus)

Megalocytiviruses cause high mortality diseases that have seriously impacted aquaculture, with the most frequent outbreaks occurring in East and South-East Asia. The international trade of juvenile fish for food and ornamental aquaculture has aided the spread of these viruses, which have spread to Europe and Australia and other regions. Australian freshwater fishes were examined for susceptibility to infection with the exotic megalocytivirus, dwarf gourami iridovirus (DGIV), which belongs to a group with the type species, Infectious spleen and kidney necrosis virus (ISKNV). Fish were held at 23 ± 1 °C and challenged by intraperitoneal (IP) injection or by cohabitation with Murray cod, Maccullochella peelii (Mitchell) infected with DGIV. A species was deemed to be susceptible to DGIV based on evidence of viral replication, as determined by qPCR, and megalocytic inclusion bodies observed histologically. Horizontal transmission occurred between infected Murray cod and golden perch, Macquaria ambigua (Richardson), Macquarie perch, Macquaria australasica (Cuvier) and Murray cod. This indicated that DGIV shed from infected fish held at 23 °C can survive in fresh water and subsequently infect these naïve fish. Further, DGIV administered IP was highly pathogenic to golden perch, Macquarie perch and Murray cod. Compared to these species, the susceptibility of southern pygmy perch, Nannoperca australis (Gunther) was lower. Freshwater catfish (dewfish), Tandanus tandanus (Mitchell), were not susceptible under the experimental conditions based on the absence of clinical disease, mortality and virus replication. This study showed the potential risks associated with naïve and DGIV-infected fish sharing a common water source.

Detection of infectious spleen and kidney necrosis virus (ISKNV) and turbot reddish body iridovirus (TRBIV) from archival ornamental fish samples

Although infections caused by megalocytiviruses have been reported from a wide range of finfish species for several decades, molecular characterisation of the viruses involved has been undertaken only on more recent cases. Sequence analysis of the major capsid protein and adenosine triphosphatase genes is reported here from formalin-fixed, paraffin-embedded material from 2 archival ornamental fish cases from 1986 and 1988 in conjunction with data for a range of genes from fresh frozen tissues from 5 cases obtained from 1991 through to 2010. Turbot reddish body iridovirus (TRBIV) genotype megalocytiviruses, previously not documented in ornamental fish, were detected in samples from 1986, 1988 and 1991. In contrast, megalocytiviruses from 1996 onwards, including those characterised from 2002, 2006 and 2010 in this study, were almost indistinguishable from infectious spleen and kidney necrosis virus (ISKNV). Three of the species infected with TRBIV-like megalocytiviruses from 1986 to 1991, viz. dwarf gourami Trichogaster lalius (formerly Colisa lalia), freshwater angelfish Pterophyllum scalare and oscar Astronotus ocellatus, were infected with ISKNV genotype megalocytiviruses from 2002 to 2010. The detection of a TRBIV genotype isolate in ornamental fish from 1986 represents the index case, confirmed by molecular sequence data, for the genus Megalocytivirus.

GSIV serine/threonine kinase can induce apoptotic cell death via p53 and pro-apoptotic gene Bax upregulation in fish cells

Previous studies have shown that GSIV induces apoptotic cell death through upregulation of the pro-apoptotic genes Bax and Bak in Grouper fin cells (GF-1 cells). However, the role of viral genome-encoded protein(s) in this death process remains unknown. In this study, we demonstrated that the Giant seaperch iridovirus (GSIV) genome encoded a serine/threonine kinase (ST kinase) protein, and induced apoptotic cell death via a p53-mediated Bax upregulation approach and a downregulation of Bcl-2 in fish cells. The ST kinase expression profile was identified through Western blot analyses, which indicated that expression started at day 1 h post-infection (PI), increased up to day 3, and then decreased by day 5 PI. This profile indicated the role of ST kinase expression during the early and middle phases of viral replication. We then cloned the ST kinase gene and tested its function in fish cells. The ST kinase was transiently expressed and used to investigate possible novel protein functions. The transient expression of ST kinase in GF-1 cells resulted in apoptotic cell features, as revealed with Terminal deoxynucleotidyl transferase biotin-dUTP nick-end labeling (TUNEL) assays and Hoechst 33258 staining at 24 h (37 %) and 48 h post-transfection (PT) (49 %). Then, through studies on the mechanism of cell death, we found that ST kinase overexpression could upregulate the anti-stress gene p53 and the pro-apoptotic gene Bax at 48 h PT. Interestingly, this upregulation of p53 and Bax also correlated to alterations in the mitochondria function that induced loss of mitochondrial membrane potential (MMP) and activated the initiator caspase-9 and the effector caspase-3 in the downstream. Moreover, when the p53-dependent transcriptional downstream gene was blocked by a specific transcriptional inhibitor, it was found that pifithrin-α not only reduced Bax expression, but also averted cell death in GF-1 cells during the ST kinase overexpression. Taken altogether, these results suggested that aquatic GSIV ST kinase could induce apoptosis via upregulation of p53 and Bax expression, resulting in mitochondrial disruption, which activated a downstream caspases-mediated cell death pathway.

Multiple Passages of Grunt Fin Cells Persistently Infected with Red Seabream Iridovirus (RSIV) at 15ºC or 30ºC to Yield Uninfected Cells

Red seabream iridovirus (RSIV), a member within genus Megalocytivirus (Iridoviridae), causes serious economic losses to marine fish aquaculture industry in East Asia. In this study, we established a Blue Striped Grunt Haemulon sciurus fin (grunt fin; GF) cell line persistently infected with RSIV (PI-GF^RSIV) by subculturing GF cells that survived RSIV inoculation. PI-GF^RSIV cells were morphologically indistinguishable from naive GF cells. They could stably produce RSIV at approximately 10^{4.9 ± 0.5} genomes per microliter after 24 passages over 18 months. The optimum temperature to produce RSIV in PI-GF^RSIV cells was 25°C. These cells also produced RSIV at 15, 20, and 30°C with multiple subcultures. The amount of RSIV yielded from PI-GF^RSIV cells decreased gradually by multiple subculturing at 15°C or 30°C. Red seabream iridovirus was no longer detected from PI-GF^RSIV cells after subcultures at these temperatures. These PI-GF^RSIV cells freed from RSIV infection exhibited a level of RSIV productivity similar to those of naive GF cells after inoculation with RSIV. Therefore, we consider that these PI-GF^RSIV cells were no longer infected with RSIV after multiple subculturing at 15°C or 30°C. Received October 15, 2015; accepted June 27, 2016.