ICTV Virus Taxonomy Profile: Iridoviridae

Complete Genome Sequences and Pathogenicity Analysis of Two Red Sea Bream Iridoviruses Isolated from Cultured Fish in Korea

In Korea, red sea bream iridovirus (RSIV), especially subtype II, has been the main causative agent of red sea bream iridoviral disease since the 1990s. Herein, we report two Korean RSIV isolates with different subtypes based on the major capsid protein and adenosine triphosphatase genes: 17SbTy (RSIV mixed subtype I/II) from Japanese seabass (Lateolabrax japonicus) and 17RbGs (RSIV subtype II) from rock bream (Oplegnathus fasciatus). The complete genome sequences of 17SbTy and 17RbGs were 112,360 and 112,235 bp long, respectively (115 and 114 open reading frames [ORFs], respectively). Based on nucleotide sequence homology with sequences of representative RSIVs, 69 of 115 ORFs of 17SbTy were most closely related to subtype II (98.48–100% identity), and 46 were closely related to subtype I (98.77–100% identity). In comparison with RSIVs, 17SbTy and 17RbGs carried two insertion/deletion mutations (ORFs 014R and 102R on the basis of 17SbTy) in regions encoding functional proteins (a DNA-binding protein and a myristoylated membrane protein). Notably, survival rates differed significantly between 17SbTy-infected and 17RbGs-infected rock breams, indicating that the genomic characteristics and/or adaptations to their respective original hosts might influence pathogenicity. Thus, this study provides complete genome sequences and insights into the pathogenicity of two newly identified RSIV isolates classified as a mixed subtype I/II and subtype II.

Isolation, Identification and Characterization of a Novel Megalocytivirus from Cultured Tilapia (Oreochromis spp.) from Southern California, USA

In spring 2019, diseased four-month-old tilapia (Oreochromis spp.) from an aquaculture farm in Southern California, USA were received for diagnostic evaluation with signs of lethargy, anorexia, abnormal swimming, and low-level mortalities. At necropsy, non-specific external lesions were noted including fin erosion, cutaneous melanosis, gill pallor, and coelomic distension. Internal changes included ascites, hepatomegaly, renomegaly, splenomegaly, and multifocal yellow-white nodules in the spleen and kidney. Cultures of spleen and kidney produced bacterial colonies identified as Francisella orientalis. Homogenized samples of gill, brain, liver, spleen, and kidney inoculated onto Mozambique tilapia brain cells (OmB) developed cytopathic effects, characterized by rounding of cells and detaching from the monolayer 6-10 days post-inoculation at 25 °C. Transmission electron microscopy revealed 115.4 ± 5.8 nm icosahedral virions with dense central cores in the cytoplasm of OmB cells. A consensus PCR, targeting the DNA polymerase gene of large double-stranded DNA viruses, performed on cell culture supernatant yielded a sequence consistent with an iridovirus. Phylogenetic analyses based on the concatenated full length major capsid protein and DNA polymerase gene sequences supported the tilapia virus as a novel species within the genus Megalocytivirus, most closely related to scale drop disease virus and European chub iridovirus. An intracoelomic injection challenge in Nile tilapia (O. niloticus) fingerlings resulted in 39% mortality after 16 days. Histopathology revealed necrosis of head kidney and splenic hematopoietic tissues.

Interaction between 038R and 125R of Cherax quadricarinatus iridovirus (CQIV) and their effects on virus replication

Iridovirids are a group icosahedral viruses containing linear double-stranded DNA, and mainly infect invertebrates and poikilothermic vertebrates. Cherax quadricarinatus iridovirus (CQIV) is a new species of the family Iridoviridae and can cause high mortality in shrimps. In CQIV genome, there are 25 conserved genes and the putative products are involved in several viral processes. In this study, three core protein including CQIV-032R, CQIV-125R and CQIV-160L were identified to interact with CQIV-038R by yeast two-hybrid (Y2H), and the interaction between CQIV-038R and CQIV-125R was further confirmed by co-immunoprecipitation (Co-IP) assays. In the expression system, EGFP-038R and mCherry-125R were colocalized in the cytoplasm when co-expressed in Sf9 cells. Moreover, silencing the expression of 038R, 125R or both of these two proteins respectively in C. quadricarinatus cells by small interfering RNAs showed significantly inhibit CQIV replication. Collectively, we identified the interaction between 038R and 125R, and demonstrated they are essential for CQIV replication.

Rapid detection of Decapod iridescent virus 1 (DIV1) by recombinase polymerase amplification

A recombinase polymerase amplification (RPA) assay was established for the rapid detection of Decapod iridescent virus 1 using primers targeted to the virus's ATPase gene (ORF114R). Optimization experiments showed that the optimal amplification temperature of the RPA assay was 37 °C and that the reaction could be completed within only 15 min. The target band of 15 min. is bright enough. In order to shorten the operational reaction time, consequently, 15 min was the optimal amplification time for our new RPA assay for DIV1. Specificity tests showed that the RPA assay did not exhibit any cross-reactivity with other shrimp pathogens(TSV, MrNV, YHV-1, WSSV, EHP, AHPND, EHNV, RSIV, RGV and IHHNV). Sensitivity tests further showed that the detection limit of the new RPA assay was 200 copies/50 μL, indicating that this assay was more sensitive than a nested polymerase chain reaction (PCR) method. A total of 509 clinical samples were assayed using the RPA and the PCR assays; analysis showed that the RPA method could detect weak-positive samples more effectively than the PCR method. Collectively, these findings indicated that the RPA assay was fast, simple, specific, sensitive and has significant potentials for clinical and on-site testing.

Genomic Sequencing of Ranavirus Isolates from a Three-Spined Stickleback (Gasterosteus aculeatus) and a Red-Legged Frog (Rana aurora)

Two ranavirus isolates were recovered during a wildlife disease investigation in California in 1996. Preliminary testing at the time of sample collection indicated that the two isolates were identical. Phylogenetic analysis of the full genomes of these two isolates revealed that they are a single strain of frog virus 3.

Patterns of infection, origins, and transmission of ranaviruses among the ectothermic vertebrates of Asia

Ranaviral infections, a malady of ectothermic vertebrates, are becoming frequent, severe, and widespread, causing mortality among both wild and cultured species, raising odds of species extinctions and economic losses. This increase in infection is possibly due to the broad host range of ranaviruses and the transmission of these pathogens through regional and international trade in Asia, where outbreaks have been increasingly reported over the past decade. Here, we focus attention on the origins, means of transmission, and patterns of spread of this infection within the region. Infections have been recorded in both cultured and wild populations in at least nine countries/administrative regions, together with mass die-offs in some regions. Despite the imminent seriousness of the disease in Asia, surveillance efforts are still incipient. Some of the viral strains within Asia may transmit across host-taxon barriers, posing a significant risk to native species. Factors such as rising temperatures due to global climate change seem to exacerbate ranaviral activity, as most known outbreaks have been recorded during summer; however, data are still inadequate to verify this pattern for Asia. Import risk analysis, using protocols such as Pandora+, pre-border pathogen screening, and effective biosecurity measures, can be used to mitigate introduction of ranaviruses to uninfected areas and curb transmission within Asia. Comprehensive surveillance using molecular diagnostic tools for ranavirus species and variants will help in understanding the prevalence and disease burden in the region. This is an important step toward conserving native biodiversity and safeguarding the aquaculture industry.

Initial Evidence That Gilthead Seabream (Sparus aurata L.) Is a Host for Lymphocystis Disease Virus Genotype I

Simple Summary

Nodular lesions were observed on the skin and fins of 95% of one and a half million juvenile gilthead seabreams cultured in Egypt, shortly after importation from Europe. We undertook a study to describe the clinical disease course, identify the causative agent, and investigate the origin of the causative agent. Preliminary diagnosis based on gross lesions and postmortem examination suggested lymphocystis disease caused by lymphocystis disease virus, Lymphocystivirus, Iridoviridae. Histopathological and ultrastructural pictures were typical of lymphocystis disease virus infections. Polymerase Chain Reaction followed by sequencing and phylogenetic analysis of the major capsid protein gene demonstrated the presence of lymphocystis disease virus genotype I, originally associated with lymphocystis disease in Northern European countries, with 99.7% and 100% nucleotide and deduced amino acid identity values, respectively. Lymphocystis disease virus genotype I has never been reported in this species or in the region. Regardless of whether it has maintained a previously undetected state of endemicity in Egypt or was introduced through importation or contamination of ship ballast water, the findings of this study add to existing knowledge about the lymphocystis disease’s ecology, and lymphocystis disease virus genotypes and their host range.

Abstract

Marine and brackish water aquacultures are rapidly expanding in the Mediterranean basin. In this context, Egypt recently received a shipment of a 1.5 million juvenile gilthead seabream (Sparus aurata L.) from European Mediterranean facility. Within a few weeks of their arrival, 95% of the imported fish developed nodules on their skin and fins that lasted for several months. This study was undertaken to describe the clinical disease course, to identify the causative agent, and to investigate its origin. Preliminary diagnosis based on gross lesions and postmortem examination suggested lymphocystis disease (LCD), caused by the lymphocystis disease virus (LCDV; genus Lymphocystivirus, family Iridoviridae). Histopathological and ultrastructural features were typical of LCDV infections. PCR followed by sequencing and phylogenetic analysis of a 306-bp fragment of the major capsid protein (MCP) gene demonstrated the presence of LCDV genotype I, originally associated with LCD in Northern European countries, with 99.7% and 100% nucleotide and deduced amino acid identity values, respectively. LCDV genotype I has neither been reported in this species nor in the region. Regardless of the source of infection, findings of this study add to existing knowledge about the ecology of LCDV genotype I and its host range.

Molecular Confirmation of Ranavirus Infection in Amphibians From Chad, Africa

Ranaviruses are DNA viruses (Family Iridoviridae; Subfamily Alphairidovirinae) and ranaviral disease is considered an emerging infectious disease of ectothermic vertebrates. Ranavirus infection can have varying pathological effects on infected amphibians, reptiles, and fish, most notably causing significant mortality events and population declines. Despite having a broad global range with reports from six continents, only a single incidental finding in Xenopus longipes from mainland Africa (Cameroon) is known and lacks molecular confirmation. Thus, there is a considerable knowledge gap concerning ranaviruses in Africa. We opportunistically obtained tissue samples from 160 amphibians representing five genera (Hoplobatrachus, Hylarana, Ptychadena, Pyxicephalus, and Xenopus) and two turtles (Pelomedusa sp.) from Chad, Africa. Samples were tested for ranavirus infection using a conventional PCR assay targeting the major capsid protein (MCP). A total of 25/160 (16%) frogs tested positive including 15/87 (17%) Hoplobatrachus occipitalis, 10/58 (17%) Ptychadena spp., 0/3 Pyxicephalus spp., 0/9 Xenopus spp., and 0/3 Hylarana spp. One of two turtles tested positive. Partial MCP gene sequences indicated all samples were >98% similar to several frog virus 3 (FV3)-like sequences. Additional gene targets (DNA polymerase [DNApol], ribonucleotide reductase alpha [RNR- α], ribonucleotide reductase beta subunit [RNR- β]) were sequenced to provide further detailed classification of the virus. Sequences of individual gene targets indicate that the ranavirus detected in frogs in Chad is most similar to tiger frog virus (TFV), a FV3-like virus previously isolated from diseased amphibians cultured in China and Thailand. Full genome sequencing of one sample indicates that the Chad frog virus (CFV) is a well-supported sister group to the TFVs previously determined from Asia. This work represents the first molecular confirmation of ranaviruses from Africa and is a first step in comparing ranavirus phylogeography on a local and global scale.

Evidence for a Novel Antiviral Mechanism of Teleost Fish: Serum-Derived Exosomes Inhibit Virus Replication through Incorporating Mx1 Protein

Exosomes are associated with cancer progression, pregnancy, cardiovascular diseases, central nervous system-related diseases, immune responses and viral pathogenicity. However, study on the role of exosomes in the immune response of teleost fish, especially antiviral immunity, is limited. Herein, serum-derived exosomes from mandarin fish were used to investigate the antiviral effect on the exosomes of teleost fish. Exosomes isolated from mandarin fish serum by ultra-centrifugation were internalized by mandarin fish fry cells and were able to inhibit Infectious spleen and kidney necrosis virus (ISKNV) infection. To further investigate the underlying mechanisms of exosomes in inhibiting ISKNV infection, the protein composition of serum-derived exosomes was analyzed by mass spectrometry. It was found that myxovirus resistance 1 (Mx1) was incorporated by exosomes. Furthermore, the mandarin fish Mx1 protein was proven to be transferred into the recipient cells though exosomes. Our results showed that the serum-derived exosomes from mandarin fish could inhibit ISKNV replication, which suggested an underlying mechanism of the exosome antivirus in that it incorporates Mx1 protein and delivery into recipient cells. This study provided evidence for the important antiviral role of exosomes in the immune system of teleost fish.

Discovery of Novel Viruses Associated With the Invasive Cane Toad (Rhinella marina) in Its Native and Introduced Ranges

Cane toads (Rhinella marina) are notoriously successful invaders: from 101 individuals brought to Australia in 1935, poisonous toads now cover an area >1.2 million km² with adverse effects on native fauna. Despite extensive research on the role of macroparasites in cane toad invasion, viral research is lagging. We compared viral prevalence and diversity between toads in their native range (French Guiana, n=25) and two introduced ranges: Australia (n=151) and Hawai'i (n=10) with a metatranscriptomic and metagenomic approach combined with PCR screening. Australian toads almost exclusively harbor one of seven viruses detected globally. Rhimavirus-A (Picornaviridae) exhibited low genetic diversity and likely actively infected 9% of sampled Australian toads extending across ~2,000km of Northern Australia and up to the current invasion front. In native range cane toads, we identified multiple phylogenetically distinct viruses (Iridoviridae, Picornaviridae, Papillomaviridae, and Nackedna-like virus). None of the same viruses was detected in both ranges, suggesting that Australian cane toads have largely escaped the viral infection experienced by their native range counterparts. The novel native range viruses described here are potential biocontrol agents, as Australian toads likely lack prior immunological exposure to these viruses. Overall, our evidence suggests that there may be differences between viruses infecting cane toads in their native vs. introduced ranges, which lays the groundwork for further studies on how these viruses have influenced the toads' invasion history.

The Molecular Mechanism of Hemocyte Immune Response in Marsupenaeus japonicus Infected With Decapod Iridescent Virus 1

As a new type of shrimp lethal virus, decapod iridescent virus 1 (DIV1) has caused huge economic losses to shrimp farmers in China. Up to now, DIV1 has been detected in a variety of shrimps, but there is no report in Marsupenaeus japonicus. In the current study, we calculated the LC₅₀ to evaluate the toxicity of DIV1 to M. japonicus and determined through nested PCR that M. japonicus can be the host of DIV1. Through enzyme activity study, it was found that DIV1 can inhibit the activities of superoxide dismutase, catalase, lysozyme, and phenoloxidase, which could be a way for DIV1 to achieve immune evasion. In a comprehensive study on the transcriptomic changes of M. japonicus in response to DIV1 infection, a total of 52,287 unigenes were de novo assembled, and 20,342 SSR markers associated with these unigenes were obtained. Through a comparative transcriptomic analysis, 6,900 differentially expressed genes were identified, including 3,882 upregulated genes and 3,018 downregulated genes. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that some GO terms related to virus invasion, replication, and host antiviral infection were promoted under DIV1 infection, such as carbohydrate binding, chitin binding, chitin metabolic process, and DNA replication initiation, and some KEGG pathways related to immune response were significantly influenced by DIV1 infection, including Toll and IMD signaling pathway, JAK-STAT signaling pathway, IL-17 signaling pathway, C-type lectin receptor signaling pathway, complement and coagulation cascades, antigen processing and presentation, necroptosis, apoptosis, NOD-like receptor signaling pathway, apoptosis-multiple species, and TNF signaling pathway. Further analysis showed that STAT, Dorsal, Relish, heat shock protein 70 (HSP70), C-type lectins, and caspase play an important role in DIV1 infection. This is the first detailed study of DIV1 infection in M. japonicus, which initially reveals the molecular mechanism of DIV1 infection in M. japonicus by using the transcriptome analysis of hemocytes combined with enzyme activity study.

Development of single nucleotide polymorphism (SNP) application for detection and genotyping of RSIV-type megalocytiviruses

Red sea bream iridovirus (RSIV) belonging to the genus Megalocytivirus of the family Iridoviridae is the cause of serious mass mortality of cultured marine fishes. RSIV-type megalocytiviruses show extremely high nucleotide sequence identities. Thus, epidemiological studies on this virus are limited. This study developed two primer sets amplifying the regions possessing single nucleotide polymorphism (SNP) to determine the relationships and divergence of RSIV-type megalocytiviruses isolated from cultured marine fishes in Japan. The two regions were designed according to the genome sequences of the representative RSIV genotype II of megalocytivirus members in GenBank. The SNP 1 and 2 regions have sequences homologous to hypothetical protein ORF 24 and ORF 31, respectively, of RSIV (accession no. AP017456.1). By sequencing the regions, 53 polymorphic sites were identified. The phylogenetic analysis of 25 RSIV-type megalocytivirus isolates, classified into RSIV cluster, was clustered into eight haplotypes (seven haplotypes from Oita, two haplotypes from Ehime, and one haplotype shared between Oita and Ehime). These findings suggested that SNP in the RSIV genome is a powerful application for the detection and identification of RSIV-type megalocytiviruses.

Genome Sequence of Lymphocystis Disease Virus 2 LCDV-JP_Oita_2018, Isolated from a Diseased Japanese Flounder (Paralichthys olivaceus) in Japan

Here, we present the genome sequence of lymphocystis disease virus 2 LCDV-JP_Oita_2018 (genus Lymphocystivirus, family Iridoviridae), which was isolated from a diseased Japanese flounder (Paralichthys olivaceus) in Japan. The LCDV-JP_Oita_2018 genome was assembled into a circular contig of 186,627 bp, with 140 predicted protein-coding genes and a GC content of 27%.

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.

Extended in vivo transcriptomes of two ascoviruses with different tissue tropisms reveal alternative mechanisms for enhancing virus reproduction in hemolymph

Ascoviruses are large dsDNA viruses characterized by the extraordinary changes they induce in cellular pathogenesis and architecture whereby after nuclear lysis and extensive hypertrophy, each cell is cleaved into numerous vesicles for virion reproduction. However, the level of viral replication and transcription in vesicles compared to other host tissues remains uncertain. Therefore, we applied RNA-Sequencing to compare the temporal transcriptome of Spodoptera frugiperda ascovirus (SfAV) and Trichoplusia ni ascovirus (TnAV) at 7, 14, and 21 days post-infection (dpi). We found most transcription occurred in viral vesicles, not in initial tissues infected, a remarkably novel reproduction mechanism compared to all other viruses and most other intracellular pathogens. Specifically, the highest level of viral gene expression occurred in hemolymph, for TnAV at 7 dpi, and SfAV at 14 dpi. Moreover, we found that host immune genes were partially down-regulated in hemolymph, where most viral replication occurred in highly dense accumulations of vesicles.

Toll protein family structure, evolution and response of the whiteleg shrimp (Litopenaeus vannamei) to exogenous iridescent virus

Whiteleg shrimp is a widely cultured crustacean, but frequent disease outbreaks have decreased production and caused significant losses. Toll-like receptors (TLRs) comprise a large innate immune family that is involved in the innate immune response. However, understanding of their regulatory mechanism is limited. In this study, PacBio sequencing and Illumina sequencing were applied to the gill and hepatopancreas tissues of whiteleg shrimp and an integrated transcript gene set was established. The upregulation of Toll1, Toll2 and Toll3 transcripts in the hepatopancreas tissue of whiteleg shrimp after iridescent virus infection implies that these proteins are involved in the immune response to the virus; simultaneously, the TRAF6 and relish transcripts in the Toll pathway were also upregulated, implying that the Toll pathway was activated. We predicted the three-dimensional structure of the five Toll proteins in whiteleg shrimp and humans and constructed a phylogenetic tree of the Toll protein family. In addition, there was a large discrepancy of Toll1 between invertebrates and vertebrates, presumably because of the loss of Toll1 protein sequence during the evolution process from invertebrates to vertebrates. Our research will improve the cognition of Toll protein family in invertebrates in terms of evolution, structure and function and provide theoretical guidance for researchers in this field.

Development of a gene-deleted live attenuated candidate vaccine against fish virus (ISKNV) with low pathogenicity and high protection

Aquaculture provides important food, nutrition, and income sources for humans. However, aquaculture industry is seriously threatened by viral diseases. Infectious spleen and kidney necrosis virus (ISKNV) disease causes high mortality and economic losses to the fish culture industry in Asia and has been listed as a certifiable disease by the International Epizootic Office. Vaccine development is urgent to control this disease. Here, a gene-deleted live attenuated candidate vaccine (ΔORF022L) against ISKNV with low pathogenicity and high protection was developed. ΔORF022L replicated well in mandarin fish fry-1 cells and showed similar structure with wild-type ISKNV. However, the pathogenicity was significantly lower as 98% of the mandarin fish infected with ΔORF022L survived, whereas all those infected with wild-type ISKNV died. Of importance, 100% of the ΔORF022L-infected fish survived the ISKNV challenge. ΔORF022L induced anti-ISKNV specific antibody response and upregulation of immune-related genes. This work could be beneficial to the control of fish diseases.

Evaluation of Gilthead Seabream (Sparus aurata) Immune Response after LCDV-Sa DNA Vaccination

Simple Summary

Lymphocystis disease is the main viral pathology in gilthead seabream aquaculture. Currently, there are no treatments or vaccines to control this disease, thus our main goal was to construct a DNA vaccine that can be used in the future to stop the spread of this pathology in sea farms. The vaccine consisted of a plasmid DNA that contains a known viral gene. Once it was established that the vaccine drives the expression of the antigenic viral protein in fish, vaccination experiments were conducted to determine if the vaccinated fish become protected against the viral infection. In addition, the immune response triggered by the vaccine was also evaluated in order to understand the mechanisms underlying such protection. The obtained results showed that in vaccinated fish an activation of several genes relating to both the inflammatory process and the mucosal immunity were produced, as well as specific anti-viral antibodies. Although limited, our results deserve further investigation to assess the efficacy of the vaccine in bigger fish populations and to confirm the mode of action of the vaccine.

Abstract

Lymphocystis disease is the main viral pathology reported in gilthead seabream. Its etiological agent is Lymphocystis disease virus 3 (LCDV-Sa), genus Lymphocystivirus, family Iridoviridae. There are no effective treatments or vaccines for LCDV control, thus the main aim of this study was to develop a DNA vaccine, and to evaluate both the protection conferred against LCDV-Sa infection and the immune response in vaccinated fish. The vaccine was constructed by cloning the mcp gene (ORF LCDVSa062R) into pcDNA3.1/NT-GFP-TOPO. Two independent vaccination trials were conducted. In the first one, 5–7 g fish were intramuscularly injected with the vaccine (pcDNA-MCP) or the empty-plasmid, and the distribution and expression of the vaccine was investigated. Furthermore, vaccinated fish were challenged with LCDV-Sa in order to access the protective capacity of the vaccine. In the second trial, 70–100 g fish were vaccinated as specified, and the immune response was evaluated analyzing the expression of 23 immune-related genes and the production of specific antibodies. The results showed that the vaccine triggers an immune response characterized by the overexpression of genes relating to the inflammatory process, but not the innate antiviral immunity relating to type I IFN (interferon), and also induces the production of specific neutralizing antibodies, which could explain the protection against LCDV-Sa in vaccinated fish.

Citizen science data opens multiple avenues for iridovirus research and prompts first detection of Invertebrate iridescent virus 31 in Australia

From citizen science data we report the first records of blue to violet-colored oniscideans (Oniscidea: Isopoda), indicating potential invertebrate iridescent virus (IIV; Betairidovirinae: Iridoviridae) infection: in Africa, South America, and Oceania; and of the new hosts Armadillidium nasatum and Balloniscus sellowii. DNA sequencing of indigo Porcellio scaber confirms the presence of Invertebrate iridescent virus 31 in Australia. Beyond the Oniscidea, new, putative IIV hosts are identified: hoverfly pupae (Eristalinae), a tortrix moth larva (Phaecasiophora niveiguttana), and a millipede (Harpaphe haydeniana). In addition, the purported positive correlation between virion diameter and wavelength of iridescence is analyzed qualitatively for the first time.

Phylogenomics of the Maverick Virus-Like Mobile Genetic Elements of Vertebrates

Mavericks are virus-like mobile genetic elements found in the genomes of eukaryotes. Although Mavericks encode capsid morphogenesis homologs, their viral particles have not been observed. Here, we provide new evidence supporting the viral nature of Mavericks and the potential existence of virions. To this end, we conducted a phylogenomic analysis of Mavericks in hundreds of vertebrate genomes, discovering 134 elements with an intact coding capacity in 17 host species. We reveal an extensive genomic fossil record in 143 species and date three groups of elements to the Late Cretaceous. Bayesian phylogenetic analysis using genomic fossil orthologs suggests that Mavericks have infected osteichthyans for ∼419 My. They have undergone frequent cross-species transmissions in cyprinid fish and all core genes are subject to strong purifying selection. We conclude that vertebrate Mavericks form an ancient lineage of aquatic dsDNA viruses which are probably still functional in some vertebrate lineages.

FATAL RANAVIRUS INFECTION IN A GROUP OF ZOO-HOUSED MELLER'S CHAMELEONS (TRIOCEROS MELLERI)

A group of five juvenile Meller's chameleons (Trioceros melleri) experienced 100% mortality over a period of 1 mo due to ranavirus infection. The index case was found dead without premonitory signs. The three subsequent cases presented with nonspecific clinical signs (lethargy, decreased appetite, ocular discharge) and were ultimately euthanatized. The final case died after initially presenting with skin lesions. Postmortem examination revealed thin body condition in all five animals and mild coelomic effusion and petechiae affecting the tongue and kidneys of one animal. Microscopically, all animals had multifocal necrosis of the spleen, liver, and kidney; four of five animals had necrosis of the nasal cavity; and two of five had necrosis of adrenal tissue, bone marrow, and skin. Numerous basophilic intracytoplasmic inclusions were present in the liver of all animals and nasal mucosa of three of the five animals. Consensus polymerase chain reaction for herpesvirus and adenovirus were negative, whereas ranavirus quantitative polymerase chain reaction was positive. Virus isolation followed by whole genome sequencing and Bayesian phylogenetic analysis classified the isolates as a strain of frog virus 3 (FV3) most closely related to an FV3 isolate responsible for a previous outbreak in the zoo's eastern box turtle (Terrapene carolina carolina) group. This case series documents the first known occurrence of ranavirus-associated disease in chameleons and demonstrates the potential for interspecies transmission between chelonian and squamate reptiles.

The Insights of Genomic Synteny and Codon Usage Preference on Genera Demarcation of Iridoviridae Family

The members of the family Iridoviridae are large, double-stranded DNA viruses that infect various hosts, including both vertebrates and invertebrates. Although great progress has been made in genomic and phylogenetic analyses, the adequacy of the existing criteria for classification within the Iridoviridae family remains unknown. In this study, we redetermined 23 Iridoviridae core genes by re-annotation, core-pan analysis and local BLASTN search. The phylogenetic tree based on the 23 re-annotated core genes (Maximum Likelihood, ML-Tree) and amino acid sequences (composition vector, CV-Tree) were found to be consistent with previous reports. Furthermore, the information provided by synteny analysis and codon usage preference (relative synonymous codon usage, correspondence analysis, ENC-plot and Neutrality plot) also supports the phylogenetic relationship. Collectively, our results will be conducive to understanding the genera demarcation within the Iridoviridae family based on genomic synteny and component (codon usage preference) and contribute to the existing taxonomy methods for the Iridoviridae family.

Development and comparison of qPCR and qLAMP for rapid detection of the decapod iridescent virus 1 (DIV1)

Decapod iridescent virus 1 (DIV1) is a new virus discovered in recent years that infects farmed shrimp. DIV1 is highly infectious and causes substantial economic loss to the aquaculture industry of China. To prevent and control the spread and outbreak of DIV1 in a timely manner, it is necessary to establish an efficient method for DIV1 diagnosis. In this study, quantitative real-time polymerase chain reaction (qPCR) and quantitative real-time loop-mediated isothermal amplification (qLAMP) detection methods were established based on the specific sequence of the viral ATPase gene. The results indicated that the minimum detection limits of qPCR and qLAMP were 1.9 × 10¹ copies/μL and 1.9 × 10² copies/μL, respectively; the designed primer had good specificity for DIV1 and did not react with 13 other viruses, including white spot syndrome virus (WSSV), Enterocytozoon hepatopenaei (EHP), acute hepatopancreatic necrosis disease (AHPND), infectious hypodermal and haematopoietic necrosis virus (IHHNV), etc. A total of 43 clinical samples suspected of DIV1 infection were diagnosed by qPCR and qLAMP. Our qPCR demonstrated results consistent with a qPCR assay published previously, and the diagnostic sensitivity (DSe) and diagnostic specificity (DSp) of qLAMP were 85.71% and 100%, respectively. This result indicates that qPCR and qLAMP have good accuracy in the detection of DIVI in clinical samples. As established in this study, qPCR and qLAMP combined with a comprehensive comparative analysis can provide effective new solutions for the detection of DIV1.

Megalocytiviruses in ornamental fish: A review

Iridoviruses, especially megalocytiviruses, are related to severe disease resulting in high economic losses in the aquaculture industry worldwide. The ornamental fish industry has been affected severely due to Megalocytivirus infections. Megalocytivirus is a DNA virus that has three genera; including red sea bream iridovirus, infectious spleen and kidney necrosis virus, and turbot reddish body iridovirus. Megalocytivirus causes non-specific clinical signs in ornamental fish. Cell culture, histology, immunofluorescence test, polymerase chain reaction (PCR) assay, and loop-mediated isothermal amplification assay have been used to diagnose megalocytiviruses. Risk factors such as temperature, transportation (export and import), and life stages of ornamental fish have been reported for the previous cases due to Megalocytivirus infections. In addition, other prevention and control methods also have been practiced in farms to prevent Megalocytivirus outbreaks. This is the first review of megalocytiviruses in ornamental fish since its first detection in 1989. This review discusses the occurrences of Megalocytivirus in ornamental fish, including the history, clinical signs, detection method, risk factors, and prevention measures.

Roles of extracellular matrix components in Tiger frog virus attachment to fathead minnow (Pimephales promelas) cells

Tiger frog virus (TFV) belongs to the genus Ranavirus (family Iridoviridae) and causes significant harm in cultured frogs, resulting in substantial losses in ecological and economic field in Southern China. Attachment is the first step in viral life cycle, which is dependent on the interactions of virions with extracellular matrix (ECM) components. Studying this process will help in understanding virus infection and controlling viral diseases. In this study, the roles of primary ECM components in TFV attachment were investigated. The results on the kinetics of virus attachment showed TFV successful attachment to the cell surface as a relatively rapid process after TFV was used to inoculate cells for 10 min at 4 °C. Western blot and quantitative PCR analyses results showed that soluble fibronectin, collagen IV, laminin, or hyaluronic acid treatment with TFV caused no significant effect on virus attachment. Soluble heparin, heparan sulfate and chondroitin sulfate A/B could inhibit TFV attachment in a dose-dependent manner. Enzymic digestion by cell surface heparin/heparan sulfate using heparinase I, II, and III could significantly prevent TFV attachment, suggesting that heparan sulfate plays an important role in TFV attachment. Furthermore, the binding assays of heparin-agarose beads and virion showed that TFV virions specifically bound with heparin in a dose-dependent manner. Given that heparin is a structural analogue of heparan sulfate, the above results suggest that heparan sulfate might serve as an attachment factor of TFV infection. Our work would be beneficial to understand the mechanisms of TFV attachment and the interactions of TFV with cellular receptor(s).

The interactome of Singapore grouper iridovirus protein ICP18 as revealed by proximity-dependent BioID approach

Singapore grouper iridovirus (SGIV) is a large double-stranded DNA virus that is a major threat to grouper aquaculture. The pathogenesis of SGIV is not well understood so far. Previous studies have revealed that ICP18, an immediate early protein encoded by SGIV ORF086R gene, promotes viral replication by regulating cell proliferation and virus assembly. In the present study, the potential functions of ICP18 were further explored by probing into its interactors using a proximity-dependent BioID method. Since our in-house grouper embryonic cells (a natural host cell of SGIV) could not be efficiently transfected with the plasmid DNA, and the grouper genome data for mass spectrometry-based protein identification is not currently available, we chosen a non-permissive cell (HEK293 T) as a substitute for this study. A total of 112 cellular proteins that potentially bind to ICP18 were identified by mass spectrometry analysis. Homology analysis showed that among these identified proteins, 110 candidate ICP18-interactors had homologous proteins in zebrafish (a host of SGIV), and shared high sequence identity. Further analysis revealed that the identified ICP18-interacting proteins modulate various cellular processes such as cell cycle and cell adhesion. In addition, the interaction between ICP18 and its candidate interactor, i.e., cyclin-dependent kinase1 (CDK1), was confirmed using Co-immunoprecipitation (Co-IP) and Pull-down assays. Collectively, our present data provides additional insight into the biological functions of ICP18 during viral infection, which could help in further unraveling the pathogenesis of SGIV.

Optimization of the efficacy of a SWCNTs-based subunit vaccine against infectious spleen and kidney necrosis virus in mandarin fish

Infectious spleen and kidney necrosis virus (ISKNV) cause a high mortality disease which brings substantial economic losses to the mandarin fish culture industry in China. This study was aimed at optimizing the efficacy of a SWCNTs-based immersion subunit vaccine (SWCNTs-M-MCP) which as a promising vaccine against ISKNV. Mandarin fish were vaccinated by immersion, then we designed an orthogonal experiment to optimize different parameters affecting vaccination such as immune duration of bath immunization, immune dose, and fish density when immunized. Our results showed that the highest relative percent survival (86.7%) was found in the group 6 with 8 h of immune duration, 20 mg/L of immune dose, and 8 fish per liter of fish density. And other immune responses (serum antibody production, enzyme activities, and immune-related genes expression) also demonstrated similar results. In addition, the expression of IRF-I in group 6 (8 h, 20 mg/L, 8 fish per liter) was significant extents, and about 16-folds increases were obtained than the control group at 21 d post-vaccination. And the highest specific antibody response was significantly increased (more than 4-folds) than control group which was found in group 6. The optimum immune duration, immune dose, and fish density of SWCNTs-M-MCP were 8 h, 20 mg/L, 8 fish per liter, respectively. Importantly, our results also showed that immune duration had the greatest effect on the immune response of our vaccine, followed by immune dose. The study reported herein provides a helpful reference for the effective use of vaccine in fish farming industry.

Conserved motifs in the invertebrate iridescent virus 6 (IIV6) genome regulate virus transcription

Invertebrate iridescent virus 6 (IIV6) is the type species of the Iridovirus genus in the Betairidovirinae subfamily of the Iridoviridae family. Transcription of the 215 predicted IIV6 genes is temporally regulated, dividing the genes into three kinetic classes: immediate-early (IE), delayed-early (DE), and late (L). So far, the transcriptional class has been determined for a selection of virion protein genes and only for three genes the potential promoter regions have been analyzed in detail. In this study, we investigated the transcriptional class of all IIV6 genes that had not been classified until now. RT-PCR analysis of total RNA isolated from virus-infected insect cells in the presence or absence of protein and DNA synthesis inhibitors, placed 113, 23 and 22 of the newly analyzed viral ORFs into the IE, DE and L gene classes, respectively. Afterwards, in silico analysis was performed to the upstream regions (200 bp) of all viral ORFs using the MEME Suite Software. The AA(A/T)(T/A)TG(A/G)A and (T/A/C)(T/G/C)T(T/A)ATGG motifs were identified in the upstream region of IE and DE genes, respectively. These motifs were validated by luciferase reporter assays as crucial sequences for promoter activity. For the L genes two conserved motifs were identified for all analyzed genes: (T/G)(C/T)(A/C)A(T/G/C)(T/C)T(T/C) and (C/G/T)(G/A/C)(T/A)(T/G) (G/T)(T/C). However, the presence of these two motifs did not influence promoter activity. Conversely, the presence of these two sequences upstream of the reporter decreased its expression. Single nucleotide mutations in the highly conserved nucleotides at the end of the second motif (TTGT) showed that this motif acted as a repressor sequence for late genes in the IIV6 genome. Next, upstream sequences of IIV6 L genes from which we removed this second motif in silico, were re-analyzed for the presence of potential conserved promoter sequences. Two additional motifs were identified in this way for L genes: (T/A)(A/T)(A/T/G)(A/T)(T/C)(A/G)(A/C)(A/C) and (C/G)(T/C)(T/A/C)C(A/T)(A/T)T(T/G) (T/G)(T/G/A). Independent mutations in either motif caused a severe decrease in luciferase expression. Information on temporal classes and upstream regulatory sequences will contribute to our understanding of the transcriptional mechanisms in IIV6.

The Role of Tape Measure Protein in Nucleocytoplasmic Large DNA Virus Capsid Assembly

Nucleocytoplasmic large DNA viruses (NCLDVs) are a group of large viruses that infect a wide range of hosts, from animals to protists. These viruses are grouped together in NCLDV based on genomic sequence analyses. They share a set of essential genes for virion morphogenesis and replication. Most NCLDVs generally have large physical sizes while their morphologies vary in different families, such as icosahedral, brick, or oval shape, raising the question of the possible regulatory factor on their morphogenesis. The capsids of icosahedral NCLDVs are assembled from small building blocks, named capsomers, which are the trimeric form of the major capsid proteins. Note that the capsids of immature poxvirus are spherical even though they are assembled from capsomers that share high structural conservation with those icosahedral NCLDVs. The recently published high resolution structure of NCLDVs, Paramecium bursaria Chlorella virus 1 and African swine fever virus, described the intensive network of minor capsid proteins that are located underneath the capsomers. Among these minor proteins is the elongated tape measure protein (TmP) that spans from one icosahedral fivefold vertex to another. In this study, we focused on the critical roles that TmP plays in the assembly of icosahedral NCLDV capsids, answering a question raised in a previously proposed spiral mechanism. Interestingly, basic local alignment search on the TmPs showed no significant hits in poxviruses, which might be the factor that differentiates poxviruses and icosahedral NCLDVs in their morphogenesis.

Host-virus interactions and defense mechanisms for giant viruses

Giant viruses, with virions larger than 200 nm and genomes larger than 340 kilobase pairs, modified the now outdated perception of the virosphere. With virions now reported reaching up to 1.5 μm in size and genomes of up to 2.5 Mb encoding components shared with cellular life forms, giant viruses exhibit a complexity similar to microbes, such as bacteria and archaea. Here, we review interactions of giant viruses with their hosts and defense strategies of giant viruses against their hosts and coinfecting microorganisms or virophages. We also searched by comparative genomics for homologies with proteins described or suspected to be involved in defense mechanisms. Our search reveals that natural immunity and apoptosis seem to be crucial components of the host defense against giant virus infection. Conversely, giant viruses possess methods of hijacking host functions to counteract cellular antiviral responses. In addition, giant viruses may encode other unique and complex pathways to manipulate the host machinery and eliminate other competing microorganisms. Notably, giant viruses have evolved defense mechanisms against their virophages and they might trigger defense systems against other viruses through sequence integration. We anticipate that comparative genomics may help identifying genes involved in defense strategies of both giant viruses and their hosts.

Emerging Infectious Diseases of Chelonians: An Update

Chelonians are increasingly challenged by anthropogenic threats and disease. This article summarizes recent literature and clinical experiences regarding 4 emerging infectious diseases in turtles and tortoises: ranaviruses, cryptosporidiosis, intranuclear coccodiosis of Testudines, and Emydomyces testavorans.

The Vaginal Virome-Balancing Female Genital Tract Bacteriome, Mucosal Immunity, and Sexual and Reproductive Health Outcomes?

Besides bacteria, fungi, protists and archaea, the vaginal ecosystem also contains a range of prokaryote- and eukaryote-infecting viruses, which are collectively referred to as the "virome". Despite its well-described role in the gut and other environmental niches, the vaginal virome remains understudied. With a focus on sexual and reproductive health, we summarize the currently known components of the vaginal virome, its relationship with other constituents of the vaginal microbiota and its association with adverse health outcomes. While a range of eukaryote-infecting viruses has been described to be present in the female genital tract (FGT), few prokaryote-infecting viruses have been described. Literature suggests that various vaginal viruses interact with vaginal bacterial microbiota and host immunity and that any imbalance thereof may contribute to the risk of adverse reproductive health outcomes, including infertility and adverse birth outcomes. Current limitations of vaginal virome research include experimental and analytical constraints. Considering the vaginal virome may represent the missing link in our understanding of the relationship between FGT bacteria, mucosal immunity, and adverse sexual and reproductive health outcomes, future studies evaluating the vaginal microbiome and its population dynamics holistically will be important for understanding the role of the vaginal virome in balancing health and disease.

Transcriptomics of Cherax quadricarinatus hepatopancreas during infection with Decapod iridescent virus 1 (DIV1)

Cherax quadricarinatus is a large-sized, highly fecund, and fast-growing species of freshwater crayfish, and has become one of the world's most intensely studied crustaceans. Decapod iridescent virus 1 (DIV1), a newly described species in the family Iridoviridae, is known to infect various crustaceans, including C. quadricarinatus, and may pose a new threat in the shrimp-farming industry. The present study performed de novo transcriptome sequencing of C. quadricarinatus hepatopancreas during DIV1 infection. A total of 114,784 transcripts and 56,418 genes were obtained; 1070 genes were upregulated and 775 genes were downregulated when compared with the uninfected samples (controls). Three pattern recognition receptor genes (fibrinogen-related protein, C-type lectin, and beta-1,3-glucan-binding protein) were upregulated during DIV1 infection. Among the top-30 upregulated unigenes, 9 unigenes were identified as vitellogenin (Vg) genes, and the top-3 upregulated unigenes were identified as involved in Vg lipid transport, lipid localization, and lipid transporter activity, which were all significantly over-representative GO terms in the GO enrichment analysis of total and upregulated differentially expressed genes (DEGs). Many genes associated with Jak-STAT signaling pathway, Endocytosis, Phagosome, MAPK signaling pathway, Apoptosis and Lysosome were positively modified after DIV1 infection. The predicted protein-protein interaction (PPI) analysis showed NF1 and TUBA, CRM1 and TUBB were involved in protein interactions. This research showed that DIV1 infection has a significant impact on the transcriptome profile of C. quadricarinatus hepatopancreas, and the results enhance our understanding of virus-host interactions. Furthermore, the high number of transcripts generated in the present study will provide information for identifying novel genes in the absence of a full C. quadricarinatus genome sequence.

The Old and the New on Viral Diseases in Sturgeon

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

Initial Virome Characterization of the Common Cnidarian Lab Model Nematostella vectensis

The role of viruses in forming a stable holobiont has been the subject of extensive research in recent years. However, many emerging model organisms still lack any data on the composition of the associated viral communities. Here, we re-analyzed seven publicly available transcriptome datasets of the starlet sea anemone Nematostella vectensis, the most commonly used anthozoan lab model, and searched for viral sequences. We applied a straightforward, yet powerful approach of de novo assembly followed by homology-based virus identification and a multi-step, thorough taxonomic validation. The comparison of different lab populations of N. vectensis revealed the existence of the core virome composed of 21 viral sequences, present in all adult datasets. Unexpectedly, we observed an almost complete lack of viruses in the samples from the early developmental stages, which together with the identification of the viruses shared with the major source of the food in the lab, the brine shrimp Artemia salina, shed new light on the course of viral species acquisition in N. vectensis. Our study provides an initial, yet comprehensive insight into N. vectensis virome and sets the first foundation for the functional studies of viruses and antiviral systems in this lab model cnidarian.

Amphibian ranaviruses in Europe: important directions for future research

Ranaviruses are an emerging group of pathogens capable of infecting all cold-blooded vertebrates. In Europe, ranaviruses pose a particularly potent threat to wild amphibian populations. Since the 1980s research on amphibian-infecting ranaviruses in Europe has been growing. The wide distribution of amphibian populations in Europe, the ease with which many are monitored, and the tractable nature of counterpart ex situ experimental systems have provided researchers with a unique opportunity to study many aspects of host–ranavirus interactions in the wild. These characteristics of European amphibian populations will also enable researchers to lead the way as the field of host–ranavirus interactions progresses. In this review, we provide a summary of the current key knowledge regarding amphibian infecting ranaviruses throughout Europe. We then outline important areas of further research and suggest practical ways each could be pursued. We address the study of potential interactions between the amphibian microbiome and ranaviruses, how pollution may exacerbate ranaviral disease either as direct stressors of amphibians or indirect modification of the amphibian microbiome. Finally, we discuss the need for continued surveillance of ranaviral emergence in the face of climate change.

Phylogenomic characterization of ranaviruses isolated from cultured fish and amphibians in Thailand

Ranaviruses are emerging pathogens associated with worldwide epizootics in farmed and wild ectothermic vertebrates. In this study, we determined the full genomes of eight ranaviruses isolated from marbled sleeper goby ( Oxyeleotris marmorata), goldfish ( Carassius auratus), guppy ( Poecilia reticulata), tiger frog ( Hoplobatrachus tigerinus), Asian grass frog ( Fejervarya limnocharis), and East Asian bullfrog ( H. rugulosus) cultured or imported into Thailand. These ranaviral isolates induced the same cytopathic effects (i.e., progression of coalescing round plaques) in epithelioma papulosum cyprini (EPC) cell cultures. Transmission electron microscopy of infected EPC cells revealed cytoplasmic viral particles with ultrastructural features typical for ranaviruses. Pairwise genetic comparisons of the complete major capsid protein coding sequences from the Thai ranaviruses displayed the highest identity (99.8%–100%) to a ranavirus (tiger frog virus; TFV) isolated from diseased tiger frogs cultured in China, a slightly lower identity (99.3%–99.4%) to a ranavirus (Wamena virus; WV) isolated from diseased green tree pythons ( Morelia viridis) illegally exported from Papua New Guinea, and a lower identity to 35 other ranaviruses (93.7%–98.6%). Phylogenomic analyses supported the eight Thai ranaviruses, Chinese TFV, and WV as a subclade within a larger frog virus 3 clade. Our findings confirm the spread of TFV among cultured fish and amphibians in Asia and likely in reptiles in Oceania. Biosecurity measures are needed to ensure TFV does not continue to spread throughout Southeast Asia and to other parts of the world via international trade.

Sublethal effects of wild-type and a vIF-2α-knockout Frog virus 3 on postmetamorphic wood frogs (Rana sylvatica): potential for a stage-specific reservoir

Ranaviruses have been associated with rising numbers of mass die-offs in amphibian populations around the globe. However, most studies on ranaviruses to date focused on larval amphibians. To assess the role of postmetamorphic amphibians in the epidemiology of ranaviruses and to determine the role of viral immune-suppression genes, we performed a bath-exposure study on post-metamorphic wood frogs ( Rana sylvatica) using environmentally relevant concentrations of wild-type Frog virus 3 (WT FV3), and a gene-knockout mutant (KO FV3), deficient for the putative immune-suppression gene vIF-2α. We observed a 42% infection rate and 5% mortality across the virus challenges, with infection rates and viral loads following a dose-dependent pattern. Individuals exposed to the knockout variant exhibited significantly decreased growth and increased lethargy compared with wild-type treatments. Although 85% of exposed individuals exhibited common signs of ranavirosis throughout the experiment, most of these individuals did not exhibit signs of infection by 40 d post-exposure. Overall, we showed that even a single short time exposure to environmentally relevant concentrations of ranavirus may cause sublethal infections in postmetamorphic amphibians, highlighting the importance of this life stage in the epidemiology of ranaviruses. Our study also supports the importance of the vIF-2α gene in immune-suppression in infected individuals.

Detection and molecular characterization of Lymphocystivirus in Brazilian ornamental fish

The aim of this study is to report the occurrence of Lymphocystivirus in Brazilian ornamental fish. From 25 ornamental fish species submitted for molecular diagnosis, only one sample (Pomacanthus imperator) was positive, and its viral nucleotide sequence obtained clustered with sequences of genotype VII. To our knowledge, this is the first report on the genetic characterization of Lymphocystivirus in Brazil.

Deletion of the Infectious spleen and kidney necrosis virus ORF069L reduces virulence to mandarin fish Siniperca chuatsi

Mandarin fish (Siniperca chuatsi) is a significant cultured species with high added value in China. With the expansion of farming, diseases of mandarin fish such as Infectious spleen and kidney necrosis virus (ISKNV) diseases are becoming more and more serious. Human endogenous retrovirus subfamily H long terminal repeat associating protein 2 (HHLA2) is a type 1 transmembrane molecule with three extracellular Ig domains (IgV-IgC-IgV) and plays important roles in the T cell proliferation and tumorigenesis. The HHLA2-homologues have not been found in virus. In this study, a viral HHLA2 protein encoded by ISKNV ORF069L was identified and the virulence of the deleted ORF069L reconstruction ISKNV strain (ΔORF069L) was investigated. ISKNV ORF069L gene was predicted to encode a 222-amino acids peptide. The bioinformation analysis revealed that ISKNV ORF069L contained an Ig HHLA2 domain and was homologous to vertebrate B7-CD28 family proteins. The recombinant virus strain of ΔORF069L was constructed by homologous recombination technology. The virus titer and growth curves between ISKNV wild type (WT) and ΔORF069L on cellular level showed no significant differences indicating that the ORF069L did not influence the ISKNV replication. The expression levels of immune-related genes (Mx1, IL-1β, IL-8, TNF-a and IgM) were increased in fish infected with ΔORF069L, compared to those in fish infected with ISKNV WT. Furthermore, the lethality caused by ΔORF069L declined by 40% compared with ISKNV WT, indicating that ORF069L was a virulence gene of ISKNV. Most importantly, the protection rate was nearly 100% for fish immunized with ΔORF069L strain. Those results suggested that ΔORF069L could be developed as a potential attenuated vaccine against ISKNV. Our work will be beneficial to promote the development of gene deletion attenuated vaccines for ISKNV disease.

Genome analysis of Ranavirus frog virus 3 isolated from American Bullfrog (Lithobates catesbeianus) in South America

Ranaviruses (family Iridoviridae) cause important diseases in cold-blooded vertebrates. In addition, some occurrences indicate that, in this genus, the same virus can infect animals from different taxonomic groups. A strain isolated from a Ranavirus outbreak (2012) in the state of Sao Paulo, Brazil, had its genome sequenced and presented 99.26% and 36.85% identity with samples of Frog virus 3 (FV3) and Singapore grouper iridovirus (SGIV) ranaviruses, respectively. Eight potential recombination events among the analyzed sample and reference FV3 samples were identified, including a recombination with Bohle iridovirus (BIV) sample from Oceania. The analyzed sample presented several rearrangements compared to FV3 reference samples from North America and European continent. We report for the first time the complete genome of Ranavirus FV3 isolated from South America, these results contribute to a greater knowledge related to evolutionary events of potentially lethal infectious agent for cold-blooded animals.

A transmembrane domain of Andrias davidianus ranavirus 13R is crucial for co-localization to endoplasmic reticulum and viromatrix

13R, a core gene of Andrias davidianus ranavirus (ADRV), encoded a protein containing a transmembrane domain (TMD) and a restriction endonuclease-like domain. However, the characterization and function of 13R and the protein it encodes remain unclear. In this study, Chinese giant salamander thymus cell (GSTC) was used to investigate the function of 13R. The results showed that the 13R transcripts were detected first at 8 h post-infection (hpi) by RT-PCR and the protein was detected first at 24 hpi by western blot, but the transcription was inhibited by cycloheximide and cytosine arabinofuranoside, indicating that 13R is a viral late gene. Subcellular localization showed that the 13R was co-localized with endoplasmic reticulum (ER) in the cytoplasm, while 13R deleting TMD (13RΔTM) was distributed in cytoplasm and nucleus. During ADRV infection, 13R was observed first in the cytoplasm and nucleus, and later aggregated into the viromatrix, whereas 13RΔTM remain dispersed in cytoplasm and nucleus. Western blot analysis suggested that 13R was a viral non-structural protein and its overexpression did not affect the viral titer in GSTC. All these indicated that the TMD of 13R is crucial for the co-localization into the ER and the viromatrix.

Frog Virus 3 Genomes Reveal Prevalent Recombination between Ranavirus Lineages and Their Origins in Canada

Ranaviruses are pathogens associated with the decline of amphibian populations across much of their distribution. In North America, frog virus 3 (FV3) is a widely distributed pathogen with wild populations of amphibians harboring different lineages and putative recombinants between FV3 and common midwife toad virus (CMTV). These recombinants have higher pathogenicity, and CMTV-derived genes associated with virulence are reported in wild strains in Canada. However, while FV3 is linked to amphibian die-offs in North America, CMTVs have been reported only in commercial frog farms in North America. We sequenced complete genomes of 18 FV3 isolates from three amphibian species to characterize genetic diversity of the lineages in Canada and infer possible recombinant regions. The 18 FV3 isolates displayed different signals of recombination, varying from none to interspersed recombination with previously isolated CMTV-like viruses. In general, most recombination breakpoints were located within open reading frames (ORFs), generating new ORFs and proteins that were a mixture between FV3 and CMTV. A combined spatial and temporal phylogeny suggests the presence of the FV3 lineage in Canada is relatively contemporary (<100 years), corroborating the hypothesis that both CMTV- and FV3-like viruses spread to North America when the international commercial amphibian trade started. Our results highlight the importance of pathogen surveillance and viral dynamics using full genomes to more clearly understand the mechanisms of disease origin and spread.IMPORTANCE Amphibian populations are declining worldwide, and these declines have been linked to a number of anthropogenic factors, including disease. Among the pathogens associated with amphibian mortality, ranaviruses have caused massive die-offs across continents. In North America, frog virus 3 (FV3) is a widespread ranavirus that can infect wild and captive amphibians. In this study, we sequenced full FV3 genomes isolated from frogs in Canada. We report widespread recombination between FV3 and common midwife toad virus (CMTV). Phylogenies indicate a recent origin for FV3 in Canada, possibly as a result of international amphibian trade.

Immune gene expression in gilthead seabream (Sparus aurata) after Lymphocystis disease virus (LCDV-Sa) challenge resulting in asymptomatic infection

AIMS

To determine the immune gene expression response of gilthead seabream (Sparus aurata) that is experimentally infected with the lymphocystivirus LCDV-Sa.

METHODS AND RESULTS

Viral DNA and transcripts were detected by qPCR in all samples from fish injected with LCDV-Sa, demonstrating that the virus establish a systemic and asymptomatic infection. The expression of 23 immune-related genes was also analysed by RT-qPCR in the head kidney (HK) and intestine at several times post-infection (dpi). In HK, the expression of five type I interferon (IFN)-related genes (ifn, irf3, mx2, mx3 and isg15), il10 and ck10 was upregulated at 1-3 dpi, while genes related to the inflammation process (tnfα, il1ß, il6, casp1) were not differentially expressed or even downregulated. The expression profile in the intestine was different regarding type I INF-related genes. An upregulated c3 and ighm expression was observed in both HK and intestine at 3-8 dpi. Finally, the transcription of nccrp1 and mhcIIα was induced in HK, whereas tcrβ expression was downregulated in both organs.

CONCLUSIONS

LCDV-Sa seems to trigger an immune response in gilthead seabream characterized by a partial activation of type I IFN system and a lack of systemic inflammatory response which may be related to viral persistence.

SIGNIFICANCE AND IMPACT OF THE STUDY

The immune response observed in gilthead seabream infected by LCDV-Sa could be implicated in the establishment of an asymptomatic persistent infection.

Viral Hormones: Expanding Dimensions in Endocrinology

Viruses have developed different mechanisms to manipulate their hosts, including the process of viral mimicry in which viruses express important host proteins. Until recently, examples of viral mimicry were limited to mimics of growth factors and immunomodulatory proteins. Using a comprehensive bioinformatics approach, we have shown that viruses possess the DNA/RNA with potential to encode 16 different peptides with high sequence similarity to human peptide hormones and metabolically important regulatory proteins. We have characterized one of these families, the viral insulin/IGF-1-like peptides (VILPs), which we identified in four members of the Iridoviridae family. VILPs can bind to human insulin and IGF-1 receptors and stimulate classic postreceptor signaling pathways. Moreover, VILPs can stimulate glucose uptake in vitro and in vivo and stimulate DNA synthesis. DNA sequences of some VILP-carrying viruses have been identified in the human enteric virome. In addition to VILPs, sequences with homology to 15 other peptide hormones or cytokines can be identified in viral DNA/RNA sequences, some with a very high identity to hormones. Recent data by others has identified a peptide that resembles and mimics α-melanocyte-stimulating hormone's anti-inflammatory effects in in vitro and in vivo models. Taken together, these studies reveal novel mechanisms of viral and bacterial pathogenesis in which the microbe can directly target or mimic the host endocrine system. These findings also introduce the concept of a system of microbial hormones that provides new insights into the evolution of peptide hormones, as well as potential new roles of microbial hormones in health and disease.

Protective immunity induced by DNA vaccine encoding viral membrane protein against SGIV infection in grouper

Singapore grouper iridovirus (SGIV) is the main grouper-infecting virus in southern China that causes serious economic losses. However, there is no effective way to control this viral disease. In this study, SGIV ORF19R (SGIV-19R) encoding a viral membrane protein was constructed into pcDNA3.1-HA and then was used to evaluate the immune protective effects in grouper Epinephelus coioides. Subcellular localization showed that SGIV-19R distributed in the cytoplasm and co-localization analysis indicated the protein partially co-localized with the endoplasmic reticulum (ER). RT-PCR and Western blot analyses confirmed the expression of the vaccine plasmids in grouper muscle tissues. Moreover, the transcription levels of tumor necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β), myxovirus resistance 1 (Mx1) and immunoglobulin M (IgM) genes were significantly up-regulated in the spleen, liver and kidney of vaccinated groupers. SGIV challenge experiments showed the relative percent survival (RPS) was significantly enhanced in fish with 49.9% at the DNA dose of 45 μg pcDNA3.1-19R, while 75.0% RPS when using 90 μg pcDNA3.1-19R. Meanwhile, vaccination with pcDNA3.1-19R significantly reduced the virus replication, evidenced by a low viral load in the spleen of survivals groupers after SGIV challenge. These results imply that pcDNA3.1-19R could induce protective immunity in grouper, and might be a potential vaccine candidate for controlling SGIV disease.

Detection and Characterization of Invertebrate Iridoviruses Found in Reptiles and Prey Insects in Europe over the Past Two Decades

Invertebrate iridoviruses (IIVs), while mostly described in a wide range of invertebrate hosts, have also been repeatedly detected in diagnostic samples from poikilothermic vertebrates including reptiles and amphibians. Since iridoviruses from invertebrate and vertebrate hosts differ strongly from one another based not only on host range but also on molecular characteristics, a series of molecular studies and bioassays were performed to characterize and compare IIVs from various hosts and evaluate their ability to infect a vertebrate host. Eight IIV isolates from reptilian and orthopteran hosts collected over a period of six years were partially sequenced. Comparison of eight genome portions (total over 14 kbp) showed that these were all very similar to one another and to an earlier described cricket IIV isolate, thus they were given the collective name lizard-cricket IV (Liz-CrIV). One isolate from a chameleon was also subjected to Illumina sequencing and almost the entire genomic sequence was obtained. Comparison of this longer genome sequence showed several differences to the most closely related IIV, Invertebrateiridovirus6 (IIV6), the type species of the genus Iridovirus, including several deletions and possible recombination sites, as well as insertions of genes of non-iridoviral origin. Three isolates from vertebrate and invertebrate hosts were also used for comparative studies on pathogenicity in crickets (Gryllusbimaculatus) at 20 and 30 °C. Finally, the chameleon isolate used for the genome sequencing studies was also used in a transmission study with bearded dragons. The transmission studies showed large variability in virus replication and pathogenicity of the three tested viruses in crickets at the two temperatures. In the infection study with bearded dragons, lizards inoculated with a Liz-CrIV did not become ill, but the virus was detected in numerous tissues by qPCR and was also isolated in cell culture from several tissues. Highest viral loads were measured in the gastro-intestinal organs and in the skin. These studies demonstrate that Liz-CrIV circulates in the pet trade in Europe. This virus is capable of infecting both invertebrates and poikilothermic vertebrates, although its involvement in disease in the latter has not been proven.

Ranaviruses Bind Cells from Different Species through Interaction with Heparan Sulfate

Ranavirus cross-species infections have been documented, but the viral proteins involved in the interaction with cell receptors have not yet been identified. Here, viral cell-binding proteins and their cognate cellular receptors were investigated using two ranaviruses, Andrias davidianus ranavirus (ADRV) and Rana grylio virus (RGV), and two different cell lines, Chinese giant salamander thymus cells (GSTC) and Epithelioma papulosum cyprinid (EPC) cells. The heparan sulfate (HS) analog heparin inhibited plaque formation of ADRV and RGV in the two cell lines by more than 80% at a concentration of 5 μg/mL. In addition, enzymatic removal of cell surface HS by heparinase I markedly reduced plaque formation by both viruses and competition with heparin reduced virus-cell binding. These results indicate that cell surface HS is involved in ADRV and RGV cell binding and infection. Furthermore, recombinant viral envelope proteins ADRV-58L and RGV-53R bound heparin-Sepharose beads implying the potential that cell surface HS is involved in the initial interaction between ranaviruses and susceptible host cells. To our knowledge, this is the first report identifying cell surface HS as ranavirus binding factor and furthers understanding of interactions between ranaviruses and host cells.