A new aquareovirus causing high mortality in farmed Atlantic halibut fry in Norway

Isolation and characterization of hirame aquareovirus (HAqRV): A new Aquareovirus isolated from diseased hirame Paralichthys olivaceus

We isolated a novel Aquareovirus (hirame aquareovirus: HAqRV) from Japanese flounder Paralichthys olivaceus suffering from reovirus-like infection. In electron microscopy, the spherical virion (75 nm in diameter) was observed with multi-layered capsid structure. The viral genome consisted of 11 segments and regions encoding 7 virion structural proteins and 5 non-structural proteins were predicted. The deduced amino acid sequences of those proteins were highly similar to those of the aquareoviruses. However, the similarity of complete genome sequence between the HAqRV and other aquareoviruses was less than 60%. Phylogenetic analyses based on the deduced amino acid sequences suggested that the HAqRV is not classified into the known species of Aquareovirus. Pathogenicity of HAqRV was clearly demonstrated in accordance with Koch's postulates by experimental infection using Japanese flounder. The results suggest that the HAqRV is a new Aquareovirus species which is highly virulent for the Japanese flounder at early life stages.

Complete genome sequence and phylogenetic analysis of a novel aquareovirus isolated from a diseased marbled eel (Anguilla marmorata)

Marbled eel reovirus (MERV) is an aquareovirus (AQRV) isolated from diseased marbled eels (Anguilla marmorata) with petechial skin hemorrhage. In this study, we propagated MERV in a cell line derived from the brain of Aequidens rivulatus and purified viral particles by using a discontinuous cesium chloride gradient. Genomic RNA sequences were obtained through next-generation sequencing. MERV, similar to most other AQRVs, showed the presence of 11 double-stranded RNA segments encoding 12 proteins; however, the genome sequence displayed very little similarity to known AQRV sequences. Furthermore, the structural proteins of MERV were most closely related to American grass carp reovirus with sequence identity values of no more than 64.89%. Phylogenetic analysis based on the sequences of structural proteins indicated that MERV shows an evolutionary history between AQRV-B and -G, which belong to the saline and freshwater environment subgroups, respectively. We also observed that MERV showed a closer relationship to orthoreoviruses based on the protein sequences of NS38 and NS73. In summary, MERV is a novel AQRV that could be classified as a member of the new proposed AQRV species "Aquareovirus H". The taxonomic assignments and evolution of AQRVs thus warrant further investigation.

Fusogenic Reoviruses and Their Fusion-Associated Small Transmembrane (FAST) Proteins

With no limiting membrane surrounding virions, nonenveloped viruses have no need for membrane fusion to gain access to intracellular replication compartments. Consequently, nonenveloped viruses do not encode membrane fusion proteins. The only exception to this dogma is the fusogenic reoviruses that encode fusion-associated small transmembrane (FAST) proteins that induce syncytium formation. FAST proteins are the smallest viral membrane fusion proteins and, unlike their enveloped virus counterparts, are nonstructural proteins that evolved specifically to induce cell-to-cell, not virus-cell, membrane fusion. This distinct evolutionary imperative is reflected in structural and functional features that distinguish this singular family of viral fusogens from all other protein fusogens. These rudimentary fusogens comprise specific combinations of different membrane effector motifs assembled into small, modular membrane fusogens. FAST proteins offer a minimalist model to better understand the ubiquitous process of protein-mediated membrane fusion and to reveal novel mechanisms of nonenveloped virus dissemination that contribute to virulence.

Isolation, characterization and molecular identification of a novel aquareovirus that infects endangered fountain darter Etheostoma fonticola

The fountain darter Etheostoma fonticola (FOD) is a federally endangered fish listed under the US Endangered Species Act. Here, we identified and characterized a novel aquareovirus isolated from wild fountain darters inhabiting the San Marcos River. This virus was propagated in Chinook salmon embryo (CHSE)-214, rainbow trout gonad-2 and fathead minnow cells at 15°C. The epithelioma papulosum cyprini cell line was refractory at all temperatures evaluated. High throughput sequencing technologies facilitated the complete genome sequencing of this virus utilizing ribosomal RNA-depleted RNA extracted from infected CHSE-214 cells. Conventional PCR primer sets were developed for the detection and confirmation of this virus to assist diagnostic screening methods. Phylogenetic analysis suggests this virus belongs to the Aquareovirus A genus. This research provides requisite initial data critical to support hatchery and refugia biosecurity measures for this endangered species.

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.

Z-FA-FMK demonstrates differential inhibition of aquatic orthoreovirus (PRV), aquareovirus (CSRV), and rhabdovirus (IHNV) replication

Benzyloxycarbonyl-phenylalanyl-alanyl-fluoromethyl ketone (Z-FA-FMK) is a protease inhibitor that has been shown to strongly inhibit mammalian orthoreovirus replication. Here we explore the ability of Z-FA-FMK to inhibit three important yet genetically discrete aquatic fish viruses: chum salmon aquareovirus (CSRV), piscine orthoreovirus (PRV), and the rhabdovirus infectious hematopoietic necrosis virus (IHNV). Z-FA-FMK significantly attenuated CSRV in vitro transcription and infectious yield following low-dose (2-20μM) exposure, yet a relatively high dose (200μM) was required to completely block CSRV replication. For PRV and IHNV, no significant attenuation of in vitro viral transcription was observed following low-dose (2-20μM) exposure; and although high dose (200μM) exposure significantly attenuated both PRV and IHNV transcription, neither was completely inhibited. These transcriptional results were similarly reflected in IHNV infectious titre observed at 7days post exposure. PRV titre is currently undeterminable in vitro; however, in vivo intra-peritoneal injection of PRV into juvenile Atlantic salmon (Salmo salar) in conjunction with 1.5mg/kg Z-FA-FMK did not affect PRV replication as measured by blood associated viral transcripts at 14days post challenge. These results indicate that aquatic ortho- and aqua-reoviruses appear to possess resilience to Z-FA-FMK relative to mammalian orthoreoviruses and suggest that environmental parameters or alternative mechanisms for viral replication may affect the efficacy of Z-FA-FMK as an antireoviral compound. Further, as Z-FA-FMK has been shown to irreversibly inhibit cysteine proteases such as cathepsins B and L in vitro at concentrations of ≤100μM, continued replication of IHNV (and possibly PRV) at 200μM Z-FA-FMK suggests that replication of these viruses can occur in a cathepsin-independent manner whereas CSRV likely requires cathepsins or similar cysteine proteases for successful replication.

Complete Genome Sequence of a Novel Aquareovirus That Infects the Endangered Fountain Darter, Etheostoma fonticola

Here, we report the complete genome of a novel aquareovirus isolated from clinically normal fountain darters, Etheostoma fonticola, inhabiting the San Marcos River, Texas, USA. The complete genome consists of 23,958 bp consisting of 11 segments that range from 783 bp (S11) to 3,866 bp (S1).

Syncytial Hepatitis of Tilapia ( Oreochromis niloticus L.) is Associated With Orthomyxovirus-Like Virions in Hepatocytes

Using transmission electron microscopy (TEM), the presented work expands on the ultrastructural findings of an earlier report on "syncytial hepatitis," a novel disease of tilapia (SHT). Briefly, TEM confirmed the presence of an orthomyxovirus-like virus within the diseased hepatocytes but not within the endothelium. This was supported by observing extracellular and intracellular (mostly intraendosomal), 60-100 nm round virions with a trilaminar capsid containing up to 7 electron-dense aggregates. Other patterns noted included enveloped or filamentous virions and virion-containing cytoplasmic membrane folds, suggestive of endocytosis. Patterns atypical for orthymyxovirus included the formation of syncytia and the presence of virions within the perinuclear cisternae (suspected to be the Golgi apparatus). The ultrastructural morphology of SHT-associated virions is similar to that previously reported for tilapia lake virus (TiLV). A genetic homology was investigated using the available reverse transcriptase polymerase chain reaction (RT-PCR) probes for TiLV and comparing clinically sick with clinically normal fish and negative controls. By RT-PCR analysis, viral nucleic acid was detected only in diseased fish. Taken together, these findings strongly suggest that a virus is causally associated with SHT, that this virus shares ultrastructural features with orthomyxoviruses, and it presents with partial genetic homology with TiLV (190 nucleotides).

Grass carp reovirus NS26 interacts with cellular lipopolysaccharide-induced tumor necrosis factor-alpha factor, LITAF

The nonstructural protein NS26 of grass carp reovirus (GCRV) is encoded by the 11th genomic dsRNA segment, homolog of which is not found in orthoreoviruses. The role of NS26 in GCRV pathogenesis is still unclear. Previously, grass carp LITAF/SIMPLE protein was identified as a putative binding partner for NS26 in a yeast two-hybrid screen. Here, we further characterized the association between NS26 and LITAF using in vivo and in vitro protein interaction assays. Soluble GST-NS26 and His₆-LITAF were expressed and purified from E. coli; recombinant NS26 tagged with myc and LITAF tagged with GFP were expressed in Ctenopharyngon idellus kidney cells (CIK) by transient transfection experiments. A GST pulldown assay demonstrated that GST-tagged NS26 efficiently bound to His₆-LITAF. Co-immunoprecipitation assays demonstrated that GCRV NS26 reciprocally precipitated endogenous LITAF in CIK cells. Double-immunofluorescent analyses revealed myc-NS26 colocalized with GFP-LITAF in CIK cells. Taken together, the current in vitro and in vivo data demonstrated the interaction between cellular LITAF and GCRV NS26.

Whole-Genome Analysis of a Novel Fish Reovirus (MsReV) Discloses Aquareovirus Genomic Structure Relationship with Host in Saline Environments

Aquareoviruses are serious pathogens of aquatic animals. Here, genome characterization and functional gene analysis of a novel aquareovirus, largemouth bass Micropterus salmoides reovirus (MsReV), was described. It comprises 11 dsRNA segments (S1–S11) covering 24,024 bp, and encodes 12 putative proteins including the inclusion forming-related protein NS87 and the fusion-associated small transmembrane (FAST) protein NS22. The function of NS22 was confirmed by expression in fish cells. Subsequently, MsReV was compared with two representative aquareoviruses, saltwater fish turbot Scophthalmus maximus reovirus (SMReV) and freshwater fish grass carp reovirus strain 109 (GCReV-109). MsReV NS87 and NS22 genes have the same structure and function with those of SMReV, whereas GCReV-109 is either missing the coiled-coil region in NS79 or the gene-encoding NS22. Significant similarities are also revealed among equivalent genome segments between MsReV and SMReV, but a difference is found between MsReV and GCReV-109. Furthermore, phylogenetic analysis showed that 13 aquareoviruses could be divided into freshwater and saline environments subgroups, and MsReV was closely related to SMReV in saline environments. Consequently, these viruses from hosts in saline environments have more genomic structural similarities than the viruses from hosts in freshwater. This is the first study of the relationships between aquareovirus genomic structure and their host environments.

First Description of a New Disease in Rainbow Trout (Oncorhynchus mykiss (Walbaum)) Similar to Heart and Skeletal Muscle Inflammation (HSMI) and Detection of a Gene Sequence Related to Piscine Orthoreovirus (PRV)

In fall 2013, anorexia, lethargy and mortalities up to 10-12,000 dead fish per week were observed in rainbow trout Oncorhynchus mykiss in three fresh water hatcheries (salinity 0-1 ‰) on the west coast of Norway. The fish (25-100 g) showed signs of circulatory failure with haemorrhages, ascites and anaemia. The histopathological findings comprised inflammation of the heart and red muscle and liver necrosis. The affected fish had a common origin. Disease and mortalities were also observed up to four months after sea water transfer. Microbiological examination did not reveal presence of any known pathogens. Based on histopathological similarities to heart and skeletal inflammation (HSMI) in Atlantic salmon, associated with piscine orthoreovirus (PRV), extended investigations to detect a virus within the family Reoviridae were conducted. By the use of primer sets targeting the PRV genome, a sequence with 85% identity to a part of segment S1 of PRV was obtained. Further analysis showed that the virus sequence could only be aligned with PRV and no other reoviruses both on amino acid and nucleotide level. Two PCR assays were developed for specific detection of the virus. High amounts of the virus were detected in diseased fish at all affected farms and low amounts were detected in low prevalence at the broodfish farms. Further investigations are needed to determine if the virus is associated with the new disease in rainbow trout and to further characterize the virus with respect to classification, relationship with PRV, virulence, pathology and epidemiology.