Detection and prevalence of the nonsyncytial American grass carp reovirus Aquareovirus G by quantitative reverse transcriptase polymerase chain reaction

Diversity, evolution, and emergence of fish viruses

The production of aquatic animals has more than doubled over the last 50 years and is anticipated to continually increase. While fish are recognized as a valuable and sustainable source of nutrition, particularly in the context of human population growth and climate change, the rapid expansion of aquaculture coincides with the emergence of highly pathogenic viruses that often spread globally through aquacultural practices. Here, we provide an overview of the fish virome and its relevance for disease emergence, with a focus on the insights gained through metagenomic sequencing, noting potential areas for future study. In particular, we describe the diversity and evolution of fish viruses, for which the majority have no known disease associations, and demonstrate how viruses emerge in fish populations, most notably at an expanding domestic-wild interface. We also show how wild fish are a powerful and tractable model system to study virus ecology and evolution more broadly and can be used to identify the major factors that shape vertebrate viromes. Central to this is a process of virus-host co-divergence that proceeds over many millions of years, combined with ongoing cross-species virus transmission.

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.

Persistent development of adomavirus and aquareovirus in a novel cell line from marbled eel with petechial skin haemorrhage

In Taiwan, a petechial haemorrhage disease associated with mortality has affected marbled eels (Anguilla marmorata). The eels were revealed to be infected with adomavirus (MEAdoV, previously recognized as a polyoma-like virus). In this study, cell line DMEPF-5 was established from the pectoral fin of a diseased eel. DMEPF-5 was passaged >70 times and thoroughly proliferated in L-15 medium containing 2%-15% foetal bovine serum at 20-30°C. Transcripts of neural cell adhesion molecule 1 and nestin genes, and nucleic acids of MEAdoV and a novel reovirus (MERV) in the cells were demonstrated by reverse transcription-polymerase chain reaction analysis. Phylogenetic analysis revealed that the AdoV LO8 proteins mostly relate to adenovirus adenain, whereas MERV is close to American grass carp reovirus in Aquareovirus G, based on a partial VP2 nucleotide sequence. DMEPF-5 cells are susceptible to additional viral infection. Taken together, the marbled eels with the haemorrhagic disease have coinfection with MEAdoV and MERV, and the pathogenic role of MEAdoV and MERV warrants research. DMEPF-5 has gene expression associated with mesenchymal stem and progenitor cells and is the first cell line persistently infected with adomavirus and aquareovirus. DMEPF-5 can facilitate studies of such viruses and haemorrhagic disease.

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.

Preliminary characterization of Tasmanian aquareovirus (TSRV) isolates

In an attempt to determine whether or not genetic variants of the Tasmanian strain of Atlantic salmon aquareovirus (TSRV) exist, 14 isolates of TSRV, originating from various locations in Tasmania, covering a 20-year period (1990-2010), obtained from various host species and tissues, and isolated on different cell lines, were selected for this study. Two categories, termed "typical" and "atypical", of variants of TSRV were identified based on preliminary genotypic and phenotypic characterization carried out on these 14 different isolates. In addition, electron microscopic examination indicated the existence of at least three variants based on viral particle size. Finally, this study demonstrated the existence of at least one new variant of TSRV isolates, other than the more commonly isolated typical TSRV isolates, in farmed Tasmanian Atlantic salmon.

Isolation of the Fathead Minnow Nidovirus from Muskellunge Experiencing Lingering Mortality

In 2011, the Fathead Minnow nidovirus (FHMNV; Genus Bafinivirus, Family Coronaviridae, Order Nidovirales) was isolated from pond-raised juvenile Muskellunge Esox masquinongy suffering from lingering mortality at the Wild Rose Hatchery in Wild Rose, Wisconsin. Moribund Muskellunge exhibited tubular necrosis in the kidneys as well as multifocal coalescing necrotizing hepatitis. The FHMNV was also isolated from apparently healthy juvenile Muskellunge at the Wolf Lake State Fish Hatchery in Mattawan, Michigan. The identity of the two syncytia-forming viruses (designated MUS-WR and MUS-WL from Wild Rose Hatchery and Wolf Lake State Fish Hatchery, respectively) as strains of FHMNV was determined based on multiple-gene sequencing and phylogenetic analyses. The pathogenicity of the MUS-WL FHMNV strain was determined by experimentally infecting naive juvenile Muskellunge through intraperitoneal injection with two viral concentrations (63 and 6.3 × 10(3) TCID50/fish). Both doses resulted in 100% mortality in experimentally infected fish, which exhibited severely pale gills and petechial hemorrhaging in eyes, fins, and skin. Histopathological alterations in experimentally infected fish were observed mainly in the hematopoietic tissues in the form of focal areas of necrosis. Phylogenetic analysis of concatenated partial spike glycoprotein and helicase gene sequences revealed differences between the MUS-WL FHMNV, MUS-WR FHMNV, and two other FHMNV originally isolated from moribund Fathead Minnows Pimephales promelas including the index FHMNV strain (GU002364). Based on a partial helicase gene sequence, a reverse transcriptase PCR assay was developed that is specific to FHMNV. These results give evidence that the risks posed to Muskellunge by FHMNV should be taken seriously. Received May 1, 2015; accepted February 8, 2016.

Development and application of monoclonal antibodies for detection and analysis of aquareoviruses

Monoclonal antibodies (mAbs) play an important role in detection of aquareoviruses. Three mAbs against grass carp reovirus (GCRV) were prepared. Isotyping revealed that all three mAbs were of subclass IgG2b. Western blot assay showed that all three mAbs reacted with GCRV 69 kDa protein (the putative VP5). In addition to the 69 kDa protein of GCRV, mAb 4B6 also recognize a 54 kDa protein. All three mAbs were used for detecting aquareovirus by Western blot assay and indirect immunofluorescence assay (IFA). All of them reacted with GCRV, and mAb 4A3 could also react with turbot Scophthalmus maximus reovirus (SMReV) and largemouth bass Microptererus salmonides reovirus (MsReV). Viral antigens were only observed in the cytoplasm of infected cells. Finally, syncytia formation was observed with light microscopy and fluorescence microscopy using fluorescein labelled 4A3 mAb at various times post-infection. Syncytia were observed at 36 hr post-infection (hpi) by light microscopy and at 12 hpi by fluorescence microscopy. The immunofluorescence based assay allowed earlier detection of virus than observation of virus-induced cytopathic effect (CPE) assay in inoculated cell cultures. The sensitivity and specificity of these mAbs may be useful for diagnosis and monitoring of aquareoviruses.

Phylogenetic analysis of newly isolated grass carp reovirus

Grass carp reovirus (GCRV) is a causative agent of haemorrhagic disease in grass carp that drastically affects grass carp aquaculture. Here we report a novel GCRV isolate isolated from sick grass carp that induces obvious cytopathic effect in CIK cells and name it as GCRV096. A large number of GCRV 096 viral particles were found in the infected CIK cells by electron microscope. The shape, size and the arrangement of this virus were similar to those of grass carp reovirus. With the primers designed according to GCRV 873 genome sequences, specific bands were amplified from sick grass carp and the infected CIK cells. The homology rates among vp4, vp6 and vp7 gene in GCRV 096 and those of some GCRV isolates were over 89%. In this study, the sequences of vp4, vp6 and vp7 were used to analyse sequence variation, phylogenetic relationships and genotypes in twenty five GCRV isolates. The results indicated these twenty five GCRV isolates should be attributed to four genotypes. And there were no obvious characteristics in the geographical distribution of GCRV genotype. The study should provide the exact foundation for developing more effective prevention strategies of grass carp haemorrhagic disease.

Viral diseases in zebrafish: what is known and unknown

Naturally occurring viral infections have the potential to introduce confounding variability that leads to invalid and misinterpreted data. Whereas the viral diseases of research rodents are well characterized and closely monitored, no naturally occurring viral infections have been characterized for the laboratory zebrafish (Danio rerio), an increasingly important biomedical research model. Despite the ignorance about naturally occurring zebrafish viruses, zebrafish models are rapidly expanding in areas of biomedical research where the confounding effects of unknown infectious agents present a serious concern. In addition, many zebrafish research colonies remain linked to the ornamental (pet) zebrafish trade, which can contribute to the introduction of new pathogens into research colonies, whereas mice used for research are purpose bred, with no introduction of new mice from the pet industry. Identification, characterization, and monitoring of naturally occurring viruses in zebrafish are crucial to the improvement of zebrafish health, the reduction of unwanted variability, and the continued development of the zebrafish as a model organism. This article addresses the importance of identifying and characterizing the viral diseases of zebrafish as the scope of zebrafish models expands into new research areas and also briefly addresses zebrafish susceptibility to experimental viral infection and the utility of the zebrafish as an infection and immunology model.