Betanodaviruses, the causative agents of viral nervous necrosis in marine fish, have bipartite positive-sense RNA genomes. Because the genomes are the smallest and simplest among viruses, betanodaviruses have been well studied using a reversed genetics system as model viruses. However, studies of virus-host interactions have progressed slowly because permissive hosts for betanodaviruses (basically larvae and juveniles of marine fish) are only available for limited periods of the year and are not suitable for the construction of a genetic engineering system. To obtain a model fish species that are not subject to these problems, 21 freshwater fish species were injected intramuscularly with a betanodavirus (redspotted grouper nervous necrosis virus) and tested for their susceptibility to the virus. Based on their responses, the tested fish were classified into 3 groups: 4 susceptible fish, 10 less susceptible fish, and 7 resistant fish. The susceptible fish, celebes rainbowfish Telmatherina ladigesi, threadfin rainbowfish Iriatherina werneri, dwarf rainbowfish Melanotaenia praecox, and medaka Oryzias latipes, exhibited erratic swimming and eventually died within 10 d post-inoculation. The virus was specifically localized in the brains, spinal cords, and retinas of the infected fish, similar to the pattern of infection in naturally infected marine fish. We believe that these susceptible freshwater fish species could act as good host models for betanodavirus-fish interaction studies. KEY WORDS: Betanodavirus · Freshwater fish · Model fish · Experimental infection · Medaka · Rainbowfish Resale or republication not permitted without written consent of the publisherDis Aquat Org 77: [119][120][121][122][123][124][125] 2007 tex and tiger puffer Takifugu rubripes, respectively, whereas BFNNV has been isolated from some coldwater species, such as barfin flounder Verasper moseri, Pacific cod Gadus macrocephalus, and Atlantic halibut Hippoglossus hippoglossus. RGNNV has a broad host range and causes disease among a variety of warm water fish species, particularly groupers and sea bass (Munday et al. 2002).Betanodaviruses have key features as model viruses, as is also the case with the insect alphanodaviruses (Ball & Johnson 1998), the other genus of the family Nodaviridae. Betanodaviruses have one of the smallest genomes (4.5 kb in total) among the known viruses, which encode only 3 viral proteins (protein A, CP, and protein B2). Furthermore, a genetic engineering system has already been established based on a cDNAmediated infectious RNA transcription strategy (Iwamoto et al. 2001(Iwamoto et al. , 2004. Qualitative and quantitative analysis of virus multiplication is possible with betanodaviruses, using cultured cells (Frerichs et al. 1996, Chi et al. 1999, Iwamoto et al. 1999, 2000. However, studies of virus-host interactions have progressed slowly because of the limited availability of host fish. Betanodaviruses basically infect marine fish larvae and juveniles, which are only available for a few months of the...

Betanodaviruses, the causal agents of viral nervous necrosis in marine fish, have bipartite, positive-sense RNA genomes. As their genomes are the smallest and simplest among viruses, betanodaviruses have been studied in detail as model viruses by using a genetic-engineering system, as has occurred with the insect alphanodaviruses, the other members of the family Nodaviridae. However, studies of virus-host interactions have been limited, as betanodaviruses basically infect marine fish at early developmental stages (larval and juvenile). These fish are only available for a few months of the year and are not suitable for the construction of a reverse-genetics system. To overcome these problems, several freshwater fish species were tested for their susceptibility to betanodaviruses. It was found that adult medaka (Oryzias latipes), a well-known model fish, was susceptible to both Striped jack nervous necrosis virus (the type species of the genus Betanodavirus) and Redspotted grouper nervous necrosis virus (RGNNV), which have different host specificities in marine fish species. Infected medaka exhibited erratic swimming and the viruses were localized specifically in the brain, spinal cord and retina of the infected fish, similar to the pattern of infection in naturally infected marine fish. Moreover, medaka were susceptible to RGNNV at the larval stage. This is the first report of a model virus-model host infection system in fish. This system should facilitate elucidation of the mechanisms underlying RNA virus infections in fish.

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