Betanodavirus of marine and freshwater fish: distribution, genomic organization, diagnosis and control measures.

The family Nodaviridae include the genera Alphanodavirus and the Betanodavirus which are non-enveloped, single stranded RNA viruses. Alphanodavirus include the insect viruses while betanodavirus include species that are responsible for causing disease outbreaks in hatchery-reared larvae and juveniles of a wide variety of marine and freshwater fish throughout the world and has impacted fish culture over the last decade. According to International Committee on Taxonomy of Viruses, the genus Betanodavirus comprises four recognized species viz barfin flounder nervous necrosis virus, red-spotted grouper nervous necrosis virus (RGNNV), striped jack nervous necrosis virus and tiger puffer nervous necrosis virus with the RGNNV being the most common. The viruses are distributed worldwide having been recorded in Southeast Asia, Mediterranean countries, United Kingdom, North America and Australia. The disease has been reported by different names such as viral nervous necrosis, fish encephalitis, viral encephalopathy and retinopathy by various investigators. The virus is composed of two segments designated RNA1 and RNA2 and sometimes possesses an additional segment designated RNA3. However, genome arrangement of the virus can vary from strain to strain. The virus is diagnosed by microscopy and other rapid and sensitive molecular methods as well as immunological assays. Several cell lines have been developed for the virus propagation and study of infection mechanism. Control of nodavirus infection is a serious issue in aquaculture industry since it is responsible for huge economic losses. In combination with other management practices, vaccination of fish would be a useful strategy to control the disease.