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

Abstract

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.

Keywords: Betanodavirus; Diagnosis; Disease control; Fish; Genome arrangement; Vaccination.

Fig. 1

Asian seabass (L. calcarifer) juveniles infected with nodavirus. A: Arrow shows dark coloration on the body surface. B & C: Highly emaciated infected fish

Fig. 2

Phylogenetic tree constructed from the variable region of coat protein sequences of betanodaviruses using MegAlign program (Windows version 5.05, DNASTAR, USA). Accession numbers: HM017076, HM017077 [Asian seabass (L. calcarifer), India] are the coat protein sequences generated in our lab and shown in rectanglular box (Shetty et al. unpublished data); EU380202, FJ617262, GQ120525: [Asian seabass (L. calcarifer), Malaysia]; AF245004: [Giant Grouper (E. lanceolatus), Taiwan]; EF591371 and EF591372: [Barramundi (L. calcarifer), Australia]; D38636: [Redspotted grouper (E. akaara), Japan]; EF558369: [Redspotted grouper (E. akaara), China]; AF499774: [Guppy (P. reticulate), Singapore]; AF175518: [Brownspotted Grouper (E. malabaricus), Thailand]; AF175516: [Barramundi (L. calcarifer), Singapore]; AF318942: [Greasy grouper (E. tauvina), Singapore]; D38637 and EU236149: [Tiger puffer (Takifugu rupripes), Japan]; D38635, EU236147 and EU826138: [Barfin flounder (V. moseri), Japan]; AF175519, D30814 and AB056572: [Striped jack (P. dentex), Japan]. The distance between sequences is represented by the length of each pair of branch and the unit below the tree indicates the number of substitution events

Fig. 3

RT–PCR detection of betanodavirus in Asian seabass (L. calcarifer) (Shetty et al. unpublished data) using T4NV-F2/R primer set [89]. M: 100 bp DNA ladder, Genei Bangalore, Lane 1 positive control. Lane 2 Negative control. Lane 3–6 positive samples