Viruses of the family Nodaviridae, genus Betanodavirus (i.e., betanodaviruses) infect many different marine and freshwater fish species worldwide and are the etiological agents of viral encephalopathy and retinopathy (VER) in fish. This disease, also called viral nervous necrosis (VNN), can cause significant fish mortality.
The betanodaviruses that cause the disease are non-enveloped and their genomes contain two single-stranded, positive-sense RNA molecules, designated RNA1 and RNA2. Infected cells contain three ssRNAs: RNA1, RNA2 and subgenomic RNA3 derived from RNA1. The T4 variable region within the RNA2 segment has been used to classify betanodaviruses into four different genotypes: striped jack nervous necrosis virus (SJNNV), tiger puffer nervous necrosis virus (TPNNV), barfin flounder nervous necrosis virus and red-spotted grouper nervous necrosis virus (RGNNV) (Nishizawa et al. 1997). The NNV part of the name refers to nervous necrosis virus and the initial part of the name refers to the fish from which it was isolated. These have become accepted species, with the Striped jack species being known as the type species, based on the characterisation of the SJNag93 isolate. (Iwamoto et al, 2001) Others which may be members of the genus Betanodavirus, but have not yet been approved as separate species, and may or may not be variants of any of the four accepted species listed above include: Atlantic cod nervous necrosis virus, Atlantic halibut nodavirus, Dicentrarchus labrax encephalitis virus, Dragon grouper nervous necrosis virus, Greasy grouper nervous necrosis virus, Japanese flounder nervous necrosis virus, Lates calcarifer encephalitis virus, Malabaricus grouper nevous necrosis virus, Seabass nervous necrosis virus, Solea senegalensis nervous necrosis virus and Turbot nodavirus. (Virus Taxonomy, 2012).
Different genotypes of betanodaviruses likely infect different host ranges of fish. However, coexistence of SJNNV and RGNNV, within single fish in the wild, has been shown in wild meager (Argyrosomus regius). Genetic analysis based on both RNA1 and RNA2 has also demonstrated the existence of reassortant viruses (RGNNV/SJNNV and SJNNV/RGNNV) in infected fish. Such reassortant virus strains or isolates, containing one genome RNA molecule from SJNNV and one genome RNA molecule from RGNNV, have been isolated from Senegalese sole and gilt-head sea bream. Reassortant strains have been associated with clinical outbreaks of disease. Reassortants could result from coinfection of the same cells with SJNNV and RGNNV. Although this evidence indicates that both SJNNV and RGNNV RNA can coexist in the same infected cells, nothing is known about how this affects replication and/or progeny virus production, or how it affects the ability to cause symptoms of disease.
SJNNV and RGNNV have separately been detected in fish species inhabiting the Iberian Peninsula, such as European sea bass (Dicentrarchus labrax), Senegalese sole (Solea senegalensis), gilt-head seabream (Sparus aurata), red-banded seabream (Pagrus auriga), common seabream (Pagrus pagrus), Shi drum, (Umbrina cirrosa) and white seabream (Diploidus sargus).
The RGNNV genotype appears to be the most common in the Mediterranean region and has been responsible for devastating losses in the fishing industry, especially in Greece. The losses in the field of European sea bass have been reported to be up to 60%. Hence there is an urgent need for ways to protect fish, especially sea bream, from the effects of RGNNV infection.