|Vacuoles in retina of Australian bass larva affected with NNV|
|Group:||Group IV ((+)ssRNA)|
Betanodavirus or nervous necrosis virus (NNV) is a viral genus classified in the family Nodaviridae. It contains four recognized species: Barfin flounder nervous necrosis virus (BFNNV), Redspotted grouper nervous necrosis virus(RGNNV), Striped Jack nervous necrosis virus (SJNNV) and Tiger puffer nervous necrosis virus (TPNNV), SJNNV being the type species. There are other 12 tentative species in the genus that haven't been recognized by the ICTV. Betanodaviruses cause a disease in fish called viral nervous necrosis (VNN) or viral encephalopathy and retinopathy (VER).
Betanodaviruses are small single-stranded positive sense RNA viruses, non-enveloped and spherical. The average diameter of the viral capsid is only 37 nm. Its RNA strand is split in two sections RNA1 and RNA2 that together account for 3.5 kbp (approx) that encode 3 genes. Betanodavires can be inactivated by contact with 50ppm of chlorine and iodine for 10 min, 50% and 60% of ethanol and methanol respectively, high alkalinities (pH12), heat treatment (60‹›C for 30 min), UV radiation and ozonation. However they are resistant to Chloroform, ether and formalin.
The virus was reported for the first time during the years 1989-1991 when it was associated with high mortalities in young marine fish and was described initially as a picornavirus-like virus. It was discovered almost simultaneously in Australia, Norway, France and Japan. However, the oldest evidence related to the virus dates back to 1984 in Queensland (Australia) where disease outbreaks with the clinical signs and histopathological lesions corresponding to NNV were referenced for the first time.
Betanodavirus is a genus of viruses, in the family Nodaviridae. Fish serve as natural hosts. There are currently four species in this genus including the type species Striped jack nervous necrosis virus. Diseases associated with this genus include: viral encephalopathy and retinopathy.
Viruses in Betanodavirus are non-enveloped, with icosahedral geometries, and T=3 symmetry. The diameter is around 30 nm. Genomes are linear and segmented, bipartite, around 21.4kb in length.
The crystal structure of a betanodavirus- T=3 Grouper nervous necrosis virus (GNNV)-like particle has been determined by X-ray crystallography. The virus-like particle contains 180 subunits of the capsid protein, and each capsid protein (CP) shows three major domains: (i) the N-terminal arm, an inter-subunit extension at the inner surface; (ii) the shell domain (S-domain), a jelly-roll structure; and (iii) the protrusion domain (P-domain) formed by three-fold trimeric protrusions. 
|Genus||Structure||Symmetry||Capsid||Genomic arrangement||Genomic segmentation|
Viral replication is cytoplasmic. Entry into the host cell is achieved by penetration into the host cell. Replication follows the positive stranded RNA virus replication model. Positive stranded RNA virus transcription, using the internal initiation model of subgenomic RNA transcription is the method of transcription. Fish serve as the natural host. Transmission routes are passive diffusion and contact.
|Genus||Host details||Tissue tropism||Entry details||Release details||Replication site||Assembly site||Transmission|
|Betanodavirus||Fish||None||Unknown||Lysis||Cytoplasm||Cytoplasm||Passive diffusion, direct contact|
The most recent common ancestor of the four extant genotypes has been dated to ~1300. An early reassortment event appears to have occurred in the early 1980s in Southern Europe.
Betanodaviruses affect teleost fish. A growing number of species are reported susceptible, one of the latest review of the disease (2009) reported more than 40 species, most of them marine. The disease is more likely to occur in larvae or juveniles but it can also affect adults.
Clinical signs and lesions
Viral nervous necrosis can have a clinical or sub-clinical presentation. Signs include: abnormal behaviour like lethargy, anorexia, spiral swimming; and change in pigmentation. Mortalities of affected populations can be of up to 100%. Microscopical lesions are mostly located in brain, retina and spinal cord where necrosis of the neurons and the presence of round empty spaces called vacoules are commonly associated with the disease.
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