| Birnaviridae | |
|---|---|
 | |
| Infectious bursal disease virus particle | |
Virus classification | |
| (unranked): | Virus |
| Realm: | Riboviria |
| Kingdom: | Orthornavirae |
| Phylum: | incertae sedis |
| Family: | Birnaviridae |
Birnaviridae is a family ofdouble-stranded RNA viruses.[1]Salmonid fish, birds and insects serve as natural hosts. There are currently 11 species in this family, divided among seven genera.[2] Diseases associated with this family includeinfectious pancreatic necrosis in salmonid fish, which causes significant losses to the aquaculture industry, with chronic infection in adult salmonid fish and acute viral disease in young salmonid fish.[2][3]
Viruses in familyBirnaviridae arenon-enveloped, with icosahedral single-shelled geometries, and T=13 symmetry. The diameter is around 70 nm.[2][3]
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The genome is composed of linear, bi-segmented, double-stranded RNA. It is around 5.9–6.9 kbp in length and codes for five to six proteins. Birnaviruses encode the following proteins:
RNA-directed RNA polymerase (VP1), which lacks thehighly conserved Gly-Asp-Asp (GDD) sequence, a component of the proposedcatalytic site of this enzyme family that exists in theconserved motif VI of the palm domain of other RNA-directed RNA polymerases.[4]
The large RNA segment, segment A, of birnavirusescodes for apolyprotein (N-VP2-VP4-VP3-C)[5] that is processed into the major structuralproteins of the virion: VP2, VP3 (a minor structural component of the virus), and into the putativeprotease VP4.[5] VP4protein is involved in generating VP2 and VP3.[5]recombinant VP3 is more immunogenic than recombinant VP2.[6]
Infectious pancreatic necrosis virus (IPNV), a birnavirus, is an importantpathogen infish farms. Analyses ofviral proteins showed that VP2 is the majorstructural and immunogenicpolypeptide of the virus.[7][8] Allneutralizingmonoclonal antibodies are specific to VP2 andbind to continuous or discontinuousepitopes. The variable domain of VP2 and the 20 adjacentamino acids of theconservedC-terminal are probably the most important in inducing animmune response for the protection ofanimals.[7]
Nonstructuralprotein VP5 is found in RNA segment A.[citation needed] The function of this small viral protein is unknown. It is believed to be involved in influencingapoptosis, but studies are not completely concurring. The protein can not be found in the virion.[citation needed]
Viral replication is cytoplasmic. Entry into the host cell is achieved by cell receptorendocytosis. Replication follows the double-stranded RNA virus replication model in the cytoplasm. Double-stranded RNA virus transcription is the method of transcription in cytoplasm. The virus is released by budding.Salmonid fish (Aquabirnavirus), young sexually immature chickens (Avibirnavirus), insects (Entomobirnavirus), andblotched snakehead fish (Blosnavirus) are the natural hosts. Transmission routes are contact.[2][3]
The following genera are recognized: