Avibirnavirus  Avibirnavirus is a genus of viruses in family "Birnaviridae". There is currently a single species in this genus, the type species, Infectious bursal disease virus, which infects chickens and other fowl. It causes severe inflammation of the bursa of Fabricius, and causes considerable morbidity and mortality. Initially, the virus was discovered in 1957 in Gumboro, Delaware, United States. Later, the disease was termed Gumboro disease. Since its discovery, the virus has been found to have a worldwide distribution. Avibirnaviruses are non-enveloped, and their single shelled icosahedral capsid exhibits a T=13 symmetry. The diameter of the capsid is around 70 nm. The double-stranded RNA genome of Avibirnavirus is linear and segmented into two segments (A and B). The combined lengths of the two segments is around 6,000 nucleotides, almost entirely composed of reading-frames. The genome codes for 4-5 proteins depending on the strain. The high degree of antigenic variation is due to a combination of high mutation rate and inter-strain homologous recombination within a hypervariable region that is located in the "vp2" gene. At approximately 3,200 bp in length, segment A is the larger of the two segments. It contains two partially overlapping open reading frames (ORFs). The first ORF in segment A encodes viral protein 5 (VP5). The second ORF in segment A encodes a polyprotein, the pVP2-VP4-VP3 precursor, which is cleaved into three proteins . Segment B encodes a single viral protein, VP1. Viral protein 1 is an RNA-dependent RNA polymerase, which cycles between the two segments and aids in the formation of ribonucleoprotein complexes with viral protein 3. Viral protein 2 is a major structural component of the virion. The protein contains three domains: base, shell, and projection. The projection domain consists of four loop structures that are exposed on the virion surface. This protein is responsible for inducing a protective immune response, which neutralize monoclonal antibodies through binding to the project domain. The viral protein in conjuction with VP5 acts as an apoptotic inducer, which causes affected cells to become cytotoxic. Also, together with VP5, it inhibits cellular protein synthesis through activation of the protein kinase R pathway (PKR).