Orbivirus

The genus Orbivirus is a member of the Reoviridae family, in the subfamily Sedoreovirinae. Unlike the other reoviruses, orbiviruses are arboviruses. This genus currently contains 22 species (including the type species Bluetongue virus) and at least 130 different serotypes. Orbiviruses can infect and replicate within a wide range of arthropod and vertebrate hosts. Orbiviruses are named after their characteristic doughnut-shaped capsomers (Orbi in Latin means ring). Many orbiviruses are transmitted by ticks or haematophagus insect vectors (Culicoides, mosquitoes and sand flies) and have a wide host range that includes cattle, goats and sheep, wild ruminants, equids, camelids, marsupials, sloths, bats, birds, large canine and feline carnivores, and humans. The three economically most important orbiviruses are Bluetongue virus, African horse sickness virus, and Epizootic hemorrhagic disease virus, all of which are transmitted by Culicoides species. The virons are nonenveloped particles that are 70–80 nm in diameter. The virus particles are spherical in appearance and have icosahedral symmetry. An outer and an inner capsid layer surround the genome, and have T=13 and T=2 symmetry, respectively. The viron is constructed of two concentric protein shells, the subcore layer which contain 120 copies/particle of the VP3 and the core-surface layer composed of 780 copies/particle of the VP7. VP1, VP4, and VP6 are minor enzymatic proteins that are packaged along with the 10 genome segments within the central space of the virus core. The orbivirus outer-capsid layer is composed of two additional structural proteins (VP2 and VP5) which mediate cell-attachment and penetration during initiation of infection. The outer-capsid proteins are more variable than the core proteins and most of the non-structural proteins and the specificity of their reactions with neutralising antibodies determines the virus serotype. These viruses have double-stranded RNA genomes, so are classified as Class III viruses. Their genome is linear and is segmented into 10 segments of various lengths. One copy of each gene segment is packaged per virion. In most cases, each gene segment encodes a single open reading frame (ORF). The genome encodes seven major structural proteins (VP1–VP7) and three major nonstructural proteins (NS1-NS3). Exceptions to the one gene-one protein rule are segment 9 (Seg-9) and segment 10 (Seg-10), both of which encode two nearly identical proteins initiated from in-phase AUG codons close together near the upstream termini (VP6 and VP6a encoded by Seg-9: NS3 and NS3a encoded by Seg-10) An ORF spans almost the entire length of genome segment 9 and encodes VP6 (the viral helicase). A second ORF (OrfX) is also present on this segment and encodes a fourth nonstructural protein (NS4), which was predicted from sequence analysis of various orbiviruses including segment 9 of Great Island virus which contained a long NS4 ORF (around 21kDa). The existence of NS4 was experimentally confirmed in both insect-borne and tick-borne orbiviruses in 2011.