Orbivirus

Orbivirus
Virus classification
Group:	Group III (dsRNA)
Order:	Unassigned
Family:	Reoviridae
Subfamily:	Sedoreovirinae
Genus:	Orbivirus
Type species
Bluetongue virus
Species
African horse sickness virus Bluetongue virus Changuinola virus Chenuda virus Chobar Gorge virus Corriparta virus Epizootic hemorrhagic disease virus Equine encephalosis virus Eubenangee virus Great Island virus Ieri virus Lebombo virus Orungo virus Palyam virus Peruvian horse sickness virus St Croix River virus Umatilla virus Wad Medani virus Wallal virus Warrego virus Wongorr virus Yunnan orbivirus

The genus Orbivirus is a member of the Reoviridae family. This genus currently contains 22 species 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).

The type species is Bluetongue virus.

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.

Virus Particles

The virons are non-enveloped particles that are between 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. The viron is constructed of two concentric protein shells, the sub-core 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 ten 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.

Genome

These viruses have double stranded RNA genomes and therefore 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 7 major structural proteins (VP1-VP7) and 3 major non structural 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 open reading frame spans almost the entire length of genome segment-9 and encodes VP6 (the viral helicase). A second open reading frame (OrfX) is also present on this segment and encodes a fourth non structural protein (NS4).^[1]

NS1 is the most abundant protein in BTV infected cells. It forms tubules that may be involved in translocation of progeny virus particles to the cell membrane. NS2 is phosphorylated by cellular kinases and is an important matrix protein of the granular viral inclusion bodies that form within the cytoplasm of infected cells. These viral inclusion bodies act as the centres of viral replication. The membrane glycoproteins NS3 and NS3a are expressed in large amounts in insect cells but not in mammalian cells. They are involved in the release of progeny virus particles from infected cells and may be involved in determination of both vector competence and virulence.

Replication

Many Orbiviruses preferentially infect vascular endothelial cells. Orbiviruses enter the host cell by endocytosis and the outer capsid is subsequently removed. The whole cycle of viral replication takes place within the cytoplasm of the host cell. Transcription of the viral genome into mRNA occurs within the core particle and mRNA is translated into proteins using the host cell ribosomes. Viral proteins are synthesized 2–14 days after initial infection. New virons self-assemble within the cytoplasm and are then released from the host cell by budding. During the budding process they transiently acquire a lipid envelope which can be detected for a short period of time following their release but this is subsequently lost.

Pathogenesis

Orbiviruses primarily cause diseases in animals. The different Orbivirus species have different host specificities. Orbiviruses are vector-borne pathogens transmitted between vertebrate hosts by vectors such as mosquitoes, midges, gnats, sandflies and ticks. Bluetongue virus (BTV) is an Orbivirus that causes bluetongue disease in sheep, cattle, goats and wild ungulates. BTV has been in the forefront of molecular studies for last three decades and now represents one of the best understood viruses at the molecular and structural levels.^[2][3] Other species of Orbiviruses are responsible for other diseases of animals such as African Horse sickness and Equine encephalosis virus ^[4].

See also

• Animal viruses

References

1. ^ Belhouchet M, Mohd Jaafar F, Firth AE, Grimes JM, Mertens PP, Attoui H (2011) Detection of a fourth orbivirus non-structural protein. PLoS One 6(10):e25697.
2. ^ Roy P (2008). "Molecular Dissection of Bluetongue Virus". Animal Viruses: Molecular Biology. Caister Academic Press. ISBN 978-1-904455-22-6. http://www.horizonpress.com/avir. 
3. ^ Roy P (2008). "Structure and Function of Bluetongue Virus and its Proteins". Segmented Double-stranded RNA Viruses: Structure and Molecular Biology. Caister Academic Press. ISBN 978-1-904455-21-9. http://www.horizonpress.com/rnav. 
4. ^ Viljoen and Huismans (1989) The Characterization of Equine Encephalosis Virus and the Development of Genomic Probes. J. gen. Virol. 70, 2007-2015.

emedicine Orbivirus [1]

Draft of the Orbivirus genus description for the ICTV seventh report [2]

Mertens, (2001) Orbiviruses and Bluetongue virus. In: Encyclopedia of Life Sciences. John Wiley & Sons, Ltd: Chichester. www.els.net [doi:10.1038/npg.els.0001010]

External links

• Viralzone: Orbivirus