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==Aetiology==

==Aetiology==

Bluetongue virus is a species of the genus Orbivirus, within the Reoviridae family. The BTV is non-enveloped, and possesses a double-stranded RNA genome within an icosohedral capsid. The genome is arranged into 10 segments and encodes 7 structural and 4 non-structural viral proteins². Since these viruses have dsRNA genomes, replication occurs exclusively in the cytoplasm and the virus encodes several proteins which are needed for replication and conversion of the dsRNA genome into (+)-RNAs. The virus can enter the host cell via a receptor on the cell surface. The receptor is not known but is thought to include sialic acid and junctional adhesion molecules (JAMs). The virus is partially uncoated by proteases in the endolysosome, where the capsid is partially digested to allow further cell entry. The core particle then enters the cytoplasm by a yet unknown process where the genome is transcribed conservatively causing an excess of (+) sense strands, which are used as mRNA templates to synthesize (-) sense strands. Viral particles begin to assemble in the cytoplasm 6–7 hours after infection.

Bluetongue virus is a species of the genus Orbivirus, within the Reoviridae family. The BTV is non-enveloped, and possesses a double-stranded RNA genome contained in an icosohedral capsid. The genome is arranged into 10 segments and encodes 7 structural and 4 non-structural viral proteins². The BTV receptor is currently unknown, but is proposed to included sialic acid and junctional adhesion molecules. After interaction with this receptor, the virus enters an endolysosome where the capsid is partially digested to allow the genome into the cell. The dsRNA genome is then transcribed conservatively, producing positive-sense RNA strands which are both translated and used as templates for produing the negative-sense strand of the genome.  

The 24 distinct serotypes of BTV are distinguished by epitopes on the outer capsid protein VP2, although VP5 also can influence neutralization through its conformational influence on VP2 [11]. The L2 gene, which encodes VP2, is the only serotype-specific BTV gene and there is considerable variation amongst all 10 genome segments of field strains of BTV within endemic areas such as California [15,25]. This variation of BTV genes in field strains of the virus has arisen as a consequence of both drift and reassortment of individual viral genes. Reassortment of BTV genes has been demonstrated after infection of either the ruminant host or insect vector with different strains or serotypes of BTV [29,30]. Individual BTV gene segments evolve and reassort independently of serotype in the field. Genetic drift of individual BTV genes occurs by the selective acquisition and amplification in vector insects of specific variants from the quasispecies virus population that arises in the blood of infected ruminants (founder effect; 6,7).

The 24 distinct serotypes of BTV are distinguished by epitopes on the outer capsid protein VP2, although VP5 also can influence neutralization through its conformational influence on VP2 [11]. The L2 gene, which encodes VP2, is the only serotype-specific BTV gene and there is considerable variation amongst all 10 genome segments of field strains of BTV within endemic areas such as California [15,25]. This variation of BTV genes in field strains of the virus has arisen as a consequence of both drift and reassortment of individual viral genes. Reassortment of BTV genes has been demonstrated after infection of either the ruminant host or insect vector with different strains or serotypes of BTV [29,30]. Individual BTV gene segments evolve and reassort independently of serotype in the field. Genetic drift of individual BTV genes occurs by the selective acquisition and amplification in vector insects of specific variants from the quasispecies virus population that arises in the blood of infected ruminants (founder effect; 6,7).