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→Aetiology
BTV is the prototype virus of the genus Orbivirus in the family Reoviridae [17]. The BTV genome includes 10 segments of double-stranded RNA, each of which encodes at least 1 viral protein (7 structural and 4 non-structural). 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).
BTV is the prototype virus of the genus Orbivirus in the family Reoviridae [17]. The BTV genome includes 10 segments of double-stranded RNA, each of which encodes at least 1 viral protein (7 structural and 4 non-structural). 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).
2.1 BTV belongs to the Orbivirus genus of the Reoviridae family. Thus far 24
serotypes are recognised. There are numerous strains - each isolate is a different strain
based on molecular analysis, regardless of serotype. The virulence of BTV strains
varies considerably. However, other factors also influence the severity of the disease in
sheep, including breed, age, exposure of animals to sunlight, walking on rough ground
and stress.
2.5 The serotypes are differentiated by serum neutralisation tests, but there are
cross-reactions between some serotypes. All BTV’s share group antigens, which can
be demonstrated by agar gel diffusion tests, fluorescent antibody tests and the group
reactive ELISA.
2.6 Several other Orbiviruses have been loosely termed ‘bluetongue-related’
viruses because of serological and other relationships to BTV. The only such viruses
known to be pathogenic for livestock are some members of the epizootic haemorrhagic
disease of deer (EHD) serogroup and the Palyam serogroup of Reoviridae.
==Hosts==
==Hosts==