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→Epidemiology
===Epidemiology===
===Epidemiology===
When ''Culicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives<sup>4</sup>. The global distribution of BTV is related only to these competent insect vectors<sup>2</sup>: although vertical and venereal transmission of bluetongue is possible, these routes do not influence the epidemiology of BTV. The vector species varies with location and is poorly characterised in some areas, including northern Europe. However, as described above, it has been demonstratated that ambient temperature has profound effects on vector survival and feeding activity, and the replication of BTV in the insect<sup>6</sup>. Therefore, the relationship between temperature and effective transmission of BTV is likely to be a limiting factor to the spread of BTV, which may potentially be overcome by global warming.
When ''Culicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives<sup>4</sup>. The global distribution of BTV is related only to these competent insect vectors<sup>2</sup>: although vertical and venereal transmission of bluetongue is possible, these routes do not influence the epidemiology of BTV.
Althbough bluetongue virus is capable of infecting any ruminant, cattle are the main amplifying and maintenance hosts, and are fed on most abundantly by ''Culicoides'' vectors.
5.2 BTV is limited to those geographical areas where competent Culicoides vectors
5.2 BTV is limited to those geographical areas where competent Culicoides vectors
are present. Its transmission is limited to those times of the year when the climatic
are present. Its transmission is limited to those times of the year when the climatic
in cattle). Otherwise the last infected vertebrate host will have died or recovered before
in cattle). Otherwise the last infected vertebrate host will have died or recovered before
new vectors arrive on the scene.
new vectors arrive on the scene.
The vector species varies with location and is poorly characterised in some areas, including northern Europe. However, as described above, it has been demonstratated that ambient temperature has profound effects on vector survival and feeding activity, and the replication of BTV in the insect<sup>6</sup>. Therefore, the relationship between temperature and effective transmission of BTV is likely to be a limiting factor to the spread of BTV, which may potentially be overcome by global warming.
Bluetongue virus infection was first confirmed in the UK in September 2007 when a veterinarian spotted suspicious clinical signs on a cattle holding near Ipswich<sup>8</sup>. There are several possibilities for how the virus spread to the British Isles: a) through transport of infected animals; b) by local vectors acquring virus from infected animals (unlikely given the island geography of the UK), or c) via movement of vectors carrying BTV. It is believed that BTV-laden vectors were dispersed to the UK on the wind, since meteorological conditions on 4th August 2007 were capable of carrying midges from northern Europe to East Anglia. This would be expected to produce disease at a point that would coincide with the first case, given the time necessary for clinical detection to occur.
==Pathogenesis==
==Pathogenesis==