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→Epidemiology
===Epidemiology===
===Epidemiology===
Althbough bluetongue virus is capable of infecting any ruminant, cattle are the main amplifying and maintenance hosts and are most abundantly fed on by ''Culicoides'' vectors. 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. This is illustrated by the fact that bluetongue virus is limited to geographical areas where competent vectors are present and that transmission only occurs at times of the year when conditions are favourably for vector activity<sup>1</sup>
Althbough bluetongue virus is capable of infecting any ruminant, cattle are the main amplifying and maintenance hosts and are most abundantly fed on by ''Culicoides'' vectors. Infection of sheep with BTV is therefore usually preceded by widespread infection of cattle and an increase in vector density<sup>1</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. This is illustrated by the fact that bluetongue virus is limited to geographical areas where competent vectors are present and that transmission only occurs at times of the year when conditions are favourably for vector activity<sup>1</sup> - in Britain, late summer and autumn. When a host becomes infected with BTV, either death ensues or an antibody response is mounted which renders the animal resistant to re-infection. This presents a challenge to persistence of bluetongue virus since animals quickly become "unavailable" for infection, particularly in areas where livestock populations are small. The virus must therefore contintually spread to new locations to find naive hosts, either by movement of viraemic animals or that of infected vectors. This means that even in endemic zones, bluetongue infection is not static and instead comprises perpetually shifting "hot spot" of infection<sup>1</sup>.
When ''Culicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives<sup>4</sup>.
When ''Culicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives<sup>4</sup>.
5.4 The incidence and geographical distribution of BTV infections are determined
5.4 The incidence and geographical distribution of BTV infections are determined
largely by the distribution of insect vectors and this can vary from year to year.
largely by the distribution of insect vectors and this can vary from year to year.