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===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. Infection of sheep with BTV is therefore usually preceded by widespread infection of cattle and an increase in vector density¹. The global distribution of BTV is related only to these competent insect vectors²: 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 activity¹ - 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 infection¹.

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 density¹.  

 

Although vertical and venereal transmission of bluetongue is possible, only to the presence of competent insect vectors influences the epidemiology of BTV². 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 activity¹. In Britain, transmission occurs mainly in late summer and autumn. Once bluetongue virus is transmitted to a vertebrate host, there are two possible outcome: either the host may die, or an immune response is mounted against the virus and renders the host resistant to re-infection. Either way, animals quickly become "unavailable" for BTV infection as the virus spreads, particularly where livestock populations are small. This presents a hurdle that must be surmounted if bluetongue virus is to persist in an area. By movement of infected vectors or viraemic animals, BTV can become established in new locations with naive hosts in order to overcome this obtascle. This means that even in zones where bluetongue virus is endemic, persistence dynamic and comprises perpetually shifting "hot spot" of infection¹.

When ''Culicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives⁴.  

When ''Culicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives⁴.