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Although vertical and venereal transmission of bluetongue is possible, only to the presence of competent insect vectors influences the epidemiology of BTV<sup>2</sup>. 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, 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 infection<sup>1</sup>.
Although vertical and venereal transmission of bluetongue is possible, only to the presence of competent insect vectors influences the epidemiology of BTV<sup>2</sup>. 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, 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 infection<sup>1</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