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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 fed on most abundantly 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>
- 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
When ''Culicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives<sup>4</sup>.
. These
movements are via the agency of viraemic hosts or infected vectors. BTV is therefore
movements are via the agency of viraemic hosts or infected vectors. BTV is therefore
a peripatetic virus and even within its enzootic zones its activity may be envisaged as a
a peripatetic virus and even within its enzootic zones its activity may be envisaged as a