Changes

===Vector Competence===

===Vector Competence===

Certain information can help inferences be made regarding BTV vectors in Britain. Both ''C. obsoletus'' and ''C. pulicaris'' have been implicated in transmission before. Previously, BTV has been isolated from ''C. obsoletus'' in Cyprus, and African horse sickness virus (another Orbivirus) in Spain. ''C. obsoletus'' and ''C. pulicaris'' were also the most abundant ''Culicoides'' species detrected in the 1999 BTV epizootic in Greece and Bulgaria, and so are strongly suspected of acting as vectors in this case. They may also have mediated outbreaks in Serbia, FYR Macedonia, Croatia and Bosnia in 2001-2002, where ''C. imicola'' has not been recorded.

Certain information can help inferences be made regarding BTV vectors in Britain. Both ''C. obsoletus'' and ''C. pulicaris'' have been implicated in transmission before. Previously, BTV has been isolated from ''C. obsoletus'' in Cyprus, and African horse sickness virus (another Orbivirus) in Spain. ''C. obsoletus'' and ''C. pulicaris'' were also the most abundant ''Culicoides'' species detrected in the 1999 BTV epizootic in Greece and Bulgaria, and so are strongly suspected of acting as vectors in this case. They may also have mediated outbreaks in Serbia, FYR Macedonia, Croatia and Bosnia in 2001-2002, where ''C. imicola'' has not been recorded. Both species are therefore contenders to transmit bluetongue virus in the UK.

3.11 Vector competence studies on a British population of C. obsoletus have

A British population of ''C. obsoletus'' has been shown to have and oral susceptibility rate of less than 2%¹, suggesting that ''C. obsoletus'' is likely to be an inefficient or minor vector of BTV in the UK. However, it is possible that a high abundance or survival rate may compensate for this low vector competence. Indeed, ''C. brevitarsis'', the major Australian vector of BTV, has an extremely low experimental competency yet is an effective vector in the field.

recorded oral susceptibility rates of less than 2% in comparison with a known major

vector C. sonorensis (19.5%). This initially suggested that C. obsoletus is likely to be

only a minor or inefficient vector of BTV. Nevertheless, the high abundance and

survival rates of C. obsoletus as exhibited in Bulgaria in 1999, and as seen on farms

and around stables in South East England, could compensate for its low levels of

vector competence. Observations of cattle exposed to midges have shown up to ten

thousand bites per hour. It should be noted that C. brevitarsis, the major vector of BTV

in Australia, has an experimental competency of only 0.3 percent when feeding on

sheep although it is quite an effective vector in the field.

===Epidemiology===

===Epidemiology===