Changes

===Epidemiology===

===Epidemiology===

When ''Cuilicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives⁴. 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. As noted above, the species of vector insects differ between regions, but are poorly characterised in Europe and Asia. Recent studies have shown that ambient temperature has a profound effect on the survival of vector insects, their feeding activity, and the replication of BTV in the insect vector [23]. Specifically, insect lifespan is inversely related to temperature, and the replication of BTV in its insect vector increases with temperature. Thus, temperature-dependent control of BTV virogenesis potentially might limit the expansion of BTV into regions outside of its current range, even into areas where apparently competent vector insects occur. Global warming, however, would be predicted to expand the northern and southern extremes of global BTV distribution.

When ''Culicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their lives⁴. 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. The vector species varies with location and is poorly characterised in some areas, including northern Europe. However, as described above, it has been demonstratated that ambient temperature has profound effects on vector survival and feeding activity, and the replication of BTV in the insect⁶. Therefore, the relationship between temperature and effective transmission of BTV is likely to be a limiting factor to the spread of BTV, which may potentially be overcome by global warming.

 

It is increasingly evident that BTV has not recently been spread globally through international trade. Rather, the virus exists in distinct, relatively stable ecosystems in different regions of the world where specific strains of the virus likely have co-evolved with different species of insect vector [17,33]. Thus, in the Americas, the serotypes of BTV that circulate in the United States are different from those in adjacent regions of the Caribbean and Central America, despite the lack of any substantial geographic barrier between the regions. The essential difference lies in the different species of vector insects in the 2 regions: Culicoides sonorensis is the vector of BTV serotypes 10, 11, 13 and 17 in the United States, whereas Culicoides insignis is the vector of BTV serotypes 1, 2, 3, 4, 6, 8, 11, 12, 13, 14 and 17 in the Caribbean and Central/South America. Movement of animals between the 2 regions has not altered the very different constellations of BTV serotypes that occur in each.

It is increasingly evident that BTV has not recently been spread globally through international trade. Rather, the virus exists in distinct, relatively stable ecosystems in different regions of the world where specific strains of the virus likely have co-evolved with different species of insect vector [17,33]. Thus, in the Americas, the serotypes of BTV that circulate in the United States are different from those in adjacent regions of the Caribbean and Central America, despite the lack of any substantial geographic barrier between the regions. The essential difference lies in the different species of vector insects in the 2 regions: Culicoides sonorensis is the vector of BTV serotypes 10, 11, 13 and 17 in the United States, whereas Culicoides insignis is the vector of BTV serotypes 1, 2, 3, 4, 6, 8, 11, 12, 13, 14 and 17 in the Caribbean and Central/South America. Movement of animals between the 2 regions has not altered the very different constellations of BTV serotypes that occur in each.

A variety of other hosts have been implicated in the lifecycle of BTV infection. Serological evidence indicates that large African carnivores are infected with BTV, whereas smaller predators that co-habit with them are not, suggesting that large carnivores are infected through feeding on BTV-infected ruminants [2]. Inadvertent contamination of a canine vaccine with BTV confirmed that dogs are susceptible to BTV infection, indeed pregnant bitches that received this contaminated vaccine typically aborted and died [1]. There is no evidence, however, that dogs or other carnivores are important to the natural cycle of BTV infection.

A variety of other hosts have been implicated in the lifecycle of BTV infection. Serological evidence indicates that large African carnivores are infected with BTV, whereas smaller predators that co-habit with them are not, suggesting that large carnivores are infected through feeding on BTV-infected ruminants [2]. Inadvertent contamination of a canine vaccine with BTV confirmed that dogs are susceptible to BTV infection, indeed pregnant bitches that received this contaminated vaccine typically aborted and died [1]. There is no evidence, however, that dogs or other carnivores are important to the natural cycle of BTV infection.