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→Epidemiology
===Epidemiology===
===Epidemiology===
Biting insects of the genus Culicoides transmit BTV. Vector insects become persistently infected with BTV for their entire lifespan after acquiring infection through feeding on a BTV-infected ruminant. Although venereal and vertical transmission of BTV can occur in ruminants, these routes are unimportant to the maintenance of BTV and the distribution of BTV in the world coincides only with that of competent vector insects [16,33]. It has been repeatedly and comprehensively shown that BTV infection of fetal cattle is unimportant to the natural epidemiology of BTV infection. Appropriate climatic conditions are also important in the maintenance of BTV, thus the virus exists in an extensive band that includes tropical, subtropical and temperate regions of the world between latitudes of approximately 40º North and 35º South. Exceptions are western North America (where infection periodically occurs as far north as the Okanagan Valley of British Columbia) and Asia (Kazakhstan and Mongolia), where BTV infection of ruminants can occurs as far as 50º North. The species of vector insects that transmit BTV differ between regions, and are especially poorly characterized 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.
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.
===Impact of Global Warming===
===Impact of Global Warming===