Changes

===Epidemiology===

===Epidemiology===

When ''Cuilicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their life². The global distribution of BTV is related only to these competent insect vectors^,6: although vertical and venereal transmission of bluetongue is possible, these routes do not influence the epidemiology of BTV.

When ''Cuilicoides'' feed on a bluetongue-infected host, they become persistently infected with the virus for the duration of their life⁴. 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 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.

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.