Bluetongue: pathogenesis and duration of viraemia

Bluetongue (BT) is a non-contagious, insect-transmitted disease of domestic and wild ruminants caused by bluetongue virus (BTV). Whereas BTV infection of the haematophagous Culicoides insect vector is persistent (life-long), BTV infection of ruminants is transient. The prolonged viraemia that occurs in many BTV-infected ruminants occurs through a novel interaction of the virus with erythrocytes and, initially, other blood cell types. The presence of BTV in ruminant blood can readily be detected by polymerase chain reaction which provides a very conservative assay for the screening of ruminants prior to movement to BTV-free regions as animals remain positive by PCR assay long after all infectious virus is cleared from their blood. BT disease occurs in sheep and some wild ruminant species and is characterised by vascular injury with haemorrhage, oedema and tissue necrosis. Inherent, species-specific differences in the susceptibility and responses of endothelial cells may be responsible for the occurrence of BT disease in BTV-infected sheep but not cattle. Although BT was once considered to be a global emerging disease that was spread by animal movement and trade, it now is clear that BTV exists throughout tropical, subtropical and some temperate regions of the world in distinct, relatively stable, ecosystems where different strains of the virus have co-evolved over long periods of time with different species of insect vector.