Animal viral diseases and global change: bluetongue and West Nile fever as paradigms

Jiménez-Clavero, Miguel Á

The causative agent, BTV, belongs to the family Reoviridae, genus Orbivirus, has a genome of segmented doublestranded RNA (dsRNA), contained in a non-enveloped capsid shell (reviewed in Schwartz-Cornil et al., 2008) . Its main route of transmission is through the bite of midges of the genus Culicoides. The BTV genome is divided into 10 dsRNA segments which encode seven structural (VP1-VP7) and four non-structural (NS1, NS2, NS3, and NS3a) proteins. BTV is closely related to other orbiviruses such as African horse sickness virus (AHSV) and epizootic hemorrhagic disease of deer virus (EHDV), to which it shares not only physico-chemical characteristics but also eco-epidemiology and vector type, although differing in host range. As for now, 26 serotypes of BTV have been described (two of them very recently; Hofmann et al., 2008; Maan et al., 2011) , with no cross-protective immunity between them. Within each serotype there is also a great variability, largely facilitated by the segmented genome, allowing genetic "reassortment." This enables these viruses to generate multiple variants that may differ in important characteristics or phenotypes, such as pathogenesis, host range, vector competence, or transmissibility (MacLachlan, 2004) .