BTV-8 (IAH reference collection number NET2006/04) was originally isolated from a naturally infected sheep during the 2006 outbreak in Northern Europe [33] . The virus was passaged once in KC cells and once in BHK 21 cells. The reference strain of BTV-1 was originally isolated at the ARC -Onderstepoort Veterinary Institute (IAH reference collection number RSArrrr/01) and was adapted to cell culture by passaging it twice in embryonated eggs and 9 times in BHK 21 cells. Both viruses were kindly provided by Peter Mertens. Virus stocks were prepared by infecting BSR cells at a multiplicity of infection (MOI) of 0.01 and collecting the supernatant when obvious cytpopathic effect (CPE) was observed. The supernatants were clarified by centrifugation at 500 g for 5 min and the resulting virus suspensions aliquoted and stored at 4uC for short term usage and at 270uC for long term storage. Virus titres were determined by standard plaque assays using BSR or CPT-Tert cells [34] .