More in vivo experiments will be needed in order to determine the role of NS4 in the interplay with the natural host of BTV infection. We observed no differences between wild type BTV-8 and BTV8-DNS4 in experimental mouse models, although it remains possible that differences could be identified in sheep. It is possible that NS4 is required for viral replication in insects, although we have established in this study that no differences are observed on the replication of the DNS4 mutants in insect cells in vitro.