Genetic characterization of bluetongue virus serotype 9 isolates from India

Recent incursions of bluetongue virus (BTV) into previously naive geographical areas have emphasised the need to better understand virus movement and epidemiology. Several bluetongue virus (BTV) serotypes are known to exist in India, and some serotype viruses have been isolated. However, the complete genome of not a single isolate is available to date. We report the complete genome sequence of one, and partial sequences of three other Indian isolates of BTV-9. Evolutionary relationships with segment-2 and -6 sequences of BTV isolates around the world, deduced using four different phylogenetic analyses and a similarity programme, show that BTV-9 (Eastern), BTV-9 (Western), and BTV-5 form a triad of equidistant, genetically distinct groups of viruses. The Indian BTV-9 isolates were closely related to Mediterranean and European BTV-9 isolates (Eastern topotype) based on segment-2 and -6 sequences. By contrast, segment-5 analyses clustered the Indian BTV-9 isolates with South African BTV-3 reference strain (98% identity), which belongs to one of the Western types. These results have implications on BTV origin and movement, genotyping, serotyping, and vaccine design.