Genomic diversity among eastern and western topotypes of bluetongue virus serotype 16 based on whole genome sequence analysis

Abstract

Bluetongue (BT) is a noncontagious, vector-borne disease of domestic and wild ruminants. The causative agent bluetongue virus (BTV) is a double stranded RNA virus belonging to genus Orbivirus within family Reoviridae. Eradication of BTV from endemic regions like India is not an easy task due to the widely distributed Culicoides spp. midge vectors, the ubiquitous distribution of vertebrate hosts and existence of a large number of serotypes of the virus (at least 26 till date). The complete genomes (19,193 base-pairs) of several strains of bluetongue virus serotype 16 (BTV-16) originated from Australia, China, Indian subcontinent, Mediterranean basin, Middle East, Africa (Nigeria) and Europe, were compared. These analysis showed that all ten genome segments of a Nigerian strain are derived from a western lineage, showing only 77% - 84% nt identity with the eastern topotype reference strain 'RSArrrr/16' (and its derivative 'RSAvvvv/16', a vaccine strain) that was originally isolated in Pakistan, 76.4% - 83% with eastern BTV-16 strain from Australia (DPP96) and 77% - 89% with a reassortant strain from India. These detailed comparisons involving global strains showed that there is a very high degree of variation (up to 24%) between BTV-16 strains from eastern and western geographical regions. These data confirm the value of whole genome sequencing for characterization of novel BTV isolates and has helped to identify representative suitable 'reference-strain' of eastern topotype (BTV-16e), western topotype (BTV-16w), as well as 'cross-topotype' reassortant strain (BTV-16r) that are generated in the field for further serological, phylogenetic and molecular epidemiology studies.