Evidence of genetic reassortment between Indian isolate of bluetongue virus serotype 21 (BTV-21) and bluetongue virus serotype 16 (BTV-16).

The genome of bluetongue virus (BTV) consists of 10 segments. Of these seg-2 encoded VP2 is the major serotype determining protein, and seg-6 encoded VP5 protein enhances the protective neutralizing activity of VP2 protein inducing higher serotype specific antibody titer than the VP2 alone. Out of the twenty-six BTV serotypes found worldwide, 22 were reported from different states of India. These include serotype 21 which was recently isolated from Andhra Pradesh, and was involved in a severe outbreak of bluetongue in Indian native sheep. BTV21 (KMNO-7) and BTV16 were circulating at the same time. This co-circulation, along with the fact that the virus genome is segmented, provides an opportunity for these two isolates of different serotypes to simultaneously infect the same animal, and even the same cell or a same vector with the potential for generation of reassortant viruses. This study was carried out to provide some insights into the outbreak. We carried out full length sequencing of genome seg-2 and seg-6 of Indian isolates VJW64 (BTV16) and KMNO-7 (BTV21). Detailed phylogenetic analysis revealed that genome seg-6 of Indian isolate KMNO-7 (BTV21) clusters with isolates of BTV16 showing maximum nucleotide similarity of 97.6% with TUR/2000/02 isolate of BTV16, which is much more than it shows with any isolate of BTV21. KMNO-7 (BTV21) significantly diverged from original strain of BTV21, and is a reassortant strain having acquired seg-6 from an isolate of BTV16. This study provides some useful insights into the epidemiology of the bluetongue disease, and undermines serotyping on genome seg-6 basis.