Isolation and Characterization of Bluetongue Virus Recovered from Blood Samples by Immunoaffinity Purification

Genetic characterization and ex-vivo neutralization behavior of bluetongue virus serotype-16 recovered from apparently healthy goat

Bluetongue virus (BTV) infects almost all the domestic and wild ruminants though the clinical disease is most commonly reported in sheep and some species of deer. Goat and cattle are the most common asymptomatic reservoir of the virus. Full genome sequencing and serological characterization of the virus isolates are emphasized for understanding the phylogenetic relationship and molecular epidemiology of bluetongue (BT). In this study, we report phylogenetic and phenotypic antigenic relationship of a BTV serotype-16 (PDP2/13/Ind) recovered from an apparently healthy goat from the state of Uttarakhand, a hilly terrain of sub-Himalayan India with four other BTV-16 isolates. The full genome sequence data was analyzed and the phylogenetic relationship of the goat isolate with other BTV-16 was established. Phylogenetic analysis revealed cluster of PDP2/13/Ind along with other Indian BTV-16 isolates indicating their close ancestral relationship. A cohesive ancestral relationship, irrespective of the genome segments analyzed, was also observed between Indian and Mediterranean BTV-16. The mean substitution rate of different segments of BTV-16 isolates varied from 3.231 × 10^-5 (seg-2) to 1.129 × 10^-3 (seg-6) substitutions per site per year. Timescale analysis indicated that all the segments had an older ancestor. No statistically significant geographic structuring of BTV-16 isolates was observed indicating frequent gene flow. The goat isolate shares highest identity (99.5%-99.8%) with G53/ABT/HSR, a BTV-16 recovered from the western part of the country whereas high level of divergence (11.9%-33.3%) at genomic segment level was observed with a Nigerian BTV-16 (NIG1982/10). Phenotypic antigenic relationship (r) of PDP2/13/Ind with other isolate-specific hyperimmune serum (HIS) determined from serum neutralization titer was 0.672 ± 0.058 to 0.948 ± 0.09. On other hand, the calculated 'r' score was 0.636 ± 0.063 to 0.814 ± 0.201 when HIS against PDP2/13/Ind was used to neutralize the other BTV-16 isolates. The percentage antigenic similarity (R) of the PDP2/13/Ind with other BTV-16 isolates was 65.39 ± 5.38-87.67 ± 14.86. Data suggests presence of subtype antigenic variation amongst the BTV-16 isolates recovered from the goats of a geographically restricted area of the state of Uttarakhand, India.

Genetic and phylogenetic analysis of the outer capsid protein genes of Indian isolates of bluetongue virus serotype-16

AIM

The aim of the study was to characterize bluetongue virus serotype 16 (BTV-16), recently isolated from different states of India. The evolutionary relationship of newly isolated BTV-16 and previously reported Indian and global BTV-16 isolates were compared using molecular analysis.

MATERIALS AND METHODS

In the present study, five (n=5) BTV-16 isolates were used to amplify gene segment-2 and segment-6 encoding the outer capsid proteins VP2 and VP5, respectively. The amplified products were purified and sequenced by the Sanger sequencing method. The phylogenetic relationship and nucleotide identity of all five BTV-16 isolates were compared with previously reported Indian and global BTV-16 isolates. Nucleotide sequence data were aligned using the CLUSTAL W algorithm implemented in the MegAlign of DNASTAR program package (MegAlign 5.00, DNASTAR Inc., Madison, USA). Phylogenetic analyses were carried out using MEGA version 6.0 software with the best nucleotide substitution model.

RESULTS

Phylogenetic analysis based on the VP2 and VP5 encoding genes, segregates Indian BTV-16 isolates in a distinct cluster with proximity to the Eastern topotype. Indian isolates make a monophyletic cluster with Eastern topotypes with Western topotype BTV-16 (BTV-16/NIG/AJ586694) occupying a separate cluster. Indian isolates were found to share 91.5%-97.5% and 96.5%-98.9% identity at the nucleotide and deduced amino acid (aa) level, respectively, to the global BTV-16 isolates. There is a high degree of variation with the Nigerian isolate with 27.0-27.7% and 26.0-26.9% at the nucleotide and aa sequence level, respectively. These data suggest that Indian BTV-16 isolates might have evolved separately within the Eastern BTV topotype.

CONCLUSION

Phylogenetic analyses and nucleotide identity of BTV-16 isolates at the VP2 and VP5 gene encoded level indicate that isolates used in the present study might have evolved from a common Eastern topotype ancestor. The data presented in this study will be helpful for future selection of reference strains in a serological and molecular epidemiology study.