Sequence analysis of VP1, VP2 and VP3 genes of Infectious Bursal Disease Virus from a field outbreak in Kerala, India

Abstract

Infectious bursal disease (IBD), caused by infectious bursal disease virus (IBDV), is one among the top five infectious diseases of poultry that discernibly affects commercial poultry industry. The mutating viral genome of IBDV accounts for disease outbreaks in fields even after following stringent biosecurity measures and vaccination protocols. The present study is focussed on the characterisation of an IBDV field virus, IBD/CVAS/6, from a vaccinated flock in Kerala, based on the sequence analysis of VP1, VP2 and VP3 genes. Bursa of Fabricius samples collected from a 26 days-old chicken flock from a suspected outbreak in the Thiruvananthapuram district of Kerala formed the subject of the study. Reverse transcriptase-polymerase chain reaction (RT-PCR) targeting VP2 gene confirmed the presence of virus in the sample. The sequence analysis revealed that the deduced amino acid sequence of VP1 gene of IBD/CVAS/6 was 100 per cent homologous with an attenuated very virulent vaccine strain of Israel, mb and the VP2 gene was 100 per cent homologous with mb and Ventri IBDV plus vaccine strain of India. The analysis of VP3 gene also revealed the similarity with vaccine strains except for a single variation S745N in its deduced amino acid sequence. The phylogenetic analysis of IBD/CVAS/6 revealed it’s close relation with mb, Ventri IBDV plus and a very virulent strain of Israel, ks. The characteristic virulent marker amino acid motifs ‘SWSASGS’ and ‘TDN’ were present in the VP2 and VP1 genes, respectively. Hence, the study revealed that the obtained virus has emerged from an attenuated very virulent vaccine strain and hence the present study is a report of involvement of the intermediate plus vaccine strain in field outbreaks in Kerala. The role of S745N in virulence cannot be accounted from the present study, however the involvement of IBD/CVAS/6 in the outbreak might be related to S745N variation or variations in other genes of the virus or due to the inefficiency of the vaccine or vaccination protocol followed, which can be defined only after further studies. The present report is the first characterisation study in Kerala focussing on the analysis of VP1, VP2 and VP3 genes of IBDV.