A Very virulent infectious bursal disease virus closely related to New York strain isolated from vaccinated small-scale broiler poultry farm in Addis Ababa, Ethiopia

Infectious bursal disease virus (IBDV) was initially identified in the USA. The first IBD case in Ethiopia was reported in 2002 and since then vaccination was employed to control the disease. However, IBD outbreaks continued to occur despite the routine vaccination. The present study investigated IBD outbreak that had occurred in a vaccinated small-scale broiler poultry farm. Eight samples (four Bursa and four spleen) were collected and all were confirmed to be IBDV positive using reverse transcription-polymerase chain reaction (RT-PCR) targeting the hypervariable region of virus protein 2 (hVP2). All the IBDV isolates of this study were identified to be very virulent IBDV (vvIBDV) strains. For the four IBDV isolates the nucleotide and deduced amino acid sequence for hVP2 was determined. The nucleotide sequence identity of the VP2 gene of the present outbreak isolates (which showed 100% homology among themselves) with the previous 19 vvIBDV characterized isolates from Ethiopia ranged from 90.8% to 96.9% but greater than 96% identity was recorded with only six isolates. The deduced amino acid (aa) sequence of the present outbreak isolates contained aa residues commonly found in vvIBDV strains: A222, I242, I256, I294 and S299 suggesting the strains belong to genogroup three (G3). The phylogenetic analysis of the isolates showed that all isolates clustered separately from classical virulent, vaccine and variant strains and also distantly related to UK661 (UK) and DV86 (Netherlands) very virulent strains but unexpectedly closely related to the New York, USA strain. In conclusion, the present study reported vvIBDV strains (G3) isolated from a vaccinated broiler flock. Further research is needed to characterize the molecular epidemiology and pathogenicity of the vvIBDV strains and evaluate protective efficacy of the current IBD vaccine used for routine vaccination.