Molecular epidemiology studies on partial sequences of both genome segments reveal that reassortant infectious bursal disease viruses were dominantly prevalent in southern China during 2000-2012

A molecular epidemiology study of infectious bursal disease viruses (IBDVs) isolated from seven provinces in southern China during the years 2000-2012 was performed based on partial sequences of genome segments A and B, namely the hypervariable region of the A-VP2 gene (A-vVP2) and the b fragment of VP1 gene (B-VP1b) from a total of 91 field isolates. Sequence analysis based on vVP2 revealed that 72 out of 91 isolates had the same characteristic amino acid (aa) sequences as vvIBDV. The mutation of D212N in A-vVP2 has become prevalent in the recent isolates. The origin of the field isolates with vvIBDV characteristic amino acid residues was complex, evidenced by the findings that more than one subgroup of strains prevailed in each province. When B-VP1b was analyzed, there were three lineages among the field isolates, and none of the isolates had a relationship to vvIBDV-related segment B. Phylogenetic analysis of both segments revealed that only a few isolates (13/91) had the same genetic relatives in consensus trees based on segments A and B, whereas the majority of the isolates (85.71%, 78/91) were identified to be naturally reassorted strains. Based on the origin of each segment, at least six types of reassortant IBDVs prevailed in southern China, three of which were shown to be dominant: segment A from vvIBDV and B from attenuated IBDV, segment A of vvIBDV and B from 002-73-like IBDV, and segment A of vvIBDV and B from HLJ0504 or a similar strain. Our findings suggest that both genomic segments of field IBDVs has been evolving, and continuous monitoring of the evolution of field IBDV genome is therefore urgently needed in the control of IBDV.