Abstract:

The objective of this study was to investigate the difference of biological characteristics between infections bursal disease virus (IBDV) field strain HQ-b and its cell-adapted virus strain HQ, and the relationship between the virulence change and mutation of VP2 and VP5 genes. Cell culture characteristics, pathogenicity to SPF chicken, sequences of VP2 and VP5 genes of the two strains were analyzed and compared. The results showed that the field strain HQ-b can not be adapted to grow in CEF, CEK, CELi, DF-1 and Vero cell lines. Cell-adapted virus strain HQ can be adapted to above cells except for Vero cell line. TCID₅₀ titers in cultured cells were stable in different lot. It was indicated that the Field Strain HQ-b was a very virulent IBDV strain (vvIBDV). Mortality of 4 -week-old SPF chicken infected with HQ-b and HQ was 80% and 0% respectively. The results of VP2 sequence (the hv regions) analyses showed that the Field Strain HQ had vvIBDV conserved amino acids (aa): 222 A, 242 I, 256 I, 294 I and 299 S. The 222th (A→P), 256th (I→V), 294th (I→L), 299th (S→N), 253th (Q→H), 279th (D→N) and 284th ( A→T) aa of HQ were all changed, which caused it having attenuated strain conserved aa: 222 P, 256 V, 279 N, 284 T, 294 L and 299 N. The analyses results of VP5 sequence showed that the HQ-b had molecular characteristics of vvIBDV. HQ virus had 12 mutation sites and 9 of them made the corresponding aa change, especially, the mutation from T to C at the second bp of VP5 gene open-reading-frame of HQ virus caused lose first “ATG” and 4 aa at the N-terminal of VP5 protein．The mutations were very similar to vaccine strains. The research provides the molecular mechanism of alteration of culture properties and pathogenicity for vvIBDV HQ-b after adaption and enriches the molecular epidemiological theory for IBDV.