| Infectious bursal disease(IBDV)virus is a double RNA virus family and avian double RNA virus.Its genome is composed of two segments of A and B,and serotypes are classified into type I and type II.The virus can cause acute and highly contagious diseases in chickens,namely infectious bursal disease(IBD).The disease mainly endangering immune organs,causing severe immunosuppression,and resulting in secondary infection of bacteria,mycoplasma,virus and parasites,and can weaken,even cause the failure of the immune response.Because of the variability of IBDV antigen and the diversity of serum subtypes,it is difficult to prevent and control diseases.Therefore,real-time understanding of the evolution of IBDV genes and antigen changes is very important for prevention and control of the disease.In this study,the suspected cases of IBD in Yulin,Nanning,Beiliu and Liuzhou in Guangxi province in 2017 were collected and taken the bursa of Fabricius for the IBDV isolation by inoculating SPF chicken embryo via the rount of chorioallantoic membrane(CAM).And 11 IBDV isolates were obtained.The vVP2 and VP1-b genes of the A and B segments of these isolates were amplified and sequenced respectively.Through the comparison of the key amino acid sites and the genetic genetic system evolution based on vVP2,it was found that the isolate NN170502 had the characteristics of 222P,279N,284T,294L and 299N,and was consistent with the characteristics of the attenuated reference strains.Phylogenetic tree analysis showed that NN170502 was in the same branch and closely related to Gt,Cu-1 M and other attenuated reference strains,and had a far genetic relationship with vvIBDV reference strains;The other 10 isolates like YL170114 had the 222A,2561,284A,2941 and 299S amino acid sites that were consistent with the vvIBDV reference strains and grouped in a branch with vvIBDV strain HLJ-0504.The analysis of the key amino acid sites based on VP1-b for the 11 isolates showed that the isolate LZ170322 had the same 242D,287T,290L and 293E as the characteristic amino acid loci of the attenuated IBDV reference strain,and the genetic evolution tree was also in the same branch with the attenuated IBDV virus reference plant.The amino acid sites of the other 10 isolates were 242D,287A,290M and 293D,which was consistent with China's new vIBDV(representing HLJ-0504).The phylogenetic tree was also closely related to HLJ-0504,and the nucleotide similarity was as high as 97.8%-98.0%.All the 11 isolates were reassotants of gene segments,and 9 of the isolates belonged to type II reassotment,that is whose vVP2(located in A segment)originated from vvIBDV strain and VP1-b(located in B segment)originated from HLJ0504-like strain;Isolate LZ170322 belonged to type I reassotment,that is its vVP2 originated from vvIBDV stain and VP1-b originated from attenuated IBDV strain;Isolate NN170502 belonged to type IV reassotment,that is its vVP2 originated from attenuated IBDV strain and VP1-b originated from HLJ0504-like strain.Monovalent antisera prepared by the laboratory were also used to run the cross neutralization test on microplate CEF cultures with the 11 isolates.The results showed that the neutralization titers of all the 11 isolates had the highest titers of 102 34-104.72 with 4 reference strains B87,040124,TSC-1 and HUN0801 of subtype 1,and the cross neutralization titers of 11 isolates with 4 reference strains were all less than 100 5,while those with the subtype 2 reference strain JS7 and subtype 3 reference strain BH11 were more than 100 5.So it can be determined that all 11 isolates belonged to the subtype 1 IBDV of serotype I.The results of this study demonstrated that all the IBDV isolates prevalent in Guangxi in 2017 were all genetically reassotants,of which HLJ0504-like strains were dominant.The serotype belonged to subtype 1 of serotype I. |
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