Isolation,identification and genetic evolution analysis on a new reassortant strain of infectious bursal disease virus from broilers
[Objective]The study aimed to determine the pathogen type and molecular characteristics of suspected infectious bursal disease vi-rus(IBDV)infections in a broiler farm in Yunnan in order to understand the prevalence and molecular evolution trends of novel reassorted IBDV strains in broilers and to provide a reference for formulating effective infectious bursal disease(IBD)prevention,control and vaccine selection strategies.[Method]Bursae of Fabricius from chickens suspected of IBDV infection were homogenized and tested using nested RT-PCR.Positive bursal tissue suspensions were inoculated into 9-day-old SPF chicken embryos for virus isolation.The vVP2 and VP1-b genes of the virus were amplified and analyzed for genetic variation.[Result]A novel reassorted IBDV strain(named YN-YX221110)was successful-ly isolated.After inoculation into 9-day-old SPF chicken embryos,the embryos showed diffuse hemorrhages,with bleeding spots on the head,neck,and back skin.Based on the vVP2 gene sequence,the YN-YX221110 isolate showed the highest nucleotide similarity with novel variant strains(nvIBDV)reference strains,ranging from 93.2％to 97.9％.The vVP2 gene of the YN-YX221110 isolate shared characteristic amino acid positions 222A with the very virulent(vvIBDV)reference strains such as UK661 and HK46,and positions 249 K,256V,279 N,284A,294L and 299S with the novel variants IBDV(nvIBDV)reference strain SHG19.In the genetic evolutionary tree based on the vVP2 gene,the YN-YX221110 isolate clustered with nvIBDV reference strains such as GX-QZ191002 and SHG19,belonging to the A2d branch.Based on the VP1-b gene sequence,the YN-YX221110 isolate showed the highest nucleotide similarity with classical virulent(cIBDV)strains,ranging from 97.8％to 98.4％.The characteristic amino acid positions of the vVP2 gene in the YN-YX221110 isolate,240D,242D,287T,390L and 393E,were the same as those in attenuated(attIBDV)reference strains such as B87 and Gt;In the phylogenetic tree based on the VP1-b gene sequence,the YN-YX221110 isolate clustered with attIBDV strains such as Gt,B87 and CEF94 in the cIBDV-like group,belonging to the B1a branch.The genotype of the YN-YX221110 isolate was A2dB1a.[Conclusion]The broiler-derived IBDV isolate YN-YX221110 is successfully isolated,with segment A derived from nvIBDV and segment B from attIBDV in cIBDV-like strains,representing a novel reassorted IBDV strain.
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