Abstract: The study focused on isolation of the cattle origin bovine viral diarrhea virus (BVDV) from persistently infected dairy cattle in Ningxia and determination of the viral genomic nucleotide sequences, which would provide theoretical basis for the studying the genetic evolution of BVDV strains from various regions of China. Two consecutive anticoagulated blood samples were collected at a two weeks interval from 240 cattle on a demonstration farm of high-yielding dairy cows in Yinchuan, Ningxia Hui Autonomous Region. The samples were tested for the presence of BVDV viral antigen by BVDV antigen detection kit to screen cattle with persistent infection (PI). Lysis of blood lymphocytes was prepared from PI cattle and inoculated onto MDBK monolayers to isolate BVDV. The viral genomic sequence was obtained by cloning and sequencing and compared with referenced viral sequences to analyze the evolutionary relationships between viral strains. A total of two cattle persistently infected with BVDV were screened from cattle on the demonstration farm. A non-cytopathogenic strain of BVDV, NX2019/01, was isolated. A genomic nucleotide sequence of 12 107 nucleotide position (nt) with an 11 703 nt open reading frame (ORF) encoding 3 898 amino acids, was determined by sequencing. At the genomic level, NX2019/01 shared higher similarity ranged from 92.17% to 93.84% with domestic strains SD-15, ZM-95, XC and LN-1 that were segregated into the 1m sub-genotype, but large differences were observed in the E^rns, E1 and E2 genes. In the viruses from acutely infected cattle on the demonstration farm, nucleotide mutations in N terminus coding region of E2 protein led to variations of amino acid at position 9 or 67. Recombination analysis demonstrated similar recombination signals in a fragment between nt 179-288 in E2 gene of NX2019/01 strain with another fragment spanning nt 168 in E1 gene and nt 332 in E2 gene of ZM-95 strain. The signals indicated that NX2019/01 and ZM-95may originated from a recombination between the major parent SD-15 strain and the minor parent LN-1, sharing close relatedness with SD-15, LN-1 strains or highly similar viruses at earlier time during the evolutionary process. The strain of BVDV-1m sub-genotype originated from persistently infected cattle was isolated. The evolutionary relationship of viruses of BVDV-1m subtype was determined at the genomic level and genomic homologous recombination between strains within a same sub-genotype was first reported in this study, which may lay the foundation for further study about evolution analysis of BVDV in China.

Key words: bovine viral diarrhea virus, persistent infection, genome sequence, homologous recombination