Objectives   To analyze the traceability and genetic characteristics of a highly pathogenic H7N9 avian influenza virus strain isolated in Nanning city, 2017 through sequencing of hemagglutinin (HA) and neuraminidase (NA) gene of the virus.

  Methods   Both HA and NA gene sequences of the isolated virus were amplified with real-time reverse transcription PCR (RT-PCR) and sequenced with Basic Local Alignment Search Tool (BLAST) database of National Center for Biotechnology Information (NCBI). The phylogenic tree analysis was drawn with Mega 5.1 and variations in dominant loci for proteins were statistically analyzed.

  Results   The established phylogenetic tree showed that the HA and NA gene sequences of the virus were mainly divided into two branches of Yangtze River Delta and Pearl River Delta; the HA and NA gene of the isolated A/Nanning/01/2017 (H7N9) virus were evolved form the Pearl River Delta Branch and were highly homologous with those of the avian influenza H7N9 virus isolated in Guangdong province. The inserted 4 amino acids at HA protein cleavage site mutated from PEIPKGR↓GLF to PEVPKRKRTAR↓GLF; the mutation involved in 5 basic amino acids and resulted in molecular characteristics of high pathogenicity of avian influenza virus. The amino acid of virulence-related site 225 changed from D-aspartic acid to glycine (D225G), resulting in enhanced virulence; the amino acid of receptor binding site (RBS) 186 changed from glycine to valine (G186V). No mutated combination was observed for on droplet propagation-related key amino acid sites and glycosylation-related sites were highly conservative. Five amino acids were absent for NA protein, which may increase virulence of the virus strain. No mutations were observed for loci related to drug resistance and glycosylation.

  Conclusion   The avian influenza A H7N9 virus isolated in Nanning city may be derived from poultry viruses isolated in Guangdong province in Pearl River Delta region and human to human transmission of the virus is unlikely to occur. The virus strain is sensitive to neuraminidase drugs, but is of the molecular characteristics of highly pathogenic avian influenza virus. Contamination of H7N9 avian influenza virus exists in Nanning external environments, suggesting that we need to strengthen surveillance, effective prevention and control of infection source.