Biological Characterization Of H7N9 Avian Influenza Viruses Isolated From 2013 To 2017 In China

Since the emergence of the H7N9 subtype avian influenza virus in China in 2013,the H7N9 viruses have not only caused great economical losses to the poultry industry but also posed a severe threat to human health.As of March 2,2018,there were 1567 reported human infections with 615 deaths.In order to understand the prevalence and biological characteristics of H7N9 subtype avian influenza viruses in China since 2013,84 H7N9 subtype influenza viruses isolated from July 2013 to January 2017 were selected and systematically analyzed.According to analysis of genetic evolution and key molecular features,the gene homology of HA and NA were 88%-100% and 90%-100%,respectively;which of the six internal gene segments PB2,PB1,PA,NP,M and NS were 84.8%-100%,88.6%-100%,86.8%-100%,86.4%-100%,88.5%-100% and 87.8%-100%,respectively.According to the evolutionary relationship of each gene,84 viruses were divided into 23 genotypes.The main genotypes were G1 and G2.Most importantly,the analysis of the molecular characteristics of the virus revealed that the highly pathogenic H7N9 subtype influenza virus emerged in China.Our study indicated that the H7N9 subtype avian influenza strain A/chicken/Guangdong/SD008/ 2017(referred to as CK/SD008)with high pathogenic avian influenza molecular characteristics exhibited high pathogenicity to chickens(IVPI=3.0)and replicated efficiently in chickens.In the replication experiment,low pathogenic H7N9 viruses and highly pathogenic H7N9 viruses showed low virulence to ducks.However,the adaptability of low pathogenic H7N9 virus to ducks and its ability to spread in ducks was gradually strengthening.Twenty-eight low-pathogenic H7N9 viruses represented different genotypes were selected to do the infectivity test in mice,it showed that the currently prevalent low pathogenic H7N9 viruses in China present different virulence to mice.All viruses were able to replicate in the turbinates and lungs of mice,15 of which caused mice to lose weight in varying degrees.The highly pathogenic H7N9 subtype avian influenza representative strain CK/SD008 was able to efficiently replicate in the turbinates and lungs of mice,but did not kill mice.However,the two PB2 mutant strains exhibited greater virulence in mice than the human H7N9 virus in 2013.The above findings were of great significance for us to understand the biological characteristics of the H7N9 subtype avian influenza virus in China after 2013.It provided theoretical and experimental evidence for the formulation of the prevention and control measures for H7N9 in China.At the same time,it also served as a warning for human infection with H7N9 virus.