Genetics And Pathogenicity In BALB/c Mice Of H5 Subtype Avian Influenza Viruses In Guangxi And Guangdong From 2015 To 2016

H5 avian influenza viruses(AIVs)have been detected in different species of wild birds and domestic poultry in more than 60 countries worldwide.Extended epizootics and panzootics of H5N1 viruses have led to the emergence of the novel 2.3.4.4 clade of H5 virus subtypes,including H5N2,H5N6,and H5N8 reassortants.However,the biological properties of this subtype,especially the infectivity and pathogenicity in mammals,are not known.Here,genetics and pathogenicity in BALB/c mice of 10 representative H5 sutype avian influenza viruses that were detected in the live poultry markets and farms in Guangxi and Guangdong between 2015 and 2016 were analyzed.Genomic sequence analysis of 10 viruses revealed 7 different genotypes.All the viruses contained five consecutive basic amino acids in cracking sites of HA protein,which possessed a typical molecular characteristics of HPAIV.The HA genes of the virus isolates all belonged to Clade 2.3.4.4,and the whole NA genes are N6.The sequence analysis of internal genes found that the six segments of virus isolates were closely related to the recent H5 subtype AIVs,which illustrated that the H5 subtype AIV is relatively conservative in the process of evolution.The origins of segments of some viruses are complex and H1、H2、H3、H4、H6、H10 and H11 subtype AIVs are covered.The infectious and pathogenic experiments in mice indicated marked differences in mice.All 10 viruses tested could replicate in the upper and lower respiratory organs of infected mice without prior adaptation.Three of the viruses could even cross the blood-brain barrier and replicate in the whole organs of the infected mice.Our study demonstrates that the current circulating H5N6 AIVs in Guangxi and Guangdong can infect and replicate in mammalian hosts,thereby posing a potential threat to human health.These findings emphasize the continual need for enhanced surveillance of H5N6 AIVs.