Reassortant H5N1 avian influenza viruses containing PA or NP gene from an H9N2 virus significantly increase the pathogenicity in mice

• Currently endemic avian influenza viruses (AIVs) of H9N2 and H5N1 subtypes in domestic poultry in many regions of China are prone to reassort with each other naturally.
• The predominant S genotype of the H9N2 virus donated their internal genes to other emerging IAVs in China.
• The parental WJ/57(H9N2) and GS/10(H5N1) viruses displayed high genetic compatibility.
• The H5N1 reassortants containing PA or NP gene from WJ/57(H9N2) virus showed significantly increased virulence in mice.

Reassortment between different influenza viruses is a crucial way to generate novel influenza viruses with unpredictable virulence and transmissibility, which may threaten the public health. As currently in China, avian influenza viruses (AIVs) of H9N2 and H5N1 subtypes are endemic in poultry in many areas, while they are prone to reassort with each other naturally. In order to evaluate the risk of the reassortment to public health, A/Goose/Jiangsu/k0403/2010 [GS/10(H5N1)] virus was used as a backbone to generate a series of reassortants, each contained a single internal gene derived from the predominant S genotype of the A/Chicken/Jiangsu/WJ57/2012 [WJ/57(H9N2)]. We next assessed the biological characteristics of these assortments, including pathogenicity, replication efficiency and polymerase activity. We found that the parental WJ/57(H9N2) and GS/10(H5N1) viruses displayed high genetic compatibility. Notably, the H5N1 reassortants containing the PA or NP gene from WJ/57(H9N2) virus significantly increased virulence and replication ability in mice, as well as markedly enhanced polymerase activity. Our results indicate that the endemicity of H9N2 and H5N1 in domestic poultry greatly increases the possibility of generating new viruses by reassortment that may pose a great threat to poultry industry and public health.