**2.5 Avian influenza A H9N2 viruses**

Influenza virus A H9N2 has been firstly known in 1966 as the infection among turkeys in Wisconsin. Until 1998–1999, influenza H9N2 viruses are less considered to be as zoonotic transmission, although this H9N2 widespread is nearly global including several Asian countries among from wild birds to domestic poultry populations [63]. H9N2 viruses are characterized as LPAI viruses that have caused mild disease which led to decreased egg production and deaths in poultry [64]. In China, natural human infections with H9N2 were firstly reported as seropositive cases in Guangdong province in 1998 and several cases with mild or asymptomatic illness in the South or mainland China and Hong Kong in 1999 [65]. H9N2 LPAI viruses, that were isolated from an infected human in Pakistan in 2012–2015 and in Northern China in 2017, have an identical amino acid residue leucine at 226 in the receptor binding site of HA, the C-terminus of PDZ ligand motif in NS1 (KSEI), NP (E372D), and L55F in the M2 protein that are mammalian host-specific markers [64, 66]. Avian influenza H9N2 viral infections have been globally distributed as mild and asymptomatic infection in humans, poultry, pet birds, and other domestic animals and may trigger weak immune responses and production of low titer of antibody to H9N2 in humans [63, 66]. These were inducted as infections from poultry to humans directly or indirectly, but not as human-to-human transmission [63, 66]. Low rates of influenza A H9N2 viral infection in poultry-exposed individuals may cause an epidemic with a longer duration or a greater magnitude than if the virus was introduced to a completely unexposed population [67]. H9N2 also demonstrates its potential to efficient transmission from avian hosts and sharing genetic materials (e.g., internal genes) with other viruses as appearance of H7N9 and H10N8 [68, 69].
