Sequence data indicated that the hemagglutinin gene of this novel subtype H7N9 virus might originate from a subtype H7N3 virus in ducks and that the neuraminidase gene probably originated from a subtype H7N9 virus in wild birds or ducks or chickens[2,3]. These data suggest that wild birds might play a role in the emergence of subtype H7N9 viruses, similar to the role they played in the geographic spread of avian subtype H5N1 viruses.
However, although avian influenza A(H7N9) viruses have been isolated from chickens and pigeons, none have been isolated from wild birds.
Tree sparrows are abundant and widely distributed in China. They are frequently in contact with humans and poultry. Prevalence of avian influenza viruses among tree sparrows could increase opportunities for them to carry influenza viruses from aquatic birds to domestic farms and even to humans. Hence, such expansion of influenza A(H7N9) virus host ranges undoubtedly increased the seriousness of the threat of this novel subtype.
Isolation of novel influenza A(H7N9) virus in a tree sparrow emphasizes the need to expand influenza surveillance to not only domestic birds but also wild and terrestrial birds.
 Liu et al: Origin and diversity of novel avian influenza A H7N9 viruses causing human infection: phylogenetic, structural, and coalescent analyses in The Lancet -2013
 Lam et al: The genesis and source of the H7N9 influenza viruses causing human infections in China in Nature - 2013
 Lee et al: A new piece in the puzzle of the novel avian-origin influenza A (H7N9) virus in Biology Direct - 2013
 Fang et al: Environmental factors contributing to the spread of H5N1 avian influenza in mainland China in PLoS ONE - 2008
 Arenas et al: Type A influenza viruses in birds in southern Spain: serological survey by enzyme-linked immunosorbent assay and haemagglutination inhibition tests in Avian Pathology - 1990
 Baihui et al: Novel Avian Influenza A(H7N9) Virus in Tree Sparrow, Shanghai, China, 2013 in Emerging Infectious Diseases - 2014