RESEARCH

phylogenetic studies indicate that three distinctChina CDC, Beijing 102206, China Full list of author information is available at the end of the articleBackground Influenza A viruses can be divided into different subtypes based upon the two surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA) [1,2]. Wild waterfowl are recognized as the natural reservoir of influenza A viruses, especially low pathogenic avian influenza virus (LPAIV) [3]. To date, all of the HA (H1–H16) and NA (N1–N9) subtype avian influenza viruses (AIVs) have been identified in wild waterfowl, with the exceptions of H17N10 and H18N11, which were isolated from bats [2,4,5]. Previous studies indicated that migratory birds play an important role in the emergence of epidemics in birds, pigs, horses, and humans [1]. Generally, AIVs are nonpathogenic in wild birds, although they sometimes cause significant morbidity and mortality when transmitted to domestic poultry [6-8]. The H9N2 subtype influenza virus was first isolated from a turkey in Wisconsin in 1966. It has been most prevalent in wild ducks and shorebirds and has shown no evidence of establishing a stable lineage in land-based poultry in North America [9]. Since the l990s, H9N2 subtype influenza vi-ruses have become prevalent in land-based poultry acros