Both the hindrance of the mucin barrier and the limited number of proper receptors on the epithelial cells of the host restrict cross-species infection by influenza viruses [6, 20] . Nevertheless, genetic mutations or recombinations in the HA of an influenza virus can render interspecies transmission by altering the receptor tropism [27, 28, 29] . The 226 and 228 residues in the HA of the influenza H3 and H2 viruses are particular important for determining the receptor specificity and host range restrictions [30, 31] . The Ser-to-Gly mutation at HA position 228 and the Leuto-Gln mutation at HA position 226 shift the receptor preference from SAa2-6Gal to SAa2-3Gal and enhance the infectivity of human influenza virus in duck intestinal cells [31] .