H9N2 influenza viruses prevalent in poultry in China are phylogenetically distinct from A/quail/Hong Kong/G1/97 presumed to be the donor of the internal protein genes of the H5N1 Hong Kong/97 virus.

Ten H9N2 influenza virus strains isolated from diseased chickens in different farms in China during 1995 to 1999 were antigenically and genetically characterized. The haemagglutinins of the isolates were not related to those of A/quail/Hong Kong/G1/97 (H9N2) (Qa/HK/G1/97), but were closely related to that of A/chicken/Hong Kong/G9/97 (H9N2) (Ck/HK/G9/97). The neuraminidase of these isolates had a deletion of three amino acid residues at positions 63 to 65 as compared with those of Ck/HK/G9/97, while that of Qa/HK/G1/97 lacked two amino acids at positions 38 and 39. The PB2 genes of the isolates were not related to those of Qa/HK/G1/97 or Ck/HK/G9/97, but showed some relationship to that of A/duck/Hong Kong/Y439/97 (H9N2) (Dk/HK/Y439/97). The PB1 genes of the isolates were not related to those of the three representative strains. The PA, NP, M, and NS genes of the isolates belonged to the same lineage as those of Ck/HK/G9/97, and were distinct from those of Qa/HK/G1/97 and Dk/HK/Y439/97. The present results indicate that H9N2 influenza viruses prevalent in chicken populations in China belong genetically to one lineage and are distinct from Qa/HK/G1/97, presumed to be the donor of the internal protein genes of the highly pathogenic H5N1 influenza virus in Hong Kong in 1997.