Antigenic Drift Of H₉N₂ Subtype Avian Influenza Viruses In Henan Province During 1998-2002

To elucidate the antigenic drift and evolution of H9N2 subtype avian influenza viruses (AIVs), five isolates from the north of Henan province during 1998-2002 were compared and analysed by cross-hemagglutinin inhibition test (HI) , cross-virus neutralization test (VN) in the chicken embryo and chicken embryo primary kidney cell (CEK) and cross protection against challenge infection test. The results revealed that antigenic drift had occurred in virus strains of the same H9N2 subtype in the north of Henan province during 1998-2002. H9N2 subtype AIVs were stable from 1998 to 1999, since the mutual protection between 98A5 and 99S approached to 100%, which was consistent with results in HI and VN tests. When challenged by 00Y, those inoculated with vaccine of virus strains during 1998-2002 could protect only 60%-80%; when challenged by 02Y, those vaccines' immuno-protection rate showed negative relationship with the length of time except 00Y, similar results could be acquired in HI and VN tests.The nucleotide sequences of cDNA of HA gene were determined by RT-PCR and sequencing . Compared with 98A5, the nucleotide of 99S changed 1-4, while 00Y, 01Q and 02 Y changed 8,5,6 respectively. Nucleotide at the 895bp site of the HA gene of 00Y mutated into A , which caused the amino acid of potential glycosylation site to become NTT; most nucleotide mutation sites of 02Y gathered at the carboxyl terminus of HA1(954, 948, 973, 1000bp), among which the mutation at 924bp site didn't chang the coding amino acid of potential glycosylation site. Fiveisolates had PARSSR ↓ GLF at the cleavage site and none of them had multiple basic amino acids that correlated with highly pathogenic avian influenza (HPAI) isolates. In addition, the amino acid residue at position 191 of receptor binding site (RBS) was His (H) in our five strains but that of others was Asn (N) in our country. Nucleotide and amino acid sequence homology among our five isolates were 97.2% to 98.7%, 96.2% to 99.2% respectively. A 399 nucleotide regions (bases 604 to 1003bp) of HA gene of 5 strains were analyzed phylogenetically with 20 H9N2 subtype viruses available in the genbank database. It is concluded that 5 strains in out study may have a common origin with Hongkong 69/97 but different from that of Hongkong G1/97.00Y owned larger genetic distance than that of others.The results of our study revealed that H9N2 subtype AIV in the north of Henan province during 1998-2002 had changed both in antigenicity and HA gene, which provided scientific reference data for prevention against H9N2 AIV and the selection of vaccine strains.