Molecular characterization of H9N2 avian influenza viruses isolated from vaccinated broiler chickens in northeast Iran.

Avian influenza is a highly contagious respiratory disease of poultry caused by influenza A viruses, family Orthomyxoviridae. H9N2 avian influenza outbreaks are a major problem of the poultry industry in Iran. To determine the genetic differences between field viruses and the vaccine strain, the genomes of four strains isolated in 2011 from vaccinated broiler flocks with a history of respiratory illness were sequenced. Genetic and serological comparisons were made. Sequence analysis of the hemagglutinin (HA) and neuraminidase (NA) genes indicated that the isolated strains shared nucleotide homologies of 91.6-93.9 and 90.2-91.7% with the vaccine strain, respectively. Phylogenetic analyses of HA and NA genes showed that all strains isolated in this study fell into the same group and belonged to the influenza A virus (A)/quail/Hong Kong/G1/97 H9N2 sublineage. Several amino acids have changed at the antigenic sites in HA in the field viruses. Extra potential glycosylation sites were observed in the HA and NA proteins expressed by the current isolates relative to those in the vaccine strain. The deduced amino acid sequence at the cleavage site of HA in recent isolates is the KSSR/GLF motif, whereas it is RSSR/GLF in the vaccine strain. A serological analysis revealed that the currently circulating strains are antigenically distinct from the vaccine strain. These results suggest that the commercial vaccine is insufficiently genetically and antigenically similar to the viruses currently circulating in the region. These findings confirm that it is important to monitor the genetic and antigenic variations in H9N2 influenza viruses when selecting a vaccine strain.