Emergence and Phylogenetic Analysis of Amantadine-Resistant Influenza A Subtype H3N2 Viruses in Dublin, Ireland, over Six Seasons from 2003/2004 to 2008/2009

Abstract

Objectives: To determine the prevalence of amantadine-resistant influenza A viruses and perform genetic analysis of isolates collected in Dublin during six seasons (2003/2004 to 2008/2009). Methods: Known mutations in the matrix 2 gene (M2) conferring amantadine resistance were screened and phylogenetic analysis of the haemagglutinin gene (HA) performed. Results: Of 1,180 samples, 67 influenza A viruses were isolated, 88% of which were subtype H3N2. Amantadine resistance was only found in subtype H3N2 and increased dramatically from 7% in 2003/2004 to 90% in 2008/2009. A maximum likelihood tree of the HA gene of influenza A H3N2 isolates differentiated them into two distinct clades, clade N and clade S, where the majority of isolates were amantadine-resistant and amantadine-sensitive, respectively. The clades were distinguished by amino acid substitutions, S193F and D225N, which probably conferred a selective advantage for the spread of such viruses. Phylogenetic analysis showed some degree of antigenic drift when compared with the vaccine strain of the corresponding season. Conclusions: This study showed that circulation in Ireland of a distinct lineage, clade N, among H3N2 viruses favoured emergence of amantadine resistance. Furthermore, comparison of circulating Irish viruses and vaccine strains used in the northern hemisphere showed high similarity.