The origin and global emergence of adamantane resistant A/H3N2 influenza viruses

Abstract

Resistance to the adamantane class of antiviral drugs by human A/H3N2 influenza viruses currently exceeds 90% in the United States and multiple Asian countries. Adamantane resistance is associated with a single amino acid change (S31N) in the M2 protein, which was shown to rapidly disseminate globally in 2005 in association with a genome reassortment event. However, the exact origin of influenza A/H3N2 viruses carrying the S31N mutation has not been characterized, particularly in South-East Asia. We therefore conducted a phylogenetic analysis of the HA, NA, and M1/2 segments of viral isolates collected between 1997 and 2007 from temperate localities in the Northern hemisphere (New York State, United States, 492 isolates) and Southern hemisphere (New Zealand and Australia, 629 isolates) and a subtropical locality in South-East Asia (Hong Kong, 281 isolates). We find that although the S31N mutation was independently introduced at least 11 times, the vast majority of resistant viruses now circulating globally descend from a single introduction that was first detected in the summer of 2003 in Hong Kong. These resistant viruses were continually detected in Hong Kong throughout 2003-2005, acquired a novel HA through reassortment during the first part of 2005, and thereafter spread globally. The emergence and persistence of adamantane resistant viruses in Hong Kong further supports a source-sink model of global influenza virus ecology, in which South-East Asia experiences continuous viral activity and repeatedly seeds epidemics in temperate areas.

Fig. 1

Phylogenetic relationships of the M1/2 segment of 226 A/H3N2 influenza viruses sampled from Hong Kong (n = 108), New York State, USA (n = 72), and New Zealand and Australia (n = 46) between 1997 and 2007, estimated using an ML method. Isolates are shaded by antigenic characteristics (inferred from HA tree): A/Wuhan/359/1995-like (WU95) isolates are blue, A/Sydney/5/1997-like (SY97) isolates are pink, A/Fujian/411/2002-like (FU02) isolates are green, A/California/07/2004-like (CA04) isolates are purple, A/Wisconsin/67/2005-like (WI05) isolates are yellow, and A/Brisbane/10/2007-like (BR07) isolates are brown. In addition, isolates bearing the S31N mutation are shaded in red. Independent introductions of the S31N replacement are numbered #1–#11. Bootstrap values (> 70%) are shown for key nodes. The tree is rooted by the oldest isolate (A/New York/564/1997, 01/02/1997), and all horizontal branch lengths are drawn to scale.

Fig. 2

Phylogenetic relationships of the HA gene segment of 231 A/H3N2 influenza viruses sampled from Hong Kong (n = 108), New York State, USA (n = 72), New Zealand and Australia (n = 46) between 1997 and 2007, and 5 influenza vaccine reference strains (A/Sydney/5/1997, A/Wyoming/3/2003 (A/Fujian/411/2002-like), A/California/7/2004, A/Wisconsin/67/2005, and A/Brisbane/10/2007), estimated using an ML method. Labels, shading, and rooting are the same as for Fig. 1, with Introductions #1–#11 referring to those identified in Fig. 1 and influenza vaccine reference strains shaded in dark green. Isolates labeled ‘#7F’ are associated with Introduction #7 and are A/Fujian/411/2002-like, and isolates labeled ‘7W’ are associated with Introduction #7 and are A/Wisconsin/67/2005-like.

Fig. 3

Phylogenetic relationships of the NA gene segment of 226 A/H3N2 influenza viruses sampled from Hong Kong (n = 108), New York State, USA (n = 72), and New Zealand and Australia (n = 46) between 1997 and 2007, estimated using an ML method. Labels, shading, and rooting are the same as for Figs. 1 and 2, with Introductions #1–#11 referring to those identified in Fig. 1.