Antigenic characterization of H3N2 influenza A viruses from Ohio agricultural fairs

Abstract

The demonstrated link between the emergence of H3N2 variant (H3N2v) influenza A viruses (IAVs) and swine exposure at agricultural fairs has raised concerns about the human health risk posed by IAV-infected swine. Understanding the antigenic profiles of IAVs circulating in pigs at agricultural fairs is critical to developing effective prevention and control strategies. Here, 68 H3N2 IAV isolates recovered from pigs at Ohio fairs (2009 to 2011) were antigenically characterized. These isolates were compared with other H3 IAVs recovered from commercial swine, wild birds, and canines, along with human seasonal and variant H3N2 IAVs. Antigenic cartography demonstrated that H3N2 IAV isolates from Ohio fairs could be divided into two antigenic groups: (i) the 2009 fair isolates and (ii) the 2010 and 2011 fair isolates. These same two antigenic clusters have also been observed in commercial swine populations in recent years. Human H3N2v isolates from 2010 and 2011 are antigenically clustered with swine-origin IAVs from the same time period. The isolates recovered from pigs at fairs did not cross-react with ferret antisera produced against the human seasonal H3N2 IAVs circulating during the past decade, raising the question of the degree of immunity that the human population has to swine-origin H3N2 IAVs. Our results demonstrate that H3N2 IAVs infecting pigs at fairs and H3N2v isolates were antigenically similar to the IAVs circulating in commercial swine, demonstrating that exhibition swine can function as a bridge between commercial swine and the human population.

Fig 1

Phylogenetic analysis of HA (A) and NA (B) gene segments of H3N2 IAV isolates from pigs at Ohio agricultural fairs (2009 to 2011) The H3N2 IAV isolates from exhibition swine from Ohio agricultural fairs are marked in red, and human H3N2 variant (H3N2v) influenza virus isolates are underlined. The H3N2 IAV isolates from commercial swine included in this study are marked in purple, and the representative strains of H3N2 antigenic clusters are marked in cyan. The phylogenetic analyses were performed using maximum likelihood by the GARLI version (39), and bootstrap resampling analyses were performed using PAUP* 4.0 Beta (40) to apply a neighborhood joining method, as described earlier (41).

Fig 2

Antigenic cartography for H3N2 swine influenza virus isolates from Ohio agricultural fairs (2009 to 2011) and human H3N2v influenza virus isolates. Four human H3N2v isolates and 12 swine influenza viruses from commercial farms were specifically labeled. Antigenic cartography was constructed using AntigenMap (http://sysbio.cvm.msstate.edu/AntigenMap) based on HI data from Table 1 (32, 33).