Global transmission of influenza viruses from humans to swine

Abstract

To determine the extent to which influenza viruses jump between human and swine hosts, we undertook a large-scale phylogenetic analysis of pandemic A/H1N1/09 (H1N1pdm09) influenza virus genome sequence data. From this, we identified at least 49 human-to-swine transmission events that occurred globally during 2009-2011, thereby highlighting the ability of the H1N1pdm09 virus to transmit repeatedly from humans to swine, even following adaptive evolution in humans. Similarly, we identified at least 23 separate introductions of human seasonal (non-pandemic) H1 and H3 influenza viruses into swine globally since 1990. Overall, these results reveal the frequency with which swine are exposed to human influenza viruses, indicate that humans make a substantial contribution to the genetic diversity of influenza viruses in swine, and emphasize the need to improve biosecurity measures at the human-swine interface, including influenza vaccination of swine workers.

Fig. 1.

Phylogenetic relationships of the six concatenated internal gene segment (PB2, PB1, PA, NP, M, NS) sequences from 100 H1N1pdm09 influenza virus isolates collected in swine globally during 2009–2011, and 1008 H1N1pdm09 influenza viruses collected globally in humans during the same time period (total dataset, n = 1108 sequences; colour-coded black). Each swine H1N1pdm09 isolate is colour-coded by the country of origin: USA, brown; Canada, light blue; Thailand, purple; South Korea, dark green; China, pink; Hong Kong, red; Taiwan, orange; Italy, olive; Mexico, light green; Singapore, yellow; Australia, dark blue. The first introduction of H1N1pdm09 identified in swine, in May 2009 in Canada, is denoted by an asterisk. An identical phylogeny with all viral introductions labelled in detail is provided in Fig. S1.