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Title: Continual re-introduction of human pandemic H1N1 influenza A viruses into US swine, 2009-2014

Author
item NELSON, MARTHA - Fogarty International Center
item Stratton, Jered
item KILLIAN, MARY LEA - Animal And Plant Health Inspection Service (APHIS)
item JANAS-MARTINDALE, ALICIA - Animal And Plant Health Inspection Service (APHIS)
item Baker, Amy

Submitted to: Journal of Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2015
Publication Date: 6/15/2015
Citation: Nelson, M.I., Stratton, J., Killian, M.L, Janas-Martindale, A., Vincent, A.L. 2015. Continual reintroduction of human pandemic H1N1 influenza A viruses into swine in the United States, 2009-2014. Journal of Virology. 89(12):6218-6226.

Interpretive Summary: Pigs can be infected with influenza A viruses (IAV) from humans, and thus serve as intermediary hosts in the evolution of IAV. Human-to-swine transmission has increased the genetic diversity IAV in swine (swIAVs) globally and is linked to the emergence of new pandemic threats. We showed that human-to-swine transmission of the 2009 pandemic H1N1 (pH1N1) continued to occur frequently in US swine in the post-pandemic period and even after the pH1N1 was declared to be the new seasonal H1N1 in the human population. An increase in human-to-swine transmission of pH1N1 during 2013-2014 coincided with a pH1N1-dominated epidemic in humans in the US, suggesting a link between viral activity in humans and inter-species spillover to pigs. These findings also indicated that although the surface proteins for the pH1N1 human virus may not be maintained in the pig population, the six internal gene segments from pH1N1 viruses are likely to be sustained long-term in the US swine population. This further contributes to the genetic diversity in swIAV with potential for producing novel swIAV with the ability to infect humans. These findings suggest that vaccinating US swine workers with the human seasonal vaccine may reduce infection of both humans and swine, and in turn limit the role of humans as sources of influenza virus diversity in pigs.

Technical Abstract: Human-to-swine transmission of pandemic H1N1 influenza viruses (pH1N1) increased the genetic diversity of influenza A viruses in swine (swIAVs) globally and is linked to the emergence of new pandemic threats, including H3N2v variants. Through phylogenetic analysis of contemporary swIAVs in the United States, we demonstrate that human-to-swine transmission of pH1N1 has continued to occur frequently in US swine, even in the post-pandemic period. An increase in human-to-swine transmission of pH1N1 during 2013-2014 coincided with a pH1N1-dominated US epidemic in humans, suggesting a link between viral activity in humans and transmission to pigs. In 2014 all 20 pandemic HA and NA segments (HA-pdm and NA-pdm) identified in US swine were newly introduced from humans during the 2013-2014 epidemic. Over the longer term, HA-pdm and NA-pdm sustained transmission in swine at low levels relative to the pandemic internal gene segments, which persist in diverse genomic combinations arising from frequent reassortment. These findings indicate that the six internal gene segments from pH1N1 viruses are likely to be sustained long-term in the US swine population, with periodic re-emergence of pandemic HA and NA segments in association with seasonal pH1N1 epidemics in humans. Vaccinating US swine workers may reduce infection of both humans and swine, and in turn limit the role of humans as sources of influenza virus diversity in pigs.