Human-like swine H3 influenza A viruses (IAV) were detected by the USDA surveillance system. We characterized two novel swine human-like H3N2 and H3N1 viruses with hemagglutinin (HA) genes similar to those in human seasonal H3 strains and internal genes closely related to those of 2009 H1N1 pandemic viruses. The H3N2 neuraminidase (NA) was of the contemporary human N2 lineage, while the H3N1 NA was of the classical swine N1 lineage. Both viruses were antigenically distant from swine H3 viruses that circulate in the United States and from swine vaccine strains and also showed antigenic drift from human seasonal H3N2 viruses. Their pathogenicity and transmission in pigs were compared to those of a human H3N2 virus with a common HA ancestry. Both swine human-like H3 viruses efficiently infected pigs and were transmitted to indirect contacts, whereas the human H3N2 virus did so much less efficiently. To evaluate the role of genes from the swine isolates in their pathogenesis, reverse genetics-generated reassortants between the swine human-like H3N1 virus and the seasonal human H3N2 virus were tested in pigs. The contribution of the gene segments to virulence was complex, with the swine HA and internal genes showing effects in vivo. The experimental infections indicate that these novel H3 viruses are virulent and can sustain onward transmission in pigs, and the naturally occurring mutations in the HA were associated with antigenic divergence from H3 IAV from humans and swine. Consequently, these viruses could have a significant impact on the swine industry if they were to cause more widespread outbreaks, and the potential risk of these emerging swine IAV to humans should be considered.
All Science Journal Classification (ASJC) codes
- Insect Science