Emergence of H3N2pM-like and novel reassortant H3N1 swine viruses possessing segments derived from the A (H1N1)pdm09 influenza virus, Korea

Background Human-to-swine transmission of the pandemic H1N1 2009 [A(H1N1) pdm09] virus in pig populations resulted in reassortment events with endemic swine influenza viruses worldwide. Objective We investigated whether A(H1N1) pdm09-derived reassortant viruses are present in South Korea and sought to determine the pathogenic potential of the novel swine viruses. Methods Pig lung tissues were collected from commercially slaughtered pigs. Isolated swine influenza viruses were genetically analyzed and characterized in vitro and in vivo. Results We identified reassortant H3N2 (H3N2pM-like) and H3N1 swine viruses containing A(H1N1) pdm09-like segments in Korean pigs that are genetically closely related to strains recently detected in pigs and humans in North America. Although the H3N2pM-like and novel H3N1 reassortants demonstrated efficient replication in mice and ferrets, all the H3N1 strains exhibited growth advantage over the representative H3N2pM-like virus in human airway cells. Interestingly, A/swine/Korea/CY02-07/2012 (H3N1) and A/swine/Korea/CY03-13/2012(H3N1) reassortants were more readily transmitted to respiratory-droplet-contact ferrets compared with the H3N2pM-like (A/swine/Korea/CY02-10/2012) isolate. Furthermore, serologic evaluation showed poor antigenicity to contemporary reference human seasonal H3N2 vaccine strains. Conclusions We report here for the first time the isolation of H3N2pM-like viruses outside North America and of novel reassortant swine H3N1 viruses with A(H1N1) pdm09-derived genes. Apart from further complicating the genetic diversity of influenza A viruses circulating in domestic pigs, our data also indicate that these strains could potentially pose threat to public health asserting the need for continuous virus monitoring in these ecologically important hosts.