Evidence of human-to-swine transmission of the pandemic (H1N1) 2009 influenza virus in South Korea

Abstract

As the pandemic (H1N1) 2009 influenza virus continues to infect human populations globally, reports on epidemiologically linked animal infections are also on the rise. Since December 2009, pandemic (H1N1) 2009-like viruses have been isolated in pigs from different swine farms of South Korea. Genetic and phylogenetic analyses of viral segments demonstrated several events of human-to-swine transmission with no apparent signs of reassortment. These events were also supported by serological surveillance in pig sera collected from April to December, suggesting that reverse transmission probably started between June and July with a drastic increase in prevalence the following months. Although molecular characterization indicates that the swine isolates are generally stable, some viruses are genetically evolving, most notably in their surface proteins. Animal studies (ferrets and mice) reveal that swine pandemic isolates epitomize biological properties attributed to the currently circulating human pandemic viruses, including replication kinetics and efficient transmission, indicating their potential to return to circulation among humans. Overall, these results indicate widespread human-to-animal transmission of pandemic (H1N1) 2009 influenza viruses in South Korea. With the significant role of pigs in the ecology of influenza viruses, these transmission events should be closely monitored and minimized to prevent the risk of generating viruses with greater human health concerns.

FIG. 1.

(a and b) Phylogenetic analysis of the HA and NA genes of pandemic (H1N1) 2009 influenza virus isolates from South Korea. Phylogenetic trees of the nucleotide sequences for the H1 (a) and N1 (b) genes of pandemic viruses isolated from pigs and humans in this study compared with nucleotide sequences from selected swine, human, and avian influenza virus strains available in GenBank are shown. The nucleotide sequences were aligned using Clustal_X (1, 38), and the phylograms were generated by the neighbor-joining method using the tree-drawing program NJ plot (31). The scale represents the number of substitutions per nucleotide. Branch labels record the stability of the branches over 1,000 bootstrap replicates. Only bootstrap values ≥400% are shown in each tree. Sw, swine; Tk, turkey.

FIG. 2.

Monitoring of body temperatures in infected and contact ferrets. Mean body temperatures of ferrets experimentally infected with Korea/CJ01/09, Sw/Korea/SCJ01/09, or the recent Korean swine H1N1 (Sw/Korea/CAN01/04) isolate, including naïve contact animals, were monitored daily starting 1 day before inoculation and continuing up to 12 days postinfection. Ranges of normal body temperatures are indicated as broken lines. Standard error bars are shown.