Isolation and characterization of novel H3N1 swine influenza viruses from pigs with respiratory diseases in Korea

Abstract

Pigs can play an important role in the genetic reassortment of influenza viruses and as a reservoir for another lineage of influenza viruses that have the ability to reassort and be transmitted between species. In March and April 2006, novel H3N1 influenza A viruses were isolated from pigs with respiratory diseases at two different commercial swine farms in Korea. Genetic and phylogenetic analyses of the sequences of all eight viral RNA segments showed that the novel H3N1 swine influenza viruses were reassortants that acquired the hemagglutinin gene from an H3 human-like virus and other genes from swine influenza viruses that are currently circulating in Korea. Serologic and virologic tests in the infected farms suggested that pig-to-pig and farm-to-farm transmissions occurred. Clinical signs in pigs and experimentally infected mice suggest the potential to transmit the virus between swine and other mammalian hosts. To our knowledge, this is the first report of the isolation of the swine H3N1 subtype from domestic pigs under field conditions in Korea. Further surveillance will be needed to determine whether this novel subtype will continue to circulate in the swine population.

FIG. 1.

Phylogenetic trees of the nucleotide sequences for the HA (a) and NA (b) genes of the two H3N1 influenza viruses isolated from pigs in Korea compared with selected swine, human, and avian influenza virus strains. The nucleotide sequences were aligned using Clustal_X (1, 29), and the phylograms were generated by the neighbor-joining method using NJplot (24). The percent bootstrap values for each node are shown in each tree. The scale represents the number of substitutions per nucleotide. Standard postal abbreviations are used for state names in the United States.

FIG. 2.

Mean viral titers of the lung homogenates (a) from five infected mice at different days after the experimental inoculation with negative control (phosphate-buffered saline) and H1N2, H3N2, and H3N1 swine influenza viruses. The mice were inoculated intranasally with 10^5.0 TCID₅₀/head of swine influenza virus strains H1N2 (Sw/Korea/JI04/05), H3N2 (Sw/Korea/CA54/04), and H3N1 (Sw/Korea/PZ72-1/06). Five mice from each group were euthanized on 1, 3, 5, and 7 days postinfection (DPI) in order to titrate virus in the lung tissues. The remaining mice were observed for clinical signs and body weight was measured until 10 days postinfection (b).