Influenza A virus infects various animal species andtransmits among different hosts, especially betweenhumans and swine. Swine may serve as a mixing vessel tocreate new reassortants that could infect humans. Thus,monitoring and characterizing influenza viruses in swineare important in preventing interspecies transmission. Wereport the emergence and characterization of a novel H3N1subtype of swine influenza virus (SIV) in the United States.Phylogenetic analysis showed that the H3N1 SIVs mayhave acquired the hemagglutinin gene from an H3N2turkey isolate, the neuraminidase gene from a humanH1N1 isolate, and the remaining genes from currently cir-culating SIVs. The H3N1 SIVs were antigenically related tothe turkey virus. Lung lesions and nasal shedding occurredin swine infected with the H3N1 SIVs, suggesting thepotential to transmit among swine and to humans. Furthersurveillance will help determine whether this novel subtypewill continue to circulate in swine populations
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