Isolation and genetic characterization of new reassortant H3N1 swine influenza virus from pigs in the midwestern United States.

Since the introduction of H3N2 swine influenza viruses (SIVs) into U.S. swine in 1998, H1N2 and H1N1 reassortant viruses have emerged from reassortment between classical H1N1 and H3N2 viruses. In 2004, a new reassortant H3N1 virus (A/Swine/Minnesota/00395/2004) was identified from coughing pigs. Phylogenetic analyses revealed a hemagglutinin segment similar to those of contemporary cluster III H3N2 SIVs and a neuraminidase sequence of contemporary H1N1 origin. The internal genes were of swine, human, and avian influenza virus origin, similar to those of contemporary U.S. cluster III H3N2 SIVs. The recovery of H3N1 is further evidence of reassortment among SIVs and justifies continuous surveillance.