Genetic reassortment of avian, swine, and human influenza A viruses in American pigs

In late summer through early winter of 1998, there were several outbreaks o f respiratory disease in the swine herds of North Carolina, Texas, Minnesot a, and Iowa. Four viral isolates from outbreaks in different states were an alyzed genetically. Genotyping and phylogenetic analyses demonstrated that the four swine viruses had emerged through two different pathways. The Nort h Carolina isolate is the product of genetic reassortment between H3N2 huma n and classic swine H1N1 influenza viruses, while the others arose from rea ssortment of human H3N2, classic swine H1N1, and avian viral genes. The hem agglutinin genes of the four isolates were all derived from the human H3N2 virus circulating in 1995. It remains to be determined if either of these r ecently emerged viruses will become established in the pigs in North Americ a and whether they will become an economic burden.