Virologic and serologic surveillance for human, swine and avian influenza virus infections among pigs in the north-central United States

Influenza virus infection in pigs is both an animal health problem and a pu blic health concern. As such, surveillance and characterization of influenz a viruses in swine is important to the veterinary community and should be a part of human pandemic preparedness planning. Studies in 1976/1977 and 198 8/1989 demonstrated that pigs in the U.S. were commonly infected with class ical swine H1N1 viruses, whereas human H3 and avian influenza virus infecti ons were very rare. In contrast, human H3 and avian H1 viruses have been is olated frequently from pigs in Europe and Asia over the last two decades. F rom September 1997 through August 1998, we isolated 26 influenza viruses fr om pigs in the northcentral United States at the point of slaughter. All 26 isolates were H1N1 viruses, and phylogenetic analyses of the hemagglutinin and nucleoprotein genes from 11 representative viruses demonstrated that t hese were classical swine H1 viruses. However, monoclonal antibody analyses revealed antigenic heterogeneity among the HA proteins of the 26 viruses. Serologically, 27.7% of 2,375 pigs tested had hemagglutination-inhibiting a ntibodies against classical swine H1 influenza virus. Of particular signifi cance, however, the rates of seropositivity to avian H1 (7.6%) and human H3 (8.0%) viruses were substantially higher than in previous studies.