Characterization of an influenza A virus isolated from pigs during an outbreak of respiratory disease in swine and people during a county fair in the United States

Abstract

In August 2007, pigs and people became clinically affected by an influenza-like illness during attendance at an Ohio county fair. Influenza A virus was identified from pigs and people, and the virus isolates were characterized as swine H1N1 similar to swine viruses currently circulating in the U.S. pig population. The swine isolate, A/SW/OH/511445/2007 (OH07), was evaluated in an experimental challenge and transmission study reported here. Our results indicate that the OH07 virus was pathogenic in pigs, was transmissible among pigs, and failed to cross-react with many swine H1 anti-sera. Naturally exposed pigs shed virus as early as 3 days and as long as 7 days after contact with experimentally infected pigs. This suggests there was opportunity for exposure of people handling the pigs at the fair. The molecular analysis of the OH07 isolates demonstrated that the eight gene segments were similar to those of currently circulating triple reassortant swine influenza viruses. However, numerous nucleotide changes leading to amino acid changes were demonstrated in the HA gene and throughout the genome as compared to contemporary swine viruses in the same genetic cluster. It remains unknown if any of the amino acid changes were related to the ability of this virus to infect people. The characteristics of the OH07 virus in our pig experimental model as well as the documented human transmission warrant close monitoring of the spread of this virus in pig and human populations.