Antigenic and genetic analyses of the hemagglutinin of influenza viruses isolated from pigs in 1993

Three strains of influenza A virus (H3N2) were isolated from pigs in Hokkaido, Japan in 1993. The hemagglutinin (HA) antigen of the three isolates was related to that of recent H3N2 viruses of human origin. The reactivity patterns of two of the isolates (A/sw/Obihiro/1/93 and A/sw/Obihiro/2/93) with monoclonal antibodies to the hemagglutinin of A/Bangkok/1/79 strain were similar to that of the human H3N2 strain isolated in Hokkaido in 1988, while that of the other one (A/sw/Obihiro/3/93) was similar to human H3N2 strains of 1993. In the phylogenetic analysis based on the nucleotide sequences of the HA1 regions, the HA genes of the two isolates were shown to be closely related to those of human H3N2 viruses isolated between 1986 and 1988. The remaining one isolate was shown to be closely related to those of current human H3N2 viruses. We have also found serological evidence that the A/sw/Obihiro/1/93 virus is circulating extensively in Obihiro swine. It is clear from these findings that pigs were infected with the recent H3N2 influenza virus during the human epidemic and that the virus has been maintained in pigs for at least five years.