Large outbreak of swine influenza in southern Japan caused by reassortant (H1N2) influenza viruses: its epizootic background and characterization of the causative viruses.

In the winter of 1989 and the spring of 1990, there were large outbreaks of respiratory disease in two swine herds in Nagasaki Prefecture, southern Japan. Serological surveillance indicated that the majority of swine possessed antibodies to swine influenza virus H1 haemagglutinin and neuraminidase of early H3N2 influenza virus strains. Eight viruses were isolated from swine that showed typical clinical symptoms of influenza. The haemagglutinin and neuraminidase of these isolates were closely related to those of swine H1N1 and early human H3N2 viruses, respectively. At least two types of haemagglutinin antigens, distinguished by two monoclonal antibodies, were involved in the outbreaks. Evolutionary analyses indicated that the haemagglutinin gene of the H1N2 reassortants was closely related to those of a recent swine lineage (A/sw/HK/1/74 and A/sw/Ehime/1/80 viruses). However, the neuraminidase genes of the H1N2 reassortants were similar to those of swine N2 viruses which in turn are related to early human H3N2 viruses. A comparison of partial nucleotide sequences revealed that the six other genes of A/sw/Nagasaki/1/89 were derived from those of swine H1N1 virus.