An epidemic of swine influenza in Japan.

In a serological survey of pigs in Japan haemagglutination inhibition (HI) antibody to swine influenza virus (Hsw1N1) was first detected in 721 of 3313 (21.1 per cent) sera in 1977 and 1578 of 4946 (32.1 per cent) in 1978. The virus was prevalent in all districts of Japan but to a different extent. In contrast to the high incidence of antibody, there were few cases of infection with clinical signs and only 13 outbreaks of swine influenza were recognised in Japan from 1978 to 1979. Thirty-seven influenza viruses were isolated from nasal swabs of diseased pigs in 13 outbreaks. The subtype of most isolates was Hsw1N1 but two strains were Hsw1N2.