A total of 6346 swine sera collected at an abattoir in the city of Obihiro, Hokkaido during the years 1978–87 were tested for the presence of antibodies to swine and human influenza viruses. A high incidence of antibody to A/New Jersey/8/76 (swine type H1N1) virus was observed throughout the 10 years except for the occasional month and a single long period of 15 months. Antibodies to human H3N2 virus in swine appeared to be related to the epidemics of human influenza which occurred in the study area during the years 1980–3, but unrelated to the epidemics during the years 1984–7. A large number of swine were found to be antibody positive to a human H1N1 virus during the period April to June 1964, and a smaller number, during the period November 1986 to June 1987. Both were in relation to human influenza epidemics. However, there were long periods where human H1N1 antibodies in swine could not be found.
The first occurrence of swine influenza in Japan was recognized in 1977, when it was presumed that the disease was introduced via imported swine (Shibata elal. 1978). Further outbreaks of swine influenza and a high prevalence of antibody to the virus in Japanese swine populations have been reported by several workers (Yamane, Sukeno & Ishida, 1978; Sugimura elal. 1981; Ogawa elal. 1983). An outbreak of influenza virus infection due to an H3N2 strain was previously seen in a herd of swine in Osaka, Japan (Sugimura etal. 1975). Later the co-existence of swine (H1N1) and human (H3N2) influenza viruses was confirmed by serological and virological studies on Japanese swine populations (Onta et al. 1978; Sugimura et al. 1980; Arikawa et al. 1982). In a previous report (Miwa et al. 1986), we suggested that the swine became infected with a human H1N1 virus as piglets during an epidemic of influenza which occurred in the human population at the same time. The present study was undertaken to evaluate the changes in the prevalence of antibodies against swine and human influenza viruses in Japanese swine during the past 10 years.