Haemagglutination-inhibiting antibodies against swine influenza and Hong Kong influenza viruses in swine sera in the USA.

Previous reports have established that swine in the midwestern states of the USA have a high incidence of classical swine influenza and that swine become infected with Hong Kong-like influenza viruses when these are prevalent in the human population. This investigation was undertaken to estimate, on the basis of 2245 sera collected randomly from swine going to slaughter in the USA during the summer months of 1974, how many of the animals had haemagglutination-inhibiting (HI) antibodies against swine influenza and Hong Kong influenza viruses. Based on HI titres of 20 or greater, our serological survey revealed that swine influenza virus infection was widespread throughout the USA, since 20.45% of the sera tested had positive HI titres. However, serological evidence of infection of swine with Hong Kong-like viruses was present in only 2.63% of the sera tested.