The possible origin H1N1 (Hsw1N1) virus in the swine population of Japan and antigenic analysis of the isolates.

Virus isolation and serological studies on swine sera collected during 1973 to 1978 showed that H1N1 (Hsw1N1) influenza viruses first appeared in the swine population of Japan about May 1977. With the exception of one strain, both haemagglutinin and neuraminidase subunits of all the H1N1 viruses isolated from swine in Japan and from pigs imported from North America were antigenically indistinguishable from those of A/NJ/8/76 virus, suggesting the introduction of swine influenza virus into Japan with imported pigs from North America as breeding stock. Antigenic analysis of a recombinant virus by neuraminidase-inhibition tests with specific antisera to the isolated neuraminidases of A/Victoria/3/75 and A/Aichi/2/68 revealed that the neuraminidase antigen of the recombinant virus, A/swine/Kanagawa/2/78 (H1N2), was closely related to those of A/Tokyo/6/73 (H3N2) and A/Kumamoto/22/76 (H3N2) viruses.