Genetic variation among human strains of influenza C virus isolated in Japan

The RNA genomes of sixteen human strains of influenza C virus isolated in Japan between 1964 and 1983 were compared by SDS-polyacrylamide gel electrophoresis and oligonucleotide fingerprinting. A high degree of genetic variation was observed among the strains analysed. However, there were some strains with the genomes closely related to one another, and they could be divided into two groups. The first group consists of C/Shizuoka/79, C/Kanagawa/1/81 and four strains of C/Yamagata/81. The 1981 strains of this group were all isolated in March of the year. The second one consists of C/Kyoto/41/82, C/Nara/82 and C/Hyogo/1/83 that were isolated between February 1982 and December 1983. Little or no difference was observed in the genomes of the same group, while the difference was evident between two groups. The Aichi/1/81 strain isolated in November 1981 had a genome distantly related to either of these two groups. Thus three different types of influenza C virus were isolated during the period of 12 mth from March 1981 to February 1982, suggesting that multiple influenza C viruses with distant genetic relationship were circulating at the same time in Japan.