In a 9-year survey from December 1990 to December 1999 in Sendai City, Japan, we succeeded in isolating a total of 45 strains of influenza C virus. These 45 strains were isolated in clusters within 4 months in a year, especially from winter to early summer. Previous studies of the hemagglutinin-esterase genes of various influenza C virus isolates revealed the existence of five distinct virus lineages (Aichi/1/81-, Yamagata/26/81-, Mississippi/80-, Sao Paulo/82-, and Kanagawa/1/76-related lineage) in Japan between 1970 and the early 1990s

The antigenic structure of influenza A/H2N2 virus haemagglutinin (HA) was analysed using 19 monoclonal antibodies (MAbs) against the HA of A/Kayano/57. The antibodies were classified into three groups : group I had both haemagglutination inhibition and neutralization activities, group II had neutralization activity but no haemagglutination inhibition activity and group III had neither activity.

, a total of 33 strains of influenza C virus were isolated from 10,726 throat swab specimens collected from children with acute respiratory illness who visited two pediatric clinics in Yamagata City, Japan. These 33 strains were isolated in clusters during two different periods, 20 strains in May to August 1996 and the remaining 13 in March to June 1998. Antigenic analysis with monoclonal antibodies to the hemagglutinin-esterase (HE) glycoprotein and phylogenetic analysis of seven RNA segments showed that the 33 influenza C viruses isolated were antigenically and genetically similar and that they were reassortant viruses which had obtained PB2, PB1, HE, M, and NS genes from a C/pig/Beijing/ 115/81-like virus and P3 and NP genes from a C/Mississippi/80-like virus. These observations suggest strongly that during the survey period of 3 years, two outbreaks of influenza C occurred in Yamagata City, both of which were caused by a reassortant virus having the genome composition described above.Influenza C virus usually causes a mild upper respiratory illness (11) but can also cause lower respiratory infections, such as bronchitis and pneumonia (19). Although seroepidemiological studies revealed that influenza C virus is widely distributed throughout the world (5, 9, 10, 22), outbreaks of illness caused by the virus have rarely been detected, and the virus has been isolated only occasionally (6,7,8,11,12,18). Therefore, information about the epidemiology of influenza C is scarce compared to that for influenza A and influenza B.The genome of the influenza C virus consists of seven RNA segments, which encode three polymerase proteins (PB2, PB1, and P3), hemagglutinin-esterase (HE) glycoprotein, nucleoprotein (NP), matrix (M1) protein, and two nonstructural proteins (NS1 and NS2) (reviewed in reference (14). Thus, reassortment characterized by exchange of genome segments between two different influenza C virus strains occurs not only in vitro (24) but also in nature (2,17,27,32). However, the epidemiological significance of genetic reassortment in influenza C viruses is unknown.To obtain more information about influenza C epidemiology, we developed a tissue culture method for primary virus isolation that is convenient for routine work with a large number of specimens (19, 23) and then initiated surveillance for influenza C virus infections in Yamagata City in 1988 and in the adjacent city of Sendai in 1990. We compared the antigenic specificity of the HE glycoprotein as well as the HE gene sequence among 42 strains isolated between 1947 and 1993 (which included 13 strains obtained by our surveillance work) and revealed the existence of six distinct virus groups, represented by strains Taylor/47, Kanagawa/1/76 (KA176), Yamagata/26/81 (YA2681), Aichi/1/81 (AI181), Sao Paulo/378/82 (SP82), and Mississippi/80 (MS80), four of which (YA2681-, AI181-, SP82-, and MS80-related lineages) circulated in Japan in the 1980s and the early 1990s (17).We report here the results of a 3-year survey (January 1996 to December 1998) performed ...

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