Antigenic and genetic characterization of influenza C viruses which caused two outbreaks in Yamagata City, Japan, in 1996 and 1998

Abstract

During the 3 years from January 1996 to December 1998, 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.

FIG. 1.

Monthly distribution of influenza C virus strains isolated in Yamagata City between 1996 and 1998.

FIG. 2.

Phylogenetic tree of influenza C virus HE genes. The region from nucleotides 64 to 1989 was used for analysis. The 1996 and 1998 isolates from Yamagata are marked by asterisks, and Yamagata and Sendai isolates having HE antigenicity identical to that of MS80 are marked by daggers. Horizontal distances are proportional to the minimum number of nucleotide differences needed to join the gene sequences. Numbers below and above the branches are the percent bootstrap probabilities of each branch, determined by the PHYLIP program (version 3.54c).

FIG. 3.

Phylogenetic trees for the PB2 (A), PB1 (B), M (C), NS (D), P3 (E), and NP (F) genes of influenza C virus isolates. The nucleotide sequences of the following regions were used for analysis: nucleotides 52 to 520 for PB2, nucleotides 50 to 425 for PB1, nucleotides 49 to 420 for P3, nucleotides 71 to 670 for NP, nucleotides 26 to 1,147 for M, and nucleotides 28 to 889 for NS. The 1996 and 1998 isolates from Yamagata are marked by asterisks, and the Yamagata and Sendai isolates having HE genes on the MS80-related lineage are marked by daggers. Horizontal distances are proportional to the minimum number of nucleotide differences needed to join the gene sequences. Numbers above the branches are the percent bootstrap probabilities of each branch, determined by the PHYLIP program (version 3.54c).

FIG. 3.

Phylogenetic trees for the PB2 (A), PB1 (B), M (C), NS (D), P3 (E), and NP (F) genes of influenza C virus isolates. The nucleotide sequences of the following regions were used for analysis: nucleotides 52 to 520 for PB2, nucleotides 50 to 425 for PB1, nucleotides 49 to 420 for P3, nucleotides 71 to 670 for NP, nucleotides 26 to 1,147 for M, and nucleotides 28 to 889 for NS. The 1996 and 1998 isolates from Yamagata are marked by asterisks, and the Yamagata and Sendai isolates having HE genes on the MS80-related lineage are marked by daggers. Horizontal distances are proportional to the minimum number of nucleotide differences needed to join the gene sequences. Numbers above the branches are the percent bootstrap probabilities of each branch, determined by the PHYLIP program (version 3.54c).