A nationwide epidemic of influenza C virus infection in Japan in 2004

Abstract

During the period from January to July 2004, a total of 131 influenza C viruses were detected by cell culture or reverse transcription-PCR (RT-PCR) from specimens that were obtained from children with acute respiratory symptoms in 10 prefectures across Japan. Influenza C virus was identified most frequently in the Miyagi (1.4%, 45 of 3,226 specimens) and Yamagata (2.5%, 31 of 1,263 specimens) prefectures, and the frequency in this year was the highest since 1990. Phylogenetic analysis of the hemagglutinin esterase gene of the 13 strains isolated in nine prefectures revealed that genetically similar strains belonging to the Kanagawa/1/76-related lineage dominantly spread throughout Japan. During the 2004 influenza season, influenza C virus coexisted with epidemics of influenza A virus (H3 strain), and 12 cases were identified from patients who had been diagnosed with influenza-like illness (7 were detected by RT-PCR, and 5 were detected by culture). A comparison of specimens that were found positive by culture with those found positive only by RT-PCR shows that the amount of virus in PCR-positive specimens tended to be lower than in isolation-positive specimens. Although the mean peak temperature in patients in the PCR-positive group was slightly lower, there were no significant differences in characteristics between specimens (i.e., kind of specimen, period from onset to specimen collection, age distribution of patients, and severity of illness). These results suggest that an epidemic of influenza C virus occurred on a national scale during this period and that RT-PCR can be an effective supplemental tool for the evaluation of clinical and epidemiological information.

FIG. 1.

Map showing the prefectures where influenza C viruses were detected and the monthly distribution of influenza C virus isolates (□) and RT-PCR positive specimens (▪) between January and July 2004. RT-PCR for the detection of influenza C virus was performed in the Yamagata, Niigata, Okayama, Hiroshima, and Fukuoka prefectures.

FIG. 2.

Phylogenetic tree for the influenza C virus based on the coding region from nucleotide 64 to 1989 of the HE gene. Representative strains of the HE gene lineage are marked by asterisks. Viruses isolated in the present study are indicated in boldface. Numbers above the branches are the percent bootstrap probabilities of each branch determined by the PHYLIP program (version 3.573c).