Department of Infectious Diseases, Yamagata University School of Medicine, Yamagata 990-9585, Japan, Department of Microbiology, Yamagata Prefectural Institute of Public Health, Yamagata 990-0031, Japan, Division of Public Health, Department of Infectious Disease Control and International Medicine, Niigata University, Graduate School of Medical and Dental Sciences, Niigata 951-8510, Japan, Department of Microbiology, Fukushima Institute for Public Health, Fukushima 960-8163, Japan, Department of Viral Diseases, Saitama Institute of Public Health, Saitama 338-0824, Japan, Department of Microbiology, Aichi Prefectural Institute of Public Health, Nagoya 462-8576, Japan, Department of Virology, Okayama Prefectural Institute for Environmental Science and Public Health, Okayama 701-0298, Japan, Division of Microbiology II, Hiroshima Prefectural Institute of Health and Environment, Hiroshima 743-0007, Japan, Fukuoka City Institute for Hygiene and the Environment, Fukuoka 810-0065, Japan, Yamanobe Pediatric Clinic, Yamanobe, Yamagata 990-0301, Japan, Virus Research Center, Clinical Research Division, Sendai Medical Center, Sendai 983-8520, Japan.

During the period from January to July 2004, a total of 131 influenza C viruses were detected by cell culture or RT-PCR from specimens that were obtained from children with acute respiratory symptoms in ten prefectures across Japan. Influenza C virus was identified most frequently in 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 coexisted with epidemics of influenza A (H3 strain) and twelve cases were identified from patients who had been diagnosed with influenza-like illness (7 were done by RT-PCR and 5 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 the virus in PCR-positive specimens tended to be lower than that 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 evaluation of clinical and epidemiological information.