Prevalence of human metapneumovirus and influenza virus infections among Japanese children during two successive winters.

BACKGROUND: Human metapneumoviruses (hMPVs) are recognized as a leading cause of respiratory infections in young infants in many countries. The objective of this study was to identify links between hMPV and influenza virus infections among children with influenza-like illness.
METHOD: This study was conducted in 2 influenza seasons (2002-2003 and 2003-2004) at 2 pediatric outpatient clinics in Niigata city, Japan. Nasopharyngeal swabs or aspirates were collected from influenza-like illness patients, and hMPV and influenza were detected by reverse transcription-polymerase chain reaction (RT-PCR). A nucleotide sequence of 352 nucleotides segment of the F gene was performed.
RESULTS: A total of 765 influenza viruses and 84 hMPV were identified from 1498 nasopharyngeal swabs or aspirates by virus isolation and RT-PCR, respectively. hMPV-positive rates in patients with influenza-like illness were 5.7 and 5.2% in the 2002-2003 and 2003-2004 seasons, respectively. Epidemic curves of influenza and hMPV patients showed similar patterns with peaks in February in 2 influenza seasons. hMPV infections occurred frequently in infants and school children. Approximately 46% of hMPV patients were coinfected with influenza A viruses, but those coinfected cases were not clinically distinct from the others. No coinfection with influenza B viruses was found. Phylogenetic analysis of the hMPV fusion gene sequences revealed that 2 distinct hMPV cocirculated and that completely identical strains in subgroup A were observed over 2 years.
CONCLUSIONS: HMPV plays an important pathogenic role in patients with influenza-like illness in winter seasons, often in coinfections with influenza A viruses.