title: Frequent Isolations of Influenza A Viruses (H1N1)pdm09 with Identical Hemagglutinin Sequences for More Than Three Months in Japan.
abstract: BACKGROUND
  Although it has been suggested that antigenic drift does not occur in a single epidemic season in temperate countries, there is not enough evidence on the circulation period of influenza virus with identical nucleotide sequences. Therefore, strains of influenza virus were isolated sequentially during five consecutive epidemic seasons in Japan and their nucleotide sequences were determined.
  
  
  METHODS
  Nasal swabs or aspirated nasal discharges were collected from influenza A virus antigen-positive individuals living in Tottori Prefecture, Japan for five consecutive winters starting in 2009-2010, and subjected to viral isolation, determination of hemagglutinin nucleotide sequence and phylogenic analyses. The nucleotide sequences were compared with each other and also with those of foreign strains in the International Nucleotide Sequence Database.
  
  
  RESULTS
  Totally 288 A(H1N1)pdm09 strains were tested and those composed 38 clusters with identical ones displaying 100% nucleotide homology. One strain showed sequential infections more than three months without any detectable mutation, and a maximum interval of two detection timings of strains was 94 days. This implies that influenza viruses mutate rarely in an epidemic season in Japan if they can be hypothesized, mutation frequency of influenza viruses being mostly the same among strains. Among these identical strains, two strains were not only identical to other Japanese isolates, but also to those isolated in Mongolia and Thailand in the same epidemic season.
  
  
  CONCLUSIONS
  These results suggest that genetic drift has occurred infrequently in Japan as shown in some other countries. The drifted strains may have generated somewhere else and entered into Japan. These results support the proposed 'sink-source' model of viral ecology in which new lineages are seeded from a persistent influenza reservoir in tropical countries to 'sink' populations in temperate regions including Japan.
authors.affiliation: Division of Virology, Department of Microbiology and Immunology, School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan; †Division of Oral and Maxillofacial Biopathological Surgery, Department of Medicine of Sensory and Motor Organs, School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan.
authors.forename: Yu
authors.initials: Y
authors.lastname: Yoshida
authors.affiliation: Division of Virology, Department of Microbiology and Immunology, School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan.
authors.forename: Akeno
authors.initials: A
authors.lastname: Tsuneki
authors.affiliation: Division of Virology, Department of Microbiology and Immunology, School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan.
authors.forename: Asao
authors.initials: A
authors.lastname: Itagaki
authors.affiliation: ‡Tottori Infectious Diseases Forum, Tottori Prefecture, Japan; §Tsuchie Internal Medicine and Pediatric Clinic, Sakaiminato 684-0011, Japan.
authors.forename: Hideaki
authors.initials: H
authors.lastname: Tsuchie
authors.affiliation: ‡Tottori Infectious Diseases Forum, Tottori Prefecture, Japan; ‖Department of Pediatrics, Tottori Prefectural Kousei Hospital, Kurayoshi 682-0804, Japan.
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Okada
authors.affiliation: ‡Tottori Infectious Diseases Forum, Tottori Prefecture, Japan; ‖Department of Pediatrics, Tottori Prefectural Kousei Hospital, Kurayoshi 682-0804, Japan.
authors.forename: Sakae
authors.initials: S
authors.lastname: Narai
authors.affiliation: ‡Tottori Infectious Diseases Forum, Tottori Prefecture, Japan; ¶Kasagi Children's Clinic for Health Service, Yonago 683-0822, Japan.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Kasagi
authors.affiliation: ‡Tottori Infectious Diseases Forum, Tottori Prefecture, Japan; *Tanaka Pediatric Clinic, Tottori 680-0844, Japan.
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Tanaka
authors.affiliation: Division of Virology, Department of Microbiology and Immunology, School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan; †Division of Oral and Maxillofacial Biopathological Surgery, Department of Medicine of Sensory and Motor Organs, School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan.
authors.forename: Akiko
authors.initials: A
authors.lastname: Ito
authors.affiliation: †Division of Oral and Maxillofacial Biopathological Surgery, Department of Medicine of Sensory and Motor Organs, School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Ryoke
authors.affiliation: Division of Virology, Department of Microbiology and Immunology, School of Medicine, Tottori University Faculty of Medicine, Yonago 683-8503, Japan; ‡Tottori Infectious Diseases Forum, Tottori Prefecture, Japan.
authors.forename: Seiji
authors.initials: S
authors.lastname: Kageyama
keywords: Japan; genetic drift; influenza A virus; sequence homology; temperate country