1）シンポジウム“鳥インフルエンザの最新の知見について—最大規模の発生にどのように取り組むべきか—”
日本および世界で今冬報告された鳥インフルエンザウイルス
2021 年 25 巻 1 号 p. 1-5
詳細
2021 年 25 巻 1 号 p. 1-5
Avian influenza virus is a causal agent of avian influenza, which is belonging to Orthomyxoviridae, and possess the two types of glycoproteins on the surface of a virion; hemagglutinin (HA) and neuraminidase (NA). Based on the reactivity to antiserum, the HA and NA of Type A influenza viruses are classified into H1 to H16, and N1 to N9, respectively. Since 1997, high pathogenicity avian influenza (HPAI), especially caused by the infection of H5HA, had been reported in Asia. Especially, since 2010, H5N8 HPAI viruses (HPAIVs) have been prevalent not only in Asia but also in other regions including Europe and Africa. As of 2020, the H5N8s were further classified into clade 2.3.4.4 according to the amino acid sequence, and H5 HPAIV subclades of a to h in the clade 2.3.4.4 are confirmed. Graduate School of Veterinary Medicine was designated as OIE reference laboratory for highly and low pathogenic avian influenza since 2005 and had conducted several actions including global surveillance of avian influenza. We had already stocked the isolates and developed the genetic database of them (https://virusdb.czc.hokudai.ac.jp).
In October 2020, we isolated the H5N8 HPAIV from a fecal sample collected at the Komuke Lake in Hokkaido under the global surveillance, which was designated as A/northern pintail/Hokkaido/M13/2020 (H5N8) (NP/Hok/20). It was revealed that NP/Hok/20 was classified into the H5 clade 2.3.4.4b in the HA of the phylogenetic tree and genetically very close to the H5N8 HPAIV isolated in Europe in the winter of 2019-2020. Furthermore, comprehensive phylogenetical analysis using the H5N8 HPAIV isolated in the Far East in the winter of 2020-2021 revealed that those isolates were classified into three categories including 1) the European isolates in the winter of 2019-2020, 2) the European isolates in the winter of 2020-2021, and 3) isolates in China and Mongolia. Since the genetic reassortment between the European H5N8 isolates and avian influenza viruses in the Asia was confirmed in the phylogenetical analysis, further spread with wider genetic variation might occur.
HPAI spread situations in the Europe and Japan in the end of 2020 were similar to ones in the end of 2016; many of the contagious viruses were brought by bird migration from the northern territories. Given the situation that very close viruses were isolated at both edges of the Eurasian continent at the same season, it is likely that the contagious viruses had been already perpetuated in the northern territory where the migratory birds nest in the summer season. Periodic updates of intensive survey at the global level as well as the reinforcement of the biosecurity measures in the poultry farm are essential to prepare for future HPAI outbreak.
