National Institute of Animal Health, NARO

Characteristics of domestic highly pathogenic avian influenza viruses in the 2021 season

National Agriculture and Food Research Organization (NARO) has conducted genetic and pathogenicity analysis of H5 highly pathogenic avian influenza virus (HPAIV) identified in domestic poultry farms from November 10, 2021, to May 14, 2022. From the analysis result it was revealed that there were two subtypes of the virus such as H5N8 subtype (2 cases) & H5N1 subtype (23 cases), and three groups of viruses entered the country during the same period based on the characteristics of the hemagglutinin (HA) gene in the 2021 season. Representative viruses from these groups all showed high lethality to chickens, but their infectivity and transmissibility differed between groups. This season's outbreak was the longest lasting to date and included the introduction of multiple virus groups and it is thought that the chances of the virus invading farms also increased much. Therefore, it is necessary to continue to monitor the trend of the HPAIV epidemic and be vigilant against the invasion of the virus into Japan and farms.


Outbreaks caused highly pathogenic avian influenza (HPAI) were confirmed at chicken farms in Akita Prefecture on November 10, 2021, and in Kagoshima Prefecture on November 13, 2021. Then the H5N8 subtype and H5N1 subtype highly pathogenic avian influenza virus (HPAIV) were isolated from dead chickens respectively (Akita strain and Kagoshima strain). Followed to these outbreaks, 23 additional outbreaks of H5N8 and H5N1 HPAIV had been reported until 14 May 2022.

Whole genome of HPAIV isolated from 25 outbreaks were sequenced and phylogenetic tree analysis for the hemagglutinin (HA) gene segment was conducted. The phylogenetic tree showed that H5N8 subtype HPAIV is closely related to "2020-2021 Winter Asian HPAIV (20A)", H5N1 subtype HPAIV is closely related to "2020-2021 Winter European HPAIV (20E)" or "2021-2022 European HPAIV (21E)", and three groups of viruses, 20A, 20E, and 21E, invaded Japan during the same period. In addition, no mutations that increase resistance to existing antiviral drugs or increase infectivity in mammals were observed in the deduced amino acid sequences of all viruses.

While analyzing the relationship between the virus subtypes/groups isolated from poultry and their outbreak times, it was found that H5N8 subtype 20A was detected in Akita and Kagoshima prefectures in early and mid-November, and H5N1 subtype 20E was detected in Kagoshima prefecture in mid-November and outbreaks from these two groups occurred at approximately the same time. 20A caused only two outbreaks in November, while 20E was detected from November to January and May 2022. In this season, 20E had the highest number of outbreaks, and there was no particular geographical bias. 21E was detected from mid-February to mid-May 2022, and its occurrence was limited to Hokkaido and Tohoku regions. Multiple groups of viruses were involved in poultry outbreaks in Hokkaido, Aomori, Akita and Kagoshima prefectures.

H5 subtype HPAIV of 20A and 21E groups was also detected in some wild bird or environmental samples from November 8 to May 14 during the same period.

For the classified three groups of viruses, H5N8 subtype Akita strain (20A), H5N1 subtype Kagoshima strain (20E) and Iwate strain (21E), an intravenous inoculation test in chickens was conducted as prescribed by the World Organization for Animal Health (OIE) and it showed a high fatality rate of 100%, exceeding the 75% fatality rate that defines hypervirulence. In addition, when intranasal inoculation tests were conducted on these three strains assuming a natural infection route, depression was observed in infected chickens, and remarkable cyanosis and neurological symptoms were observed in some chickens. As a result of intranasal inoculation test, it was clarified that (1) The difference in infection lethality is about 8 times at the maximum (2) The mean death days for chickens inoculated with a viral titer in which all chickens were infected and died was from 2.2 days at the shortest to 3.5 days at the longest, with a difference of 1.3 days (3) Since the transmissibility from one chicken inoculated with each virus to six chickens cohousing together is 33.3-100%, the infectivity and transmissibility differ among the three groups of virus strains.

From the analysis of outbreak information and detected viruses in poultry and wild birds in the 2021 season, it has become clear that this season's outbreak in poultry continued for the longest period to date, and viruses with different genetic backgrounds and properties entered the country. Therefore, it was thought that the virus brought into the country by wild birds had more opportunities to invade farms.

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