Topics

• During the 2024 season, a total of six genotypes of High Pathogenicity Avian Influenza (HPAI) viruses were identified in Japan, including one genotype detected continuously for four consecutive seasons and five genotypes newly identified during this season.
• Whole-genome analysis revealed viral sharing and genetic reassortment among wild bird populations associated with different migratory routes.

The National Agriculture and Food Research Organization (NARO) conducted whole-genome analysis of 225 strains of HPAI viruses collected from domestic poultry, wild birds, and environmental samples in Japan during the 2024 season. As a result, NARO identified a total of six genetic genotypes of HPAI viruses: one genotype that has been continuously detected for four consecutive seasons, and five genotypes that were confirmed for the first time in this season.

Particularly noteworthy was the finding that genetic reassortment of the virus had occurred among wild bird populations migrating from different breeding grounds. Previous reports indicated that the HA gene of HPAI viruses carried by wild birds was relatively stable, and there had been no known cases of replacement by HA genes from different wild bird populations. Therefore, HA genes could be grouped according to the birds' breeding grounds, and migratory routes could be inferred from the HA gene sequences. However, in this analysis, a virus was identified in which only the HA gene had been replaced by that from a different population, while all the other genome segments originated from the same group. Furthermore, the replaced HA gene belonged to a different genetic group, suggesting that genetic reassortment had occurred between viruses originating from wild birds of different breeding areas. In other words, a virus carried by one wild bird population traveled with the birds and, upon reaching Japan or nearby regions, was shared with birds from a different migratory route. The virus was then carried back to each breeding ground, suggesting that a new genotype had formed. Such events are believed to accelerate viral diversification and increase the variety of viruses introduced into Japan.

Viral genetic diversity is closely linked to the movement of wild birds, highlighting the importance of continued monitoring of viral dynamics among wild bird populations. Some viral genotypes detected in wild birds have also been found at poultry farms, indicating that continued vigilance will remain necessary. NARO will continue conducting whole-genome analyses to contribute to a better understanding of virus transmission trends both in Japan and abroad, and to help strengthen disease control measures.