Introduction of Multiple Novel High Pathogenicity Avian Influenza (H5N1) Virus of Clade 2.3.4.4b into South Korea in 2022.

Since October 2020, H5N1 clade 2.3.4.4b high pathogenicity avian influenza (HPAI) viruses have spread to many countries. Although these viruses evolved from Eurasian ancestors, reassortant with other LPAI viruses has generated various genotypes. Here, we identified three H5N1 HPAI viruses belonging to clade 2.3.4.4b; these viruses were isolated from mandarin duck, common teal, and domestic breeder ducks in October 2022 during an avian influenza (AI) active surveillance program. Two of the H5N1 viruses (MD/WA496 and BD/H493) have been found sporadically in China, Russia, and Korea. It is presumed that two viruses with a similar gene constellation isolated in Russia, China, and Korea were introduced into the breeding area during the spring migration, and were introduced newly to Korea during the autumn migration. Due to international bird migration, the other virus (CT/WA537) is most similar (99.3-99.8%) to a virus detected in North Dakota, USA in April 2022. These results suggest that H5N1 viruses with at least two genotypes were introduced at the same time into Korea during the autumn of 2022, and that they originated from Eurasian breeding grounds and North America. Thus, multiple 2.3.4.4b H5N1 viruses were introduced into Korea during the autumn season of 2022.