#19,347
Last December - in addition to reporting numerous biosecurity breaches on farms - South Korea's MAFRA Reported Increased Infectivity & Pathogenicity of This Year's Avian Flu Strains, which - for the first time - included 3 HPAI strains (H5N1, H5N6, and H5N9).
Of particular concern, their Animal and Plant Quarantine Agency conducted an evaluation of the infectivity and pathogenicity of one highly pathogenic avian influenza virus (serotype H5N1) and found that the infectivity was more than 10 times higher than in previous years.
Details at the time were understandably scant, but today we've a research article which sheds new light on the panoply of viruses behind last year's difficult avian flu season in South Korea.
While this is a fairly technical report, and the use of regional nomenclature for H5N1 genotypes further complicates matters, the gist is pretty simple.
Last year South Korea was visited by a diverse panoply of H5Nx genotypes (n=16), with at least two - H5N9 N9-G2 and H5N1 25G2 - displaying unusually high infectivity in chickens.
H5N1 25G2 was also deadly to ducks, whereas H5N9 N9-G2 spread efficiently in ducks, albeit without producing serious illness.
H5N1 25G2 was the source of nearly half (n=30) of last year's 62 South Korean poultry outbreaks, but it was only detected in environmental samples once, making its prevalence in wild birds unknown.
Whether 25G2 (or H5N9 N9-G2) will return to South Korea this fall - or turn up in other regions - remains to be seen.
Due to its technical nature, I've only provided the link, abstract, and a few excerpts. Follow the link to read the report in its entirety.
Yunyueng Jang, Ra Mi Cha, Min-Ji Park, Jong-Min Kim, Eui Hyeon Lim, Gyeong-Beom Heo, Se-Hee An, Bina Lee, Hyun-ji Seo, Kwang-Nyeong Lee, Youn-Jeong Lee, Eun-Kyoung Lee
10.1016/j.virusres.2026.199807
Highlights
- During 2025-2026, clade 2.3.4.4b H5N1, H5N6, and H5N9 HPAIVs were detected in Korea, causing 62 H5Nx HPAI outbreaks in poultry, and 63 cases in wild birds.
- Ten genotypes (24G1, 25G2–25G7, and N9-G1–3) identified in poultry, among which 25G2 was the dominant genotype.
- Most genotypes were reassortants containing gene segments derived from East Asian clade 2.3.4.4b H5N1 HPAIVs since 2022 and LPAIVs from wild birds.
- The 25G2 and N9-G2 genotype showed high infectivity in chickens with 103.3 and 103.2 EID50 of LD50 values, respectively.
- The 25G2 caused rapid duck mortality, whereas H5N9 caused asymptomatic infection with efficient transmission.
ABSTRACT
Clade 2.3.4.4b H5N1, H5N6, and H5N9 high pathogenicity avian influenza viruses (HPAIVs) were detected in poultry and wild birds during the 2025–2026 winter season in Korea. During this period, 62 H5Nx HPAI outbreaks occurred in poultry, and 63 cases were reported in wild birds.
Genome constellation analysis revealed substantial genotype diversity. Most H5N1 and H5N9 genotypes were reassortants containing gene segments derived from East Asian clade 2.3.4.4b H5N1 HPAIVs since 2022 and low-pathogenicity avian influenza viruses from wild birds.
Notably, the European H5N1 genotype EA-2024-DI.2.1, which spread rapidly across Europe in 2025, and its reassortants were identified in Korea. The novel poultry genotypes were also detected in wild birds, supporting wild bird to poultry spillover, whereas one genotype detected in previous season was identified in small-scale poultry farms.
H5N1 (C588) of major 25G2 genotype in poultry and H5N9 (C722) of N9-G2 genotype showed high infectivity in chickens with 103.3 and 103.2 EID50 of median lethal dose (LD50) values, respectively. The predominance of the 25G2 genotype in poultry may have been influenced by efficient infection and virus introduction into high-density poultry production areas, potentially facilitating horizontal transmission under field conditions.
In young ducks, H5N1 caused rapid mortality, whereas H5N9 induced limited clinical signs without mortality. Both viruses were transmitted to all contact ducks, increasing the risk of transmission to poultry. Despite intensified surveillance and early detection efforts to limit virus spread, H5Nx HPAIV outbreaks occurred during the 2025–2026 winter season. These findings highlight the need for continued surveillance, together with rapid genomic and pathogenic characterization of newly introduced viruses.
(SNIP)
Genotype 25G2 was the predominant genotype detected in poultry during the 2025–2026 season, accounting for 30 outbreaks. However, it was identified in only one environmental sample collected during an epidemiological investigation of migratory bird habitats. This outbreak pattern differed from those observed during previous seasons in Korea, in which the predominant genotypes were generally similar between wild birds and poultry (Cha et al., 2023; Cha et al., 2025; Cha et al., 2026).
The predominance of genotype 25G2 in poultry may have been influenced by higher infectivity under field conditions, particularly in layer farms located in high-density poultry farming areas where HPAI outbreaks were concentrated. However, introduction through environmental contamination associated with wild birds cannot be excluded, and the potential influence of sampling bias resulting from spatial and temporal variations in bird migration and surveillance should also be considered (Hayes et al., 2025; Llanos-Soto, Yaffy, Pavlak and Ivanek, 2025).
This study revealed multiple introductions of genetically diverse H5 viruses during the 2025–2026 winter season and demonstrated their relatively high infectivity in chickens. Epidemiological investigations of concurrent outbreaks, together with virus detection and genetic analyses, helped identify potential sources of virus introduction in the affected regions and provided evidence of viral contamination in the surrounding environment. Despite enhanced surveillance and early detection efforts to limit virus spread, H5 HPAI outbreaks still occurred in poultry farms. These findings highlight the importance of continued surveillance in wild birds and poultry, coupled with the rapid genomic and pathogenic characterization of newly emerging viruses, to support the implementation of effective avian influenza control disease.
