Commonground
Senior Moderator
[bolding is mine]
June 5, 2025
Abstract
The highly pathogenic avian influenza virus (HPAIV) H5N1 clade 2.3.4.4b has rapidly disseminated globally, with mammalian infections reported in multiple species. Recent evidence of mammal-to-mammal transmission has heightened concerns about the virus’s potential adaptation to mammals. The polymerase basic 2 (PB2) protein E627K mutation appears to be of key importance for mammalian adaptation. We isolated an HPAI H5N1 clade 2.3.4.4b virus from wild birds in Korea with 96% E and 4% K at amino acid position 627 of PB2. To investigate the genomic characteristics of this clade regarding mammalian adaptation, we studied the replication and transmission of the H5N1 virus in mice. Two experiments with different challenge-to-contact ratios were conducted to assess transmission dynamics and mutation development. In experiment 1, a 4:1 challenge-to-contact ratio resulted in 100% transmission among direct-contact mice, with all mice succumbing to the infection. In experiment 2, a 1:1 ratio yielded 50% transmission, with all challenged mice also succumbing. High viral loads were observed in the lungs and brains in both experiments, with viral titers increasing over time. Notably, the PB2-E627K variant, initially present at 4% in the virus stock, was selected and reached near-fixation (~ 100%) in the lungs and brains by 6 days post-challenge and was subsequently transmitted. No other mammalian-adaptive mutations were identified, emphasizing the pivotal role of PB2-E627K in early stages of mammalian adaptation. These findings highlight the need for continuous genomic monitoring to detect mammalian adaptation markers and assess interspecies transmission risks.
The H5N1 subtype of highly pathogenic avian influenza (HPAI) virus, first identified in a goose in Guangdong, China, in 1996 (Gs/GD), has since spread globally, infecting various domestic and wild bird species and occasionally crossing the species barrier to infect mammals, including humans [1]. In autumn 2020, novel reassortant clade 2.3.4.4b H5N1 HPAI viruses were detected and became predominant among poultry and wild birds in Europe, subsequently spread to Africa, the Middle East, and Asia [2]. Since then, numerous mammalian infections have been reported worldwide in species such as black bears, bobcats, coyotes, and ferrets. Most cases were ‘dead-end’ infections, resulting from animals consuming infected birds [3]. However, recent reports indicate mammal-to-mammal transmission, including New England seals in the United States, a mink farm in Spain, and sea lions in Peru [4]. These findings suggest an increasing threat of mammalian adaptation in H5N1 clade 2.3.4.4b. A key factor limiting the cross-species transmission of this virus is its low polymerase activity in mammalian cells. However, viruses have evolved the ability to overcome this barrier by acquiring adaptive mutations in the polymerase and NP proteins [5]. Previous studies have shown that most AIVs rapidly adapt to new hosts and become highly pathogenic by acquiring PB2 E627K or D701N mutations after only 2–3 consecutive passages. Mutations in the PB2 subunit, a key component of the polymerase complex, have been shown to significantly enhance viral replication and adaptability in mammalian hosts. Among these, the E627K substitution is particularly significant, replacing glutamic acid (E) with lysine (K) at position 627 (Fig. 1). This mutation is crucial for efficient replication in human hosts [6].
Continued: https://virologyj.biomedcentral.com/articles/10.1186/s12985-025-02811-w
June 5, 2025
Abstract
The highly pathogenic avian influenza virus (HPAIV) H5N1 clade 2.3.4.4b has rapidly disseminated globally, with mammalian infections reported in multiple species. Recent evidence of mammal-to-mammal transmission has heightened concerns about the virus’s potential adaptation to mammals. The polymerase basic 2 (PB2) protein E627K mutation appears to be of key importance for mammalian adaptation. We isolated an HPAI H5N1 clade 2.3.4.4b virus from wild birds in Korea with 96% E and 4% K at amino acid position 627 of PB2. To investigate the genomic characteristics of this clade regarding mammalian adaptation, we studied the replication and transmission of the H5N1 virus in mice. Two experiments with different challenge-to-contact ratios were conducted to assess transmission dynamics and mutation development. In experiment 1, a 4:1 challenge-to-contact ratio resulted in 100% transmission among direct-contact mice, with all mice succumbing to the infection. In experiment 2, a 1:1 ratio yielded 50% transmission, with all challenged mice also succumbing. High viral loads were observed in the lungs and brains in both experiments, with viral titers increasing over time. Notably, the PB2-E627K variant, initially present at 4% in the virus stock, was selected and reached near-fixation (~ 100%) in the lungs and brains by 6 days post-challenge and was subsequently transmitted. No other mammalian-adaptive mutations were identified, emphasizing the pivotal role of PB2-E627K in early stages of mammalian adaptation. These findings highlight the need for continuous genomic monitoring to detect mammalian adaptation markers and assess interspecies transmission risks.
The H5N1 subtype of highly pathogenic avian influenza (HPAI) virus, first identified in a goose in Guangdong, China, in 1996 (Gs/GD), has since spread globally, infecting various domestic and wild bird species and occasionally crossing the species barrier to infect mammals, including humans [1]. In autumn 2020, novel reassortant clade 2.3.4.4b H5N1 HPAI viruses were detected and became predominant among poultry and wild birds in Europe, subsequently spread to Africa, the Middle East, and Asia [2]. Since then, numerous mammalian infections have been reported worldwide in species such as black bears, bobcats, coyotes, and ferrets. Most cases were ‘dead-end’ infections, resulting from animals consuming infected birds [3]. However, recent reports indicate mammal-to-mammal transmission, including New England seals in the United States, a mink farm in Spain, and sea lions in Peru [4]. These findings suggest an increasing threat of mammalian adaptation in H5N1 clade 2.3.4.4b. A key factor limiting the cross-species transmission of this virus is its low polymerase activity in mammalian cells. However, viruses have evolved the ability to overcome this barrier by acquiring adaptive mutations in the polymerase and NP proteins [5]. Previous studies have shown that most AIVs rapidly adapt to new hosts and become highly pathogenic by acquiring PB2 E627K or D701N mutations after only 2–3 consecutive passages. Mutations in the PB2 subunit, a key component of the polymerase complex, have been shown to significantly enhance viral replication and adaptability in mammalian hosts. Among these, the E627K substitution is particularly significant, replacing glutamic acid (E) with lysine (K) at position 627 (Fig. 1). This mutation is crucial for efficient replication in human hosts [6].
Continued: https://virologyj.biomedcentral.com/articles/10.1186/s12985-025-02811-w