Introduction
Highly pathogenic avian influenza (HPAI) A(H5N1) clade 2.3.4.4b has emerged as a critical global concern due to its rapid geographic spread, expanding host range, and increasing spillover into mammalian species. Once largely restricted to avian hosts, this virus has demonstrated a growing capacity to infect wildlife, livestock, and occasionally humans, raising alarms for animal health, food security, and pandemic preparedness. The unprecedented mortality event among pinnipeds on Tyuleniy Island in 2023 underscores the evolving ecology of HPAI H5N1 and highlights the urgent need to understand its pathobiology, transmission dynamics, and adaptive potential in non-avian hosts.
Mass Mortality Event in Pinnipeds on Tyuleniy Island
During July–August 2023, Tyuleniy Island in the Sea of Okhotsk experienced a catastrophic mortality event involving more than 3,500 northern fur seals and at least one Steller sea lion. The scale and rapid progression of deaths suggested an infectious etiology with high virulence. This event represents a rare and alarming example of large-scale HPAI-associated mortality in marine mammals, emphasizing the vulnerability of densely populated pinniped colonies to emerging viral pathogens and the potential for significant ecosystem-level impacts.
Virological Characterization and Pathogenicity
Two HPAI A(H5N1) viruses isolated from northern fur seal carcasses demonstrated high pathogenicity in established animal models. Both strains showed severe virulence in chickens and mice, yet differed in disease progression, tissue tropism, and histopathological outcomes. Notably, one strain induced more pronounced pulmonary and neurological damage in mice, suggesting strain-specific differences in virulence mechanisms. These findings highlight the biological diversity of H5N1 viruses even within a single outbreak and stress the importance of detailed phenotypic characterization.
Genomic Features and Mammalian Adaptation
Whole-genome sequencing revealed that the isolates were closely related to H5N1 viruses circulating in the Russian Far East and Japan between 2022 and 2023. Importantly, several mutations associated with mammalian adaptation were identified, including NP-N319K and PB2-E627K in one isolate. These genetic markers are known to enhance viral replication and virulence in mammals, suggesting that the viruses infecting fur seals had already acquired traits facilitating cross-species transmission and adaptation beyond avian hosts.
Spillover Dynamics and Host Susceptibility
The evidence indicates that northern fur seals acted as spillover hosts rather than primary reservoirs for HPAI H5N1. However, their high population density, close physical contact, and shared environments may facilitate sustained transmission and viral amplification. The susceptibility of pinnipeds to clade 2.3.4.4b viruses reinforces concerns that marine mammals could serve as intermediate hosts, potentially enabling further viral evolution and adaptation to mammals.
Implications for Surveillance and One Health
This first documented case of HPAI H5N1 in pinnipeds in the North Pacific region has profound implications for wildlife surveillance and One Health strategies. It highlights the necessity of integrated monitoring across avian, terrestrial, and marine ecosystems to detect early signs of viral spillover and adaptation. Understanding the role of pinnipeds in influenza virus ecology is crucial for assessing zoonotic risk and preventing future outbreaks with broader ecological and public health consequences.
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