# Pathobiology of Highly Pathogenic Avian Influenza A (H5N1 Clade 2.3.4.4b) Virus from Pinnipeds on Tyuleniy Island in the Sea of Okhotsk, Russia

## TL;DR

This study documents the first case of H5N1 bird flu in seals in the North Pacific, showing how the virus spreads to mammals and causes severe disease.

## Contribution

The first documented case of HPAI H5N1 in pinnipeds in the North Pacific region is reported, with insights into viral pathogenicity and adaptation.

## Key findings

- Two HPAI H5N1 viruses were isolated from northern fur seal carcasses and showed high pathogenicity in chickens and mice.
- Strain A/74/2023 caused more severe pulmonary and neurological lesions in mice compared to A/75/2023.
- Genomic analysis revealed mutations linked to mammalian adaptation, including NP-N319K and PB2-E627K in one isolate.

## Abstract

Highly pathogenic avian influenza (HPAI) A(H5N1) clade 2.3.4.4b has recently emerged as a major threat to wildlife, agriculture, and public health due to its expanding host range and the increasing frequency of spillover into mammals. In July–August 2023, the mass death of over 3500 northern fur seals (Callorhinus ursinus) and at least one Steller sea lion (Eumetopias jubatus) was recorded on Tyuleniy Island in the Sea of Okhotsk, Russia. Two HPAI A(H5N1) viruses were isolated from fur seal carcasses and designated A/Northern_fur_seal/Russia_Tyuleniy/74/2023 and A/Northern_fur_seal/Russia_Tyuleniy/75/2023. Both viruses exhibited high pathogenicity in chickens (IVPI 2.7–3.0) and mice (MLD50 1.9–2.5 log10EID50/mL), with distinct differences in disease progression, histopathology, and organ tropism. Experimental infection of mice revealed that strain A/74/2023 induced more severe pulmonary and neurological lesions than A/75/2023. Whole-genome sequencing and phylogenetic analysis demonstrated close relatedness to HPAI H5N1 strains circulating in the Russian Far East and Japan from 2022 to 2023, with several mutations associated with mammalian adaptation, including NP-N319K and, in one isolate, PB2-E627K. According to our findings, northern fur seals (Callorhinus ursinus) on Tyuleniy Island acted as spillover hosts for the highly pathogenic avian influenza (HPAI) H5N1 virus of clade 2.3.4.4b. Furthermore, the high population density of fur seals and the extensive mortality observed during the outbreak highlight these animals’ potential role as another vessel for the evolution of avian influenza viruses. This study represents the first documented case of HPAI H5N1 in pinnipeds in the North Pacific region and supports previous reports indicating that pinnipeds, including northern fur seals, are highly susceptible to HPAI H5N1 clade 2.3.4.4b viruses.