Emergence of highly pathogenic avian influenza viruses H5N1 and H5N5 in white-tailed eagles, 2021-2023

Abstract

Highly pathogenic avian influenza (HPAI) poses a substantial threat to several raptors. Between 2021 and 2023, HPAI viruses (HPAIVs) of the Goose/Guangdong lineage H5 clade 2.3.4.4b became widespread in wild birds in Norway, and H5N1 and H5N5 viruses were detected in 31 white-tailed eagles (Haliaeetus albicilla, WTEs). Post-mortem examinations of four WTEs revealed no macroscopic pathological findings. Microscopic examinations showed the presence of myocardial and splenic necroses and a few lesions in the brain. In situ hybridization revealed the presence of the virus in several organs, suggesting a multisystemic infection. The detection of HPAIV H5N5 in a WTE in February 2022 marked the first recorded occurrence of this subtype in Norway. Since then, the virus has persisted, sporadically being detected in WTEs and other wild bird species. Phylogenetic analyses reveal that at least two distinct incursions of HPAIV H5N1 Eurasian (EA) genotype C affected WTEs, likely introduced by migratory birds from Eurasia and seabirds entering from Western and Central Europe. Some WTE isolates from 2021 to 2022 clustered with those from Canada and Ireland, aligning with the transatlantic spread of H5N1. Others were related to the 2021 mass mortality of great skuas in the UK or outbreaks in seabird populations, including gannets, gulls and terns, during 2022 in the North Sea region. This suggests that the WTEs were likely preying on the affected birds. Our study highlights that WTEs can act as sentinels for some HPAIV strains, but the absence of several known circulating genotypes in WTEs suggests varying pathogenic effects on this species.

Keywords: H5N1; H5N5; HPAI; Haliaeetus albicilla; histopathology; surveillance.

Fig. 1.. WTEs and other wild birds in Norway sampled and tested for AIV from January 2020 to December 2023. The results of subtyping and pathotyping are shown for the positive samples by colour codes in the square boxes. Negative samples are represented in light blue. Some samples had insufficient amounts of virus for complete NA-subtyping or pathotyping, and these are labelled Nx or lack the label HPAI, respectively.

Fig. 2.. Detection of HPAIVs in WTEs and other wild bird species on mainland Norway from December 2021 to December 2022. (a) HPAIV H5N1 in WTEs (red dots) and municipalities where one or more other wild birds were tested positive for H5N1 (red patches). (b) HPAIV H5N5 in WTEs (purple dots) and municipalities where one or more other wild birds were tested positive for H5N5 (purple patches).

Fig. 3.. In early February 2022, an adult male WTE (no. 4, Table S1) was observed in northern Norway, exhibiting neurological signs such as torticollis, hyperextension of neck/opisthotonos and impaired flying ability. Infection with H5N1 was later confirmed by PCR. Sequencing of the H5 cleavage site to determine pathogenicity was not successful due to the low viral load in the samples. A video featuring the affected WTE can be accessed through the Supplementary Material on Figshare or the NVI’s YouTube account: https://youtu.be/bD0Kb-OUSk4. Photo: Jørn Johnsen.

Fig. 4.. Microscopic findings by ISH of the brain, heart and spleen in WTEs infected with HPAIV. The presence of influenza A virus gene M1 appears red. (a–c) Brain, eagle no. 8. (a) Widespread positive signal in neurons. (b) Positive signal in neurons and glial cells. (c) Viral RNA was detected in endothelial cells and the vessel wall of a venule. (d–f) Heart. (d) Scattered positive signals in myocytes colocalized to areas of myocyte degeneration and necrosis (haematoxylin and eosin stained slide not included). (e) Positive signal in heart myocytes. (f) Viral RNA was detected in the arteriolar wall. (g–h) Spleen. (g) Splenic necrosis with viral RNA in mononuclear inflammatory cells within the necrosis. (h) Scattered positive signal in mononuclear inflammatory cells within the necrosis.

Fig. 5.. Consensus ML phylogenetic trees inferred from the HA gene segments of HPAI H5N1 (a) and H5N5 (b) viruses detected worldwide 2020−2022. The trees are rooted. The outgroups used for rooting have been pruned due to large distances to the rest of the tree bodies and are therefore not displayed. The light green dots in (a) represent the node of origin of the two groups presented as subtrees (Fig. S1d, f). Norwegian WTE isolates are highlighted in pink (two isolates appear as duplicates as they were also sequenced by the EURL and deposited to GISAID under different internal IDs: EPI_ISL_12754 536 and EPI_ISL_12754535). Isolates from other birds in Norway are highlighted in blue. The inner circle shows the year the isolates were obtained. The outer circle depicts the originating continent (a) or country (b) of the isolates. Nordic countries include Norway, Denmark, Finland, Iceland and Sweden. This figure can be viewed in greater detail on the Figshare link.