Abstract

The highly pathogenic avian influenza HPAI A(H5N1) outbreak in spring 2022 caused a mass mortality of seabirds in the St. Lawrence Estuary and Gulf, Québec, Canada. Concurrently, an unusually high number of marine mammal strandings (mainly harbor seals) were reported in the Estuary. A total of 61 carcasses of marine mammals were submitted for post-mortem examination. Nasal/anal swabs were also taken on 14 additional carcasses. The post-mortem examinations and/or PCR analyses identified infections by an HPAI A(H5N1), clade 2.3.4.4b, in 21 harbor seals, 1 grey seal, and 1 white-sided dolphin. Histopathological lesions suggestive of H5N1 infections were documented in 17 animals (57% juvenile animals). All (8/8) infected adult harbor seals were females. Antemortem respiratory and neurological signs were observed in some animals. Grossly, the affected animals were in good nutritional condition, indicative of an acute mortality. Generalized lymphoadenomegaly was the predominant macroscopic finding. The most frequent histological lesions were acute to subacute mixed meningoencephalitis (16/17), pneumonia, including suppurative alveolitis (13/17), necrotizing lymphadenitis (8/16), adrenocortical necrosis (6/13), necrotizing hepatitis (5/16), and necrotizing splenitis (4/16). The geographical and temporal distributions of the cases, which coincided with the peak of the outbreak in bird colonies, are suggestive of a transmission from birds to marine mammals through direct contact with sick birds at seal haul-out sites during the pupping season.¹ Because influenza viruses are known for their zoonotic potential, there are persistent and growing concerns about the endemic presence of a mammal-adapted subtype of HPAI in Canadian seals that are consumed by hunters.

Acknowledgments

The authors wish to thank the Réseau québécois d’urgences pour les mammifères marins for the stranding archives and their work in the field responding to stranding reports.

*Presenting author
+Student presenter

Literature Cited

1.  Reperant LA, Rimmelzwaan GF, Kuiken T. Avian influenza viruses in mammals. Rev Sci Tech. 2009;28(1):137–159.