In a recent early-release article published in the journal _Emerging Infectious Diseases, researchers from the Centers for Disease Control and Prevention (CDC) investigate and describe an unusual mortality event among a cohort of gray (Halichoerus grypus) and harbor (Phoca vitulina) seals infected by a highly pathogenic strain (clade 2.3.4.4b) of the avian influenza (HPAI) A (H5N1) virus. The mortality event, identified in the St. Lawrence Estuary, Quebec, Canada, involved 15 dead seals, and necropsies confirmed that they succumbed to the viral infection.
The researchers conducted detailed postmortem examinations of the seal carcasses and subsequently performed molecular sequencing, revealing that the phylogenetic origins and subtypes of the H5N1 lineages were either exclusively Eurasian or a combination (reassortment) of Eurasian with North American genome constellations. The concurrent presence of a large number of HPAI H5N1-infected seabird carcasses at seal haul-out sites suggests that this event represents a host jump event from birds to seals, raising concerns about the potential establishment of a marine mammalian viral reservoir for this deadly disease and the epidemiological potential for zoonotic spillover to humans and other mammalian taxa.
H5N1 is a subtype of the influenza A virus (IAV) that frequently infects birds (both wild and farmed populations) and has recently been found to spill over to cows and other animals living close to these infected birds. First discovered in farmed poultry in Southern China in 1996, the virus is a fast-evolving pathogen that has since been observed to sporadically infect marine mammals, most commonly pinnipeds such as harbor seals and gray seals in the United States and Europe.
Even though mammalian infections, particularly with High-Pathogenic Avian Influenza (HPAI) virus strains, are rare, a growing body of literature suggests an increase in the disease’s prevalence, highlighting the need for preventive measures to restrict transmission and prevent a potential future human epidemic. Research aimed at characterizing circulating viral strains suggests an avian-variant origin that has since mutated, allowing it to cross-species transmit to mammalian hosts from wild aquatic birds, some of which carry the disease across vast swaths of geography during their annual winter migrations.
The latest identified HPAI H5N1 clade has been named ‘2.3.4.4b A/goose/Guangdong/1/1996 (Gs/GD),’ with its first confirmed North American Atlantic coast victim being a gull carcass found in November 2021 in eastern Canada. Alarmingly, following its discovery, the virus has been observed to rapidly spread across North America, even reassorting with native American IAV strains, increasing the cross-infectivity of the latter and causing unprecedented mortality in both avian and wild terrestrial mammalian hosts. Notably, the summer of 2022 saw widespread harbor and gray seal mortality across eastern Quebec, Canada, and Maine, USA, due to this virus.
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