In 2022, an unusual mortality event occurred among harbor seals (Phoca vitulina) and gray seals (Halichoerus grypus) in the St. Lawrence Estuary, Quebec, Canada. The cause was a highly pathogenic avian influenza (HPAI) A (H5N1) virus clade 2.3.4.4b.
Affected seals: Fifteen animals submitted for necropsy were considered fatally infected by HPAI H5N1. This represented 56% of the seals examined.
Genome constellations: The infected seals carried HPAI H5N1 containing fully Eurasian or Eurasian/North American genome constellations.
Source of infection: Concurrently, large numbers of bird carcasses infected with HPAI H5N1 were found at seal haul-out sites, likely contributing to the spillover of infection to the seals.
Histological changes: Histologic changes in the infected seals included meningoencephalitis (100%), fibrinosuppurative alveolitis, and multiorgan acute necrotizing inflammation.
Public health implications:
The report raises concerns about the expanding host range of the HPAI H5N1 virus.
It highlights the potential for the establishment of a marine mammal reservoir for the virus.
Spillover to mammals poses public health risks.
Phylogenetic analyses:
The presence of HPAI H5N1 clade 2.3.4.4b viruses with different genome constellations suggests multiple sources of infection.
The outbreak likely involved more than one introduction of the virus into the seal population.
Geographic distribution:
The infected pinniped were found mainly on the south shore of the St. Lawrence Estuary, between the towns of Baie-Comeau and Notre-Dame-du-Portage.
Discussion
Investigation of increased deaths among the pinniped populations of the St. Lawrence Estuary and Gulf during the summer of 2022 detected HPAI H5N1 infections in harbor and gray seals.
All necropsied seals that were positive for H5 by PCR also manifested histologic lesions consistent with IAV infection.
The infection of mammal species such as seals by HPAI H5N1 viruses raises concern about recent viral mutations making possible entry and replication within mammalian cells.
From a human health perspective, such changes in viral host range warrant continued vigilance to detect a potentially deadly epidemic before its emergence.
In addition, marine mammals, such as seals or other pinnipeds, might act as reservoirs for this virus, which could contribute to increasing risk for mutations and viral reassortment, favoring the infection of new mammal hosts.
Therefore, monitoring the occurrence and molecular characteristics of this HPAI virus in populations of wild marine mammals is essential for assessing the public health risk associated with this emerging pathogen–host dynamic.
In summary, this outbreak underscores the need for vigilance in monitoring avian influenza viruses in wildlife populations and understanding their potential impact on public health. The emergence of HPAI H5N1 in seals adds complexity to the dynamics of influenza transmission and highlights the interconnectedness of ecosystems.
