Outbreak of Highly Pathogenic Avian Influenza Virus H5N1 in Seals in the St. Lawrence Estuary, Quebec, Canada

We describe an unusual mortality event caused by a highly pathogenic avian influenza virus (HPAI) H5N1 clade 2.3.4.4b involving harbor ( Phoca vitulina ) and grey ( Halichoerus grypus ) seals in the St. Lawrence Estuary, Quebec, Canada. Fifteen (56%) of the seals submitted for necropsy were considered to be fatally infected by H5N1 containing fully Eurasian (EA) or Eurasian/North American genome constellation. Concurrently, presence of large numbers of bird carcasses infected with H5N1 at haul-out sites most likely contributed to the spill-over of infection to the seals. Histologic changes included meningoencephalitis (100%), fibrinosuppurative alveolitis, and multi-organ acute necrotizing inflammation. This is the first report of fatal H5N1 infection in pinnipeds in Canada, raising concerns about the expanding host of this virus, potential for establishment of a marine mammal reservoir, and the public health risks associated with spillover to mammals.