Pathologic Characterization of Highly Pathogenic H5N1 Avian Influenza Virus Infections in Wild Mammals in Alaska, USA

This report describes highly pathogenic avian influenza virus (HPAI) H5N1 infections in carnivores in Alaska, USA between 2022 and 2024, including a black bear (Ursus americanus), a brown bear (Ursus arctos), and the first known report of HPAI in an ermine (Mustela ermina). The two bears were cubs, and the ermine was a young adult. The black bear and ermine were euthanized after demonstrating neurologic signs, including circling, blindness, ataxia, or seizures. The brown bear was found dead. Gross lesions included swelling and congestion of the brain in both bears. Microscopically, all three individuals exhibited severe, nonsuppurative necrotizing meningoencephalitis, which was most prominent in the gray matter. Nonneuropathic microscopic lesions shared between both bears were multifocal random hepatic necrosis with lymphoplasmacytic hepatitis and mild interstitial pneumonia with bronchitis. The spectrum of additional lesions in individual animals consisted of acute necrosis within multiple other organs, including skeletal muscle, pancreas, kidney, retina, adrenal glands, and intestine. Immunohistochemistry demonstrated positive staining within neurons and glial cells in the brain, enterocytes and neurons of the myenteric plexus in the intestine, rare bronchiolar epithelial cells, renal tubular epithelial cells, and myocyte nuclei. Two HPAI-positive red foxes (Vulpes vulpes) and one positive Arctic fox (Vulpes lagopus) with incomplete histopathology are briefly discussed, as they demonstrate potential for co-infections with HPAI and canine distemper virus or rabies virus.