Pathology and Tissue Distribution of Highly Pathogenic Avian Influenza Virus Clade 2.3.4.4b in Free-Ranging Mountain Lions (Puma concolor) and Bobcats (Lynx rufus) from Colorado, USA.

During a 1-mo period in early 2023, four adult free-ranging mountain lions (Puma concolor) and two adult free-ranging bobcats (Lynx rufus) died or were euthanized during an outbreak of H5N1 highly pathogenic avian influenza virus (HPAIV) in Colorado, USA. All six animals tested positive for HPAIV clade 2.3.4.4b, and necropsy findings resembled those described by others for domestic cats and wild felids infected with HPAIV clade 2.3.4.4b in North America. The most consistent findings in the Colorado wild felids were necrotizing and lymphoplasmacytic meningoencephalitis (6/6) and interstitial to bronchointerstitial pneumonia (3/6). Necrotizing lesions were also identified in other tissues, including liver (3/6) and adrenal gland (2/4). Immunohistochemistry demonstrated localization of influenza A-specific antibody to necrotizing lesions. Sequencing of the HPAIV PB2 gene did not reveal mutations to suggest mammalian adaptation, and resolution of the outbreak in wild felids coincided with a decline in cases in wild birds. The presumed mechanism of transmission to wild felids was exposure at the time of ingestion of infected avian carcasses. Immunohistochemistry of tongue, pinna, and skeletal muscle did not indicate utility of these samples for noninvasive sampling of hunter-harvested carcasses for HPAI surveillance. We recommend testing brain tissue for HPAIV surveillance in wild felids.