Disease susceptibility and biological vulnerability of black vultures to fatal clade 2.3.4.4b highly pathogenic avian influenza A(H5N1) virus infection.

High numbers of black vultures (Coragyps atratus) have died from clade 2.3.4.4b highly pathogenic (HP) avian influenza A(H5N1) virus (IAV) since its initial detection in North America in November 2021. Year-round mortality in 2022 may have been maintained by conspecific scavenging. HP IAV was detected in samples from 113/134 (84.3%) dead vultures collected from seven southeastern states in 2022–23. Results indicated that 53/73 (72.6%) of those with gross evaluation in 2022–23 consistently had grossly enlarged and mottled spleen and liver. Histologically, spleens and livers of 14/14 black vultures had widespread, severe necrosis concurrent with immunohistochemical IAV labeling. Additional lesions included segmental digestive tract ulceration, necrosis, and hemorrhage from the oropharynx to the large intestine. Influenza immunolabeling was common in gonad, adrenal gland, kidney, and digestive tract. This pattern, concurrent with field observations, substantiates the digestive tract as the initial site of viral entry during ingestion of high viral doses with strong epitheliotropism. Viral spread outside of seasonal migration is concerning but likely self-limiting in black vulture populations. This efficient, self-perpetuating transmission system may warrant conservation consideration in vultures and other near-obligate scavengers. Continued monitoring of HP IAV in black vultures is needed to evaluate population trends and ecological impacts.