Histopathological changes associated with infection with monkeypox virus. (a) Lung, animal M965A, euthanized on day 2 postchallenge. Sloughing of respiratory bronchiolar epithelial cells (arrow) close to the junction with a bronchus. H&E. (b) Lung, animal M029F, euthanized on day 4 postchallenge. Focal epithelial cell necrosis (arrow), with sloughed cells and neutrophil infiltration at the junction of a bronchus and bronchiole. H&E. Inset, lung, animal I089K, euthanized on day 4 postchallenge. Focal, positive immunostaining of bronchial epithelium for monkeypox viral antigen. IHC. (c) Tracheobronchial lymph node, animal M029F, euthanized on day 4 postchallenge. Infiltration of superficial cortex by macrophages (arrow). HE. Inset, scattered, positively stained cells for monkeypox viral antigen. IHC. (d) Spleen, animal M293E, euthanized on day 4 postchallenge. Scattered, positively stained cells for monkeypox viral antigen in the white pulp. IHC. (e) Lung, animal M865B, euthanized 6 days postchallenge. Focal, bronchial epithelial necrosis (arrow). HE. (f) Spleen, animal I430G, euthanized on day 6 postchallenge. Positive immunostaining for monkeypox viral antigen in a periarteriolar lymphoid sheath (PALS) within the white pulp. IHC.