Fig. 4: Comparative lung pathology of clades IIb and Ib after intranasal infection.

Groups of CAST mice (n = 5) were intranasally infected with 10⁵ or 10⁶ pfu of clades IIb and Ib and lung tissue examined at 5 dpi for both clades and at 7 dpi exclusively for clade IIb. a, b Mean and SD of cumulative histopathological lesion scores in lung samples from mock and intranasally infected mice. *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001. c Representative histopathological lung sections (haematoxylin and eosin staining) from mock and MPXV infected mice at 5 dpi. A general view of the lung area along with histopathological details from selected lung areas (black and red boxes) are shown. Note the absence of significant lesions in mock-infected mice, where alveolar spaces without inflammatory changes and empty airways without airway epithelial involvement predominate compared to mice infected at different doses. Legend: b, bronchioles; green arrowheads, small alveolar haemorrhages; red arrowheads, alveolar cellular infiltrates mainly of macrophages and occasional lymphocytes; yellow arrowheads, bronchiolitis; black arrows, moderate perivascular and peribronchiolar mononuclear infiltrates; red arrows, occasional degenerative and viable neutrophils; black asterisks, severe perivascular oedema; green arrows, haemorrhages in the bronchiolar lumen; black arrowheads, necrosis and desquamation of alveolar septa; red arrows, abundant perivascular and peribronchiolar degenerative and viable neutrophils; blue arrowheads, perivascular and alveolar oedema.