Fig. 2 Histopathological findings in chickens (scheduled euthanasia at dpi 2) characteristic for HPAI.a Chicken, P17–882, group C5.7, respiratory mucosa. Moderate, multifocal, acute, necrotizing rhinitis with epithelial degeneration characterized by attenuation, loss of cilia (arrow), and sloughing. b Chicken P17–882, group C5.7, respiratory mucosa with coalescing foci of influenza A matrixprotein-immunoreactive (“antigen-positive”) morphologically intact and degenerated epithelial cells (arrow). Notably, there are scant immunoreactive granules in some of the submucosal nerves (arrowhead), suggestive of axonal spread. c Chicken, P17–903, group M5.7, cecum. Mild, oligofocal, acute, crypt epithelial degeneration with nuclear pyknosis (arrow), necrosis and sloughing. d Chicken, P17–903, group M5.7, cecum. Antigen-positive morphologically intact and degenerated crypt epithelia (arrow). e Chicken, P17–903, group M5.7, brain. Mild, oligofocal, acute, necrotizing polioencephalitis characterized by neuroglial cytoplasmic hypereosinophilia, nuclear pyknosis, karyorrhexis and loss (arrow), as well as an associated status spongiosus interpreted as inflammatory edema. f Chicken, P17–903, group M5.7, brain. Antigen-positive neuroglial cells within necrotizing lesion. g Chicken, P17–903, group M5.7, spleen. The periarteriolar lymphoid sheaths and follicles display moderate, coalescing apoptotic lymphocytes characterized by cytoplasmic hypereosinophilic shrinkage and nuclear karyorrhexis (arrows). h Chicken, P17–903, group M5.7, spleen. Rare, individual antigen-positive round cells can be detected in the white and red pulp. a, c, e, g: Hematoxylin eosin. b, d, f, h: Influenza A virus-matrixproteinIHC. a–f: bar = 50 µm. g, h: bar = 20 µm