Subcellular localization of NS1 in duck ileal epithelial cells in vivo. A) Ileum collected 6 days post-infection from a Pekin duck infected orally with 107 pfu of the 977 virus, formalin fixed, sectioned at 3 μm. Immunohistochemical anti-NS1 staining detected with a peroxidase-coupled secondary antibody revealed with diaminobenzidine, hematoxylin counterstained. Arrowheads point to NS1 positive enterocytes. Scale bar = 20 μm. B) Magnification of the dotted region shown in panel A. NS1 staining is detected in the cytoplasm and in subnuclear structures, corresponding to nucleoli. Scale bar = 10 μm.