Immunohistochemistry of the infundibulum, magnum and uterus of challenged birds in experiment 2. Representative 3-µm sections of oviducts collected during the acute phase of infection, in experiment 1. Sections were incubated with a monoclonal antibody targeting the Influenza A virus nucleoprotein. Positivity was revealed with 3, 3-diaminobenzidine (DAB), forming a brown precipitate. A Strong positivity of an infundibulum collected on day 4 pi, showing severe necrosis and erosion of the epithelial mucosa. The arrow indicates positivity of the mesenchymal cells lining the serosa. Epithelial staining of B magnum and C uterus collected on days 6 pi.