Immunohistochemical stain was performed on 5 mm thick deparaffinized sections using monoclonal antibodies against the nucleoprotein of influenza A (Serotec, UK) by a two-step peroxidase method. For controls, we used unrelated antibodies in place of the primary antibody. Briefly, sections were deparafinized by 2 washes in xylene and were rehydrated through decreasing concentrations of ethanol. After washing in PBS at pH 7.6 for 5 min at room temperature (RT), sections were heattreated with antigen-retrieval solution (TRIS/EDTA buffer, pH 9, Dako, Denmark) for 10 min using microwave antigen retrieval method. After blocking with 10% normal horse serum for 10 min at RT, the sections were incubated with the specific antibody (1:100) for 30 min at RT. Unbound antibody was removed by 3 washes in PBS before adding HRP-labelled polymer for 30 min at RT (Dako CSA Detection System, Denmark). After washing unbound labeled polymer in PBS 3 times, peroxidase staining in tissue sections was revealed by DAB solution (CSA Detection Systems, DAKO, Denmark). After stopping the reaction in running water, sections were counter-stained with a quick rinse in Mayer's hematoxylin solution. After dehydrating with increasing concentrations of ethanol and xylene, the sections were mounted with DPX and examined by light microscopy (Olympus BX51, Japan).