Avian Metapneumovirus (aMPV) is associated with swollen head syndrome in chicken and induces rhinotracheitis in turkeys leading to significant losses in poultry production world wide. Without complicating secondary pathogens aMPV mainly affects the upper respiratory tract. Clinical signs include swollen sinuses, nasal discharge, and foamy conjunctivits. But also the reproductive tract of laying birds may be affected. The infection takes place at mucosal surfaces such as of the eye, nasal cavity, trachea as well as the areas of the primary and secondary bronchi. aMPV infection and replication leads to destruction of the respiratory epithelium with ciliostasis, loss of cilia, and mononuclear cell infiltration of the submucosa. Vaccines have to induce immunity at the upper respiratory tract in order to protect against challenge infection. aMPV induces neutralizing antibodies measurable in the lachrymal fluid and trachea, as well as infiltration of activated T cell po