Avian metapneumovirus (aMPV), known as swollen head syndrome in chickens, is a highly contagious upper respiratory tract infection. Infection with this virus can lead to secondary infections including bacteria (E. coli, ORT, Pasteurella spp., B. avium, R. anatipestifer), mycoplasma (MG), aspergillosis, and viruses (e.g. IBV) resulting in potential development of airsacculitis and pneumonia. The mortality rate of aMPV depends on virulence of virus strain, species, age of birds, breeding conditions, immune status, and secondary infections.