Avian metapneumovirus (aMPV), also known as turkey rhinotracheitis or swollen head syndrome, causes a variety of disease syndromes in birds, depending on the bird species and virus type (A, B, C or D). First detected in 1978 in South Africa, the virus has spread all over the world except Australia. It is mostly found in turkeys, pheasants, Muscovy ducks and guinea fowls. Turkeys are the most susceptible species for all subtypes. It is presumed that other species are immune.