Avian metapneumovirus

Avian metapneumovirus (aMPV) is the type species of the genus Metapneumovirus in the subfamily Pneumovirinae within the family Paramyxoviridae. The genome encodes eight discrete transcription units, which are generally assumed to code for nine proteins. Genes are flanked by a 3? leader and 5? trailer in the order 3?-N (nucleocapsid)-P (phosphoprotein)-M (matrix)-F (fusion)-M2 (second matrix)-SH (small hydrophobic)-G (glycoprotein)-L (large polymerase)-5?. Virions are pleomorphic, with sizes ranging from 50 nm to more than 200 nm. Based on the level of genetic variation and antigenic difference, aMPV isolates have been classified into four distinct subtypes: A, B, C and D. Turkeys and chickens are the most susceptible species and show an upper respiratory disease and reproductive disorders when infected. Pheasants, guinea fowls and ducks can also be affected and develop clinical disease. Since its first appearance in South Africa in the late 1970s, aMPV has been reported worldwide. The main approach to control of aMPV is by the use of live attenuated and inactivated vaccines. Several research groups have attempted development of a novel generation of vaccines, but for each of these approaches protection has been inferior to that afforded by live vaccines.