The demographic history of class II aPMV-1 determined by Bayesian skyline plots indicated that there was an abrupt decline in population size during 1997-98. Although the factors responsible for such an abrupt decline in class II aPMV-1 are not known, the impact of a severe El-Nino event during that time frame [60] or the slaughter of millions of domestic fowl during the first outbreak of H5N1 avian influenza virus in 1997 [61, 62] could be possible explanations.