Molecular Dating of the Epidemiological Events in Relation to the Emergence of Very Virulent Infectious Bursal Disease Virus

Abstract

The Very Virulent (vv) strains of Infectious Bursal Disease Virus (IBDV) were isolated first in Europe during the late 80s and soon after causing worldwide pandemic outbreaks in the early 90s. The enhanced virulence of vvIBDV was later proposed to be related to the unique mutations on both genome segments A and B. Using sequence data of both genome segments collected locally during 2001-04 and available database sequences with known sampling dates, here we estimate the time of divergence of the most recent common ancestor (TMRCA) of all vvIBDV pandemic strains and its demographic history in a Bayesian framework using Markov Chain Monte Carlo sampling (BMCMC) under both strict and relaxed molecular clock methods. Based on the VP2 sequences, the TMRCA of all vvIBDV strains was estimated to be at around 1965 (1956-1975), more than 20 years before the onset of its worldwide pandemic outbreaks. The VP1 sequence of all vvIBDVs formed a mono-phylogenetic group that excludes all the other serotype I and II IBDVs, indicating that VP1 of vvIBDV may be reassorted from an unidentified reservoir and this reassortment event was estimated to be at around 1977 (1966-1979) coincided with the TMRCA of most of the vvIBDV pandemic strains at 1979 (1975-1983). Moreover, our estimates on the demographic behavior of vvIBDVs showed a transition from a constant population size to rapid exponential population growth at around 1982 (1978-1985) coincided with the early reported outbreaks and its explosive transmission manner in the late 80s. Based on these findings, we conclude that vvVP2 may have been emerged more than 20 years before the observed vvIBDV outbreaks in Europe. The rapidly worldwide spread of vvIBDV in the late 80s is likely to be related to a single epidemiological event, which is proposed to be the reassortment of the vvVP1