Mapping a viral phylogeny onto outbreak trees to improve host transmission inference

Background: Developing methods to reconstruct transmission histories for viral outbreaks could provide critical information to support locating sources of disease transmission. Phylogenetic methods used to measure the degree of relatedness among sequenced viral samples have proven useful in identifying potential outbreak sources. The complex nature of infectious disease, however, makes it difficult to assign a rigorously defined quantitative confidence value assessing the likelihood of a true direct transmission event using genetic data alone. Results: A new method is presented to calculate a confidence value assessing the likelihood of a transmission event using both phylogenetic inference and limited knowledge of incubation and infectious duration times. The method is applied to simulations of a foot and mouth disease (FMD) outbreak to demonstrate how the combination of both phylogenetic and epidemiology data can be used to strengthen the assessment of the likelihood of direct transmission over methods using just phylogenetic data or infection timing data alone. The method is applied to a previous FMD outbreak to identify areas where over confidence in previously inferred direct transmission may exist. Conclusion: Combining knowledge from viral evolution and epidemiology within a single integrated transmission inference framework is an important approach to assess the potential likelihood of transmission events and makes clear how specific features of a virus’ spread through the course of an outbreak will directly determine the potential for confidence in inferred host transmission links.