Foot-and-mouth disease virus (FMDV) causes the most contagious transboundary disease of animals, affecting both wild and domestic cloven-hoofed animals. Similarly to other RNA viruses, FMDV is highly variable as a result of the inherent low fidelity of the viral RNA-dependent RNA polymerase. The accumulation of this variability and relatedness between FMDV sequences was used to provide evidence for modes of transmission (fomite) as well as a constant clock rate across two FMDV topotypes (~8.70 x 10-3 substitutions/site/year), during the 1967 UK FMD epidemic, using full genome consensus sequencing. However, during an epidemic, virus replicates within multiple animals, where it is also replicating and evolving within different tissues and cells. Each scale of evolution, from a single cell to multiple animals across the globe, involves evolutionary processes that shape the viral diversity generated below the level of the consensus. During this PhD project, next-generation sequencing (NGS) was used to dissect the fine scale viral population diversity of FMDV. Collaboration with the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of Glasgow provided the specialist bioinformatic and statistical capabilities required for the analysis of NGS datasets. As part of this collaboration, a new systematic approach was developed to process NGS data and distinguish genuine mutations from artefacts. Additionally, evolutionary models were applied to this data to estimate parameters such as the genome-wide mutation rate of FMDV (upper limit of 7.8 x 10-4 per nt). Analysis of the mutation spectra generated from a clonal control study established a mutation frequency threshold of 0.5% above which there can be confidence that 95% of mutations are real in the sense that they are present in the sampled virus population. This threshold, together with an optimized protocol, was used for the more extensive investigation of within and between host viral population dynamics during transmission. Analysis of mutation spectra and site-specific mutations revealed that intra-host bottlenecks are typically more pronounced than inter-host bottlenecks. NGS analysis has distinguished between the population structure of multiple samples taken from a single host, which may provide the means to reconstruct both intra- and inter-host transmission routes in the future. A more sophisticated understanding of viral diversity at its finest scales could hold the key to the better understanding of viral pathogenesis and, therefore development of effective and sustainable disease treatment and control strategies
