23 Foot-and-mouth disease (FMD) is a highly contagious viral disease that severely impacts global 24 food security and is one of the greatest constraints on international trade of animal products.25 Extensive viral population diversity and rapid, continuous mutation of circulating FMD viruses 26 (FMDVs) pose significant obstacles to the control and ultimate eradication of this important 27 transboundary pathogen. The current study investigated mechanisms contributing to within-host 28 evolution of FMDV in a natural host species (cattle). Specifically, vaccinated and non-29 vaccinated cattle were infected with FMDV under controlled, experimental conditions and 30 subsequently sampled for up to 35 days to monitor viral genomic changes as related to phases of 31 disease and experimental cohorts. Consensus-level genomic changes across the entire FMDV 32 coding region were characterized through three previously defined stages of infection: early, 33 transitional, and persistent. The overall conclusion was that viral evolution occurred via a 34 combination of two mechanisms: emergence of full-genomic minority haplotypes from within 35 the inoculum super-swarm, and concurrent continuous point mutations. Phylogenetic analysis 36 indicated that individuals were infected with multiple distinct haplogroups that were pre-existent 37 within the ancestral inoculum used to infect all animals. Multiple shifts of dominant viral 38 haplotype took place during the early and transitional phases of infection, whereas few shifts 39 occurred during persistent infection. These insights into FMDV population dynamics have 40 important implications for virus sampling methodology and molecular epidemiology. 41 42 43 3 44 Introduction 45Foot-and-mouth disease (FMD) is a highly contagious viral disease that affects wild and 46 domestic even-toed ruminants [1, 2]. FMD is a major global concern for livestock owners and 47 managers, and the disease has substantial impact on regulation of international trade in animal 48 products [3]. The classical signs of disease include oral and pedal vesicles and erosions, often 49 associated with lameness, pyrexia, and obtundation [2, 4]. The causative agent, FMD virus 50 (FMDV) is extremely contagious and disseminates rapidly amongst susceptible animals. 51 Although the disease is rarely fatal, FMD-endemic regions incur substantial economic burdens 52 associated with production losses and disease control [5]. Sporadic outbreaks in countries that 53 are normally free from FMD result in costly mediations including culling of large numbers of 54 animals and animal movement restrictions, as well as massive economic losses due to 55 implications on trade in animal products. 56 FMDV (family: Picornaviridae, genus: Aphthovirus) is a single-stranded RNA virus which 57 exists in 7 defined serotypes; O, A, C, Asia-1, and Southern African Territories (SAT) 1-3. The 58 FMDV genome encodes a total of 12 mature proteins translated from a single polyprotein coding 59 region approximately 7 kilobases in length. The structu...