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Colic is an important cause of death in managed horse populations and is a health issue of great concern to horse owners. Epidemiological studies that have investigated risk factors for colic have identified a number of factors that increase the likelihood of colic, most of which are related to change in management including diet. The precise relationship between diet and colic is currently unknown but it has been hypothesised that changes in gut microbiota composition are responsible for the development of colic associated with change in management. The work presented in this thesis has further investigated risk factors for colic and postoperative intestinal dysfunction (postoperative colic and postoperative reflux) and has explored the gut microbiota in specific groups of horses at increased risk of colic. Risk factors for colic were investigated in a previously unstudied working horse population in Egypt using a cross-sectional epidemiological study. The study recruited 350 working horses from two provinces in Egypt and data about their management and history of colic in the preceding 12 months were collected. Risk factors identified included stereotypic behaviour, severe dental disease, feeding ground corn during the ‘dry season’, anthelmintic treatment in previous six months and feeding rice bran during the ‘green season’. A prospective cohort study of 264 surgical colic patients that were admitted to the Philip Leverhulme Equine Hospital, University of Liverpool in 2012–2014 was conducted. The study investigated risk factors for morbidity and mortality following laparotomy. Factors identified to be significantly associated with altered risk of recurrent postoperative colic episodes included strangulating large colon volvulus, epiploic foramen entrapment, postoperative reflux (POR), repeat laparotomy and heart rate on admission. A subset of these horses were used to investigate risk factors for POR in horses that underwent laparotomy for treatment of small intestinal lesions. The effect of systemic perioperative lidocaine treatment on the prevalence, duration and net volume of POR was compared with a previous cohort of horses (2004–2006). Risk factors associated with POR included packed cell volume on admission, pedunculated lipoma obstruction, pelvic flexure enterotomy, side-to-side jejunocaecal anastomosis, duration of surgery and the period of hospital admission. Lidocaine treatment had no effect on the prevalence, duration and net volume of POR nor was it a risk factor for postoperative survival. The relationship between gut microbiota composition and previously reported risk factors for colic/postoperative colic were investigated in a series of faecal microbiota studies. Temporal variation in faecal microbiota composition in a group of horses managed at pasture was investigated. Faecal samples were collected every 14 days from 7 horses for a total of 52 weeks. Samples were processed to extract bacterial DNA before creation of amplicon libraries and sequencing using the Ion Torrent PGM next-generation sequencing technology. Analysis of these sequence data revealed that the horse faecal microbiota is dominated by members of the phylum Firmicutes and Bacteroidetes. Significant effects of season, diet and ambient weather conditions on the composition of the horse faecal microbiota were identified. The study concluded that the horse faecal microbiota is in a continuous state of adaptive process to external stimuli and that fluctuations in the composition of horse faecal microbiota over time is normal. The temporal stability of the faecal microbiota of periparturient mares was also investigated. Weekly faecal samples were collected from 7 broodmares for a total of 17 weeks (5 weeks pre-foaling – 12 weeks post-foaling). Foaling had minimal effect on the composition of the horse faecal microbiota and variation in data over time was associated with changes in management. This suggested that minimising changes in management during the periparturient period may be important in stabilising the gut microbiota of periparturient mares. The faecal microbiota of surgical colic patients that underwent laparotomy for treatment of primary large colon diseases was compared to a group of emergency orthopaedic patients that underwent surgery under general anaesthesia. Samples were collected from both groups on admission, during hospitalisation and following hospital discharge for a period of 3 months. The results suggested that a 3-month period was not enough to fully investigate changes in faecal microbiota composition of surgical colic patients compared with samples collected on admission. This thesis provides new information about risk factors for colic in a working horse population, risk factors for complications following intestinal surgery and has questioned the efficacy of lidocaine in management of POR. The equine faecal microbiota in 3 different horse populations has been studied and has generated new hypotheses about the relationship between the equine gut microbiota and the risk of colic.