Small intestinal ileus affects up to half of all horses undergoing small intestinal surgery, leading to prolonged gastrointestinal (GI) transit time which can be life-threatening. Various prokinetic medications have been associated with varying side effects, questionable efficacy, and increased cost. Gum chewing as a form of sham feeding is used as a safe, effective, well-tolerated, and inexpensive way to ameliorate ileus following GI surgery in humans. Bit chewing for horses, an analogous activity, has been shown to significantly decrease GI total transit time (TTT); however, a direct effect of bit chewing on gastric emptying time (GET), small intestinal transit time (SITT), and total orocecal transit time (OCTT) has not been investigated. Our objective was to determine whether bit chewing increased small intestinal motility and decreased GE, SITT, and OCTT in clinically normal horses. Gastrointestinal motility was compared in horses that were bit chewing compared to control conditions (no bit chewing) in a prospective crossover design study using acetaminophen as a marker for GET and video endoscopy (ALICAM) capsules to determine GET, SITT, and OCTT. Bit chewing was well tolerated by all horses with no side effects noted. Bit chewing led to a shorter GET, SITT, and significantly shorter OCTT when compared to the control group (P = 0.015). Median times for bit chewing conditions were as follows: GE 2.86 hr, SITT 3.65 hr, and OCTT 6.15 hr whereas the median times for control conditions were as follows: GE: 5 hr, SITT 4.4 hr, and OCTT 9.92 hr. In summary, bit chewing proves to be a potential tool to hasten the motility of the oral GIT. It is safe, inexpensive, and potentially effective prokinetic treatment to horses suffering from postoperative ileus and further investigation is warranted. / Master of Science / Ileus, or a temporary lack of intestinal motility, is a common life-threatening problem in horses, especially following abdominal surgery. Current treatments have questionable efficacy and high cost. In human patients suffering from ileus, sham feeding with gum chewing that mimics normal food consumption with no food ingested has shown promising results in improving clinical signs and restarting gastrointestinal motility. Bit chewing, an analogous activity in horses, also decreases gastrointestinal (GI) total transit time (TTT). However, ileus in horses typically affects the small intestine, which is a part of the GI tract that has not been investigated in regards to bit chewing. Our objective was to determine whether bit chewing shortens gastric emptying time (GET), small intestinal transit time (SITT), and total orocecal transit time (OCTT) in clinically normal horses. Gastrointestinal motility was compared in horses that were bit chewing compared to control (no bit chewing) conditions in a prospective crossover design study using acetaminophen serum samples as a marker for GET and video endoscopy (ALICAM) capsules to determine GET, SITT, and OCTT. There were no adverse effects to bit chewing and OCTT was significantly shortened in horses when bit chewing compared to the control group. In summary, bit chewing is a tool to hasten gastrointestinal motility, specifically small intestinal motility, and it may be a safe, inexpensive, and effective treatment to improve small intestinal motility in horses suffering from ileus and further investigation is warranted.

Post-operative ileus (POI) is the functional inhibition of propulsive intestinal motility which is a frequent occurrence following abdominal surgery in the horse and in humans. Rodent and human-derived data have shown that manipulation-induced activation of the resident muscularis externa (ME) macrophages in the intestine contributes to the pathophysiology of the disease. Most studies of the disease, specifically in the horse, have focussed on identification of risk factors, descriptive studies of the disease or the assessment of the efficacy of various therapeutic and prophylactic interventions. As a result, the proposed pathogenesis of equine POI is largely reliant on the translation of data from rodent models. The aims of this thesis were to identify macrophage populations in the normal equine gastrointestinal tract (GIT) and to study equine macrophage activation by stimulating equine bone marrow-derived macrophages (eqBMDMs) with lipopolysaccharide (LPS) as a model for intestinal macrophage activation. Firstly, the normal population of macrophages in the equine GIT was determined. Using CD163 as an immunohistochemical marker for macrophages. CD163+ve cells were present in all tissue layers of the equine intestine: mucosa, submucosa, ME and serosa. CD163+ve cells were regularly distributed within the ME, with accumulations adjacent to the myenteric plexus, and therefore to intestinal motility effector cells such as neurons and the Interstitial Cells of Cajal. The differentiation and survival of intestinal macrophages depends upon signals from the macrophage colony-stimulating factor (CSF-1) receptor. LPS translocation from the gut lumen is thought to be a key activator of ME macrophages. To provide a model for gut macrophages, a protocol was optimised to produce pure populations of equine bone marrow-derived macrophages (eqBMDMs) by cultivation of equine bone marrow in CSF-1. Macrophage functionality was assessed using microscopy, flow cytometry and phagocytosis assays. EqBMDMs responded to LPS stimulation with increases in expression of positive control genes, tumour necrosis factor alpha (TNF-α) and Indoleamine 2,3-dioxygenase (IDO1). The same mRNA was subjected to transcriptomic (RNA-Seq) analysis. Differential gene expression and network cluster analysis demonstrated an inflammatory response characterised by the production of pro-inflammatory cytokines such as interleukin 1 beta (IL-1β) and interleukin 6 (IL-6). However, in contrast to rodent macrophages, eqBMDMs failed to produce nitric oxide in response to LPS, showing species-specific variation in innate immune biology. Using these data, we compared gene expression in normal equine intestine and in intestine from horses undergoing abdominal surgery for colic (abdominal pain). Horses undergoing abdominal surgery showed evidence of increased expression of IL-1β, IL-6 and TNF-α in the mucosa and ME when compared to control tissue. Horses with post-operative reflux (POR), a clinical sign of POI, had increased gene expression of IL-1β, IL-6 and TNF-α compared to horses that did not develop POR following abdominal surgery. These preliminary data suggest that there is macrophage activation within the ME of the intestine during abdominal surgery in the horse, and that a greater activation state is present in horses that subsequently develop POR. The final part of this study was to investigate the effect of a long-acting form of CSF- 1, an Fc fusion protein (CSF1-Fc), as a potential treatment for POI using a mouse model. This work, performed in collaboration with another research group, found that mice lacking the C-C chemokine receptor type 2 (CCR2) gene, which is required for monocyte recruitment into tissues, had a longer recovery period following intestinal manipulation (IM) than wild type (WT) mice. With the administration of CSF1-Fc, infiltration of neutrophils to the ME was reduced and the number of macrophages in the ME was increased in both WT and CCR2-/- mice following IM. Administration of CSF1-Fc in CCR2-/- mice improved recovery of gastrointestinal transit three days following IM, to the same extent as WT mice. Network cluster analysis and RT-qPCR of the ME revealed clusters of genes induced and downregulated by CSF1-Fc, with increased expression of anti-inflammatory and pro-resolving genes after IM in WT and CCR2-/- mice following treatment with CSF1-Fc.