Neutrophils, the chief phagocytic cells in most mammals, are critical in the inflammatory response. Regulation of neutrophil activity occurs through several mechanisms, including apoptosis. Dysfunction of neutrophil apoptosis has been implicated as a cause of organ damage in hyper-inflammatory conditions in human patients. This pilot study investigated apoptosis in circulating neutrophils from horses with surgical lesions in the large and small intestine. We hypothesized that delayed neutrophil apoptosis occurs in peripheral blood of horses undergoing surgery with acute abdominal disease, compared with elective orthopedic cases. Adult horses undergoing surgery for acute abdominal disease (N=10) and elective orthopedic surgery (control) (N=10) were studied. Peripheral blood was collected preoperatively and postoperatively. Neutrophils were isolated using Percoll gradient. Cells undergoing apoptosis were determined by flow cytometry using a commercially available staining kit (Annexin V-PE Apoptosis Detection Kit I, BD Pharmingen™). The Mann-Whitney U test was used to detect significant differences in neutrophil apoptosis between the two groups as well as between lesion types in the abdominal surgery group. Correlations between neutrophils in apoptosis and postoperative parameters were detected using Spearman's rank correlation coefficient. No significant differences in percentages of apoptotic neutrophils between groups were found; however, a significantly lower percentage of neutrophil apoptosis was present in horses with strangulating intestinal lesions versus nonstrangulating lesions. Current investigations about neutrophil apoptosis in human medicine may result in therapeutic intervention to prevent organ damage in hyper-inflammatory states. Understanding the role of neutrophil apoptosis in equine acute abdominal disease may guide the use of new treatments as they become available. / Master of Science