Equine Metabolic Syndrome: A Complex Disease Influenced by Genetics and the Environment

Equine metabolic syndrome (EMS) refers to a cluster of clinical abnormalities including insulin resistance, generalized obesity, and/or increased adiposity in specific locations (regional adiposity) associated with an increased risk of laminitis. However, descriptions of the metabolic phenotype of laminitis-prone horses and ponies have varied among published studies. The metabolic phenotypes routinely measured (e.g., insulin, insulin responses, adipokines, adiposity, and so forth) are highly influenced by the environment and vary due to physiologic factors such as age, breed, and sex, even in normal individuals. Furthermore, not all components of the syndrome (e.g., obesity) may be present in individuals with underlying metabolic derangements. The complexity of the “EMS” phenotype has lead to an ongoing debate as to the canonical features that define the syndrome. Furthermore, little is understood about its pathophysiology. A better understanding of EMS requires a clearer phenotypic definition of EMS; a better grasp of the effects of both intrinsic and extrinsic factors that influence metabolic measures; and an appreciation of the complex interactions between the phenotypic components of the syndrome and the proposed risk factors. In this review, we will summarize the understanding of the EMS phenotype and propose a complex disease model that may help explain the variability in EMS.