Insulin Resistance in Horses

Insulin resistance (IR) is important to the equine practitioner because of its association with laminitis. Older horses with pituitary pars intermedia dysfunction (PPID) are pre-disposed to both IR and laminitis, and it is likely that these conditions are related. Horses or ponies that exhibit delayed shedding of the haircoat or hirsutism, loss of skeletal muscle mass, or polyuria/polydipsia should, therefore, be tested for PPID and IR and managed accordingly. There is also a second condition associated with IR in horses, and this syndrome is more challenging to define. Affected horses are described by owners as “easy keepers,” because body weight is maintained on a relatively low caloric intake compared with other horses. In our experience, this condition is most common in ponies, Morgans, Paso Finos, and Norwegian Fjords, but it also occurs in Arabians, Quarter Horses, American Saddlebreds, and Tennessee Walking Horses. Horses with this condition can often be recognized by their appearance. Some exhibit generalized obesity; others are thinner throughout the mid region of the body but suffer from regional adiposity in the form of a cresty neck or enlarged fat pads next to the tailhead. These horses were previously assumed to suffer from hypothyroidism because of perceived similarities between this condition and canine hypothyroidism and the detection of low or low-normal resting serum total triiodothyronine (tT3) and total thyroxine (tT4) concentrations. However, it is now accepted that lower circulating tT3 and tT4 concentrations are a consequence rather than a cause of the horse’s metabolic state, and these concentrations can be attributed to secondary hypothyroidism (decreased thyroid-stimulating hormone release from the pituitary gland) or interference from drugs such as phenylbutazone. Therefore, alternative terms have been adopted to describe this condition in horses and ponies, including peripheral Cushing’s disease, equine metabolic syndrome (EMS), and pre-laminitic metabolic syndrome (PLMS). For the purpose of this discussion, we will use the term EMS to signify this condition, but we will require that all three of the following criteria be met before the term is applied: (1) insulin resistance, (2) history of laminitis or pre-disposition to the disease (presence of abnormal growth rings on the hooves), and (3) regional adiposity in the form of a cresty neck or enlarged fat pads.