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Middle-aged obesity (body condition score 7-9 on a scale of 1 to 9) accompanied by insidious-onset laminitis is a syndrome that has been recognized by equine practitioners for decades. Equine metabolic syndrome (EMS) is a recently coined name that has gained acceptance to describe this condition. Clinical signs of laminitis commonly develop while horses are grazing spring pasture but can also occur at other times of the year and in horses without pasture access. Affected horses tend to be aged between 10-to-20 years but there does not appear to be a sex predilection. Occasionally, the syndrome can occur in younger animals that have been overfed. Pony breeds, domesticated Spanish mustangs, Peruvian Pasos, Paso Finos, Andalusians, European Warmbloods, American Saddlebreds, and Morgan horses are more commonly affected than Thoroughbreds, Standardbreds, and Quarter Horses. This breed disparity is supportive of a genetic predisposition. In the past, this syndrome was commonly attributed to hypothyroidism or pituitary pars intermedia dysfunction (PPID or classic equine Cushing's disease); however, most affected horses do not manifest additional clinical signs or endocrinologic test results to support these conditions. It is now recognized that insulin resistance is the primary endocrinopathy induced by obesity in EMS-affected horses. However, a number of additional metabolic and endocrinologic alterations can occur in affected equids making the pathophysiology of EMS an increasingly complex subject. Finally, a subgroup of EMS-affected equids may only have abnormal fatty deposits (e.g., a cresty neck or fat deposits behind the shoulders, over the tail head, and in the sheath of male horses) without generalized obesity and these patients are often more challenging to manage than those with generalized obesity.

Prevalence of laminitis and obesity in horses

Figure 1 Causes of laminitis reported in the 2000 USDA NAHMS study; note that grazing lush pasture was the most common reported cause with a peak incidence in May.

Laminitis is a devastating clinical problem for horses and their owners. In fact, data collected in the 2000 USDA-NAHMS study revealed that laminitis was reported on 13% of horse operations. Further, the leading cause of laminitis was reported to be grazing lush pasture (Figure 1). Similarly, in the United Kingdom more than 8,000 cases of laminitis are estimated to occur annually, representing 7% of the equine population, and more than 60% of cases were classified as pasture-associated disease. In both reports, pasture-associated laminitis had a peak incidence in May, followed by October and November.

Also, as horses have transitioned from beasts of burden to recreational companions, the physical condition of many horses has paralleled that of their human counterparts: they have been overfed and become more sedentary. As a consequence, obesity is becoming a significant problem in the equine species. In a recently studied cohort of horses aged 4-20 years in Virginia, 19% of 300 horses were classified as obese, defined as a body condition score (BCS) of 7.5 or greater on a scale of 1 through 9.