Cushing's or Metabolic Syndrome?
A horse with equine metabolic syndrome (EMS) is typically middle-aged with either generalized or regional adiposity (fat deposits). Regional adiposity might manifest as a cresty neck, a fat tailhead, fat on either sides of the withers or fat in the sheath or udder region. Horses with EMS are easy keepers and, thus, they are referred to as “thrifty.” In addition to having this body type, horses with EMS have insulin resistance and a propensity for laminitis.
The horse with Cushing’s or pituitary pars intermedia dysfunction (PPID) is typically older than the horse with EMS; most affected animals are older than 15 years of age. Horses with PPID often have abnormal hair coats and muscle wasting, particularly along the topline. The hair coat might be hirsute (long and curly), slow to shed, or shed incompletely. Many horses with PPID have regional fat deposits and laminitis, similar to horses with EMS, and it is suspected that EMS over time leads to the horse developing PPID, although this has not been proven at this time.
If a horse that has had EMS for several years starts to develop muscle wasting and/or an abnormal hair coat, this suggests the horse is developing PPID.
Equine metabolic syndrome is diagnosed by demonstrating insulin resistance in a horse with the thrifty phenotype (observable characteristics). Veterinarians can screen horses for insulin resistance by measuring glucose and insulin concentrations in the blood. Affected horses usually have high insulin concentrations, whereas their glucose concentrations are often within reference range. However, high glucose concentrations are occasionally detected, and this indicates a more serious condition that progresses to diabetes mellitus in some cases. More advanced testing might be required if the horse suffers from mild or early insulin resistance because serum insulin concentrations can fall within reference range in these patients. The combined glucose-insulin test can be used in these cases to diagnose insulin resistance.
Pituitary pars intermedia dysfunction is best diagnosed with an overnight dexamethasone suppression test or by measuring plasma adrenocorticotropin (ACTH) concentration. False negative tests are common early in the disease, so retesting is recommended. In the fall a negative test is strong evidence that the horse does not have PPID.
It is strongly encouraged that owners manage EMS by inducing weight loss in obese horses, controlling dietary sugar intake, and increasing exercise. Horses with EMS can be treated with levothyroxine sodium to accelerate weight loss, but this treatment should not be used as a substitute for good management practices. Metformin treatment is an option for horses that remain insulin resistant in the face of appropriate management or in the short term while new practices are being established.
Treatment of PPID is best accomplished using pergolide mesylate. Ideally, repeat diagnostic testing is necessary to titrate the dose to the amount needed to normalize the results.
A question that has yet to be answered in clinical research trials is whether it is beneficial to start horses on pergolide that have mild clinical signs, but are negative on diagnostic tests; for these horses I recommend testing twice a year.