Recognizing and treating the horse with endocrine disease was the focus of the lively discussion at the table topic "Cushing's or Metabolic Syndrome?" The session, which was held at the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif., started off with definitions and clinical descriptions of the two endocrine diseases.

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, or fat in the sheath 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 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.

Facilitators and attendees discussed the diagnosis of both conditions, and they emphasized that both conditions require the presence of clinical signs for accurate diagnosis. 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.

Practitioners should encourage owners to 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, the veterinarian repeats diagnostic testing to titrate the dose to the amount needed to normalize the results.

A question that has yet to be answered in a clinical research trial is whether it is beneficial to start horses on pergolide that have mild clinical signs, but are negative on diagnostic tests.

This Table Topic was facilitated by led by Nicholas Frank, DVM, PhD, Dipl. ACVIM, of the University of Tennessee and University of Nottingham, and Dianne McFarlane, DVM, PhD, Dipl. ACVIM, ABVP, of Oklahoma State University.

About the Author

Nicholas Frank, DVM, PhD, Dipl. ACVIM

Nicholas Frank, DVM, PhD, Dipl. ACVIM, is a professor of large animal internal medicine and chair of the department of clinical sciences at the Cummings School of Veterinary Medicine, Tufts University, Massachusetts. His research interests include laminitis, metabolic disorders, PPID/equine Cushing’s, and many other internal-medicine related areas.

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