The two major endocrine disorders affecting horses--equine metabolic syndrome (EMS) and pituitary pars intermedia dysfunction (PPID or equine Cushing's disease)--were hot topics at the 2009 American Association of Equine Practitioners (AAEP) Convention in Las Vegas, Nev., Table Topic session on Equine Endocrine Disorders.

The more recently described EMS is a condition that first develops in younger (less than15 years of age) horses and is likely to have a genetic basis. Affected animals show enhanced metabolic efficiency and might be described as "easy keepers." They readily become obese when overfed and develop enlarged fat deposits in the neck and tailhead regions.

Horses and ponies with EMS suffer from insulin resistance, which is an endocrine disorder associated with increased laminitis susceptibility. Obesity and insulin resistance should therefore be managed to lower the risk of laminitis.

In contrast, PPID, which is also called equine Cushing's disease, is an endocrine disorder that usually develops in older (greater than 15 years of age) horses. Classic signs of advanced PPID include a long, curly haircoat (hirsutism), delayed shedding of the winter haircoat, loss of muscle mass, increased drinking and urination, pot-bellied appearance, and increased susceptibility to infections.

Finally, it was noted that several horses with EMS have subsequently developed PPID as they progress through middle age, and these animals experience exacerbation of insulin resistance at the same time. The layering of one endocrine disorder on top of another might raise the risk of laminitis.

Audience participation was invited and a list of questions was formulated. Many of the questions focused upon diagnostic testing for PPID. It was pointed out that PPID is difficult to detect in its earliest stages, while advanced disease is relatively easy to recognize. The two most commonly used tests for PPID--resting adrenocorticotropin hormone( ACTH) concentrations and the overnight dexamethasone suppression test--can be used to confirm the diagnosis. However, these tests are less likely to yield positive results when PPID is first developing, so clinical judgment must be relied upon in these cases.

Early signs of PPID include delayed shedding of the winter haircoat for a few additional weeks, retention of hair in patches, and shifts in metabolism and attitude. Loss of muscle mass in the face of adequate nutrition can also signal the onset of PPID. New diagnostic tests are being developed for PPID and might allow earlier detection of the disorder. These include the combined dexamethasone suppression/thyrotropin-releasing hormone (TRH) stimulation test, TRH response test (with ACTH concentrations measured 10 and 30 minutes post-injection), and oral domperidone challenge test.

Treatment and dietary management of PPID was also discussed. Pergolide was identified as the most commonly used treatment for the disorder, with an initial dosage of 0.5 or 1.0 mg per day (total dose) that can be increased to a maximum of 5.0 mg/day.

Insulin resistance has been associated with PPID, but there are many affected horses that show normal insulin sensitivity, which highlights the importance of assessing this situation in each patient.

Horses with PPID that have normal insulin sensitivity can be fed the same way as other geriatric horses, with senior complete feeds provided as part of the diet. More effort is required for horses with PPID that are also insulin resistant. These animals must be provided with energy without exacerbating insulin resistance.

For both EMS and PPID, the importance of wellness evaluations was emphasized. These evaluations should be performed in the spring and/or summer in order to avoid the late summer and autumn periods when hormones are up-regulating in horses. History and physical examination findings should be combined with results of resting ACTH, glucose, and insulin concentrations to assess horses that are at risk for endocrine disorders, including animals that are obese or those over 15 years of age.

Other topics discussed in this session included the diurnal cortisol rhythm test, monitoring of treatment, trilostane administration, and the use of levothyroxine sodium to accelerate weight loss in obese horses.

This Table Topic report was submitted by facilitators Nicholas Frank, DVM, PhD, Dipl. ACVIM, from the University of Tennessee, and Frank Andrews, DVM, MS, Dipl. ACVIM, from Louisiana State University.

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