Diagnosis and Feed Management for Equine Cushing’s Disease

Guidelines for detecting equine Cushing’s disease (piuitary pars intermedia dysfunction) have been updated and refined, allowing veterinarians to give an earlier and more reliable diagnosis for this malady. The changes were made by the Equine Endocrinology Group (EEG), which is made up of veterinarians and researchers with experience in the field of equine endocrinology.

Horses with equine Cushing’s disease show characteristic signs such as a long nonshedding hair coat, excessive thirst, frequent urination, and a lowered resistance to disease. Clinical diagnosis has been based on the resting level of adrenocorticotropic hormone (ACTH) or results of dexamethasone suppression testing. In early stages of the disease, horses showing signs of Cushing’s do not always have test results that confirm the illness, leaving owners unsure of how to manage horses.

The revised guidelines still recommend use of the ACTH test. In place of the dexamethasone suppression test, however, the EEG recommends the thyrotropin-releasing hormone (TRH) stimulation test instead. This simple test requires two blood samples and an injection, and can be done wherever the horse is kept. It is especially helpful in making a diagnosis early in the course of the disease when ACTH levels may not yet be abnormal. Cutoff values for the TRH test have been established only for the months of December through June, so the test must be administered during that period of the year.

Horses that have equine Cushing’s disease can be managed best on a diet that produces a low glycemic response, limiting or slowing the rate at which glucose enters the bloodstream. Owners of insulin resistant horses should avoid unlimited grazing, feed little or no grain, and base the diet on low-carbohydrate hay and a ration balancer pellet. Vegetable oil, rinsed beet pulp, or rice bran are safer ways to provide extra energy for thin horses with Cushing’s disease. Senior feeds often contain high-carbohydrate ingredients and should not be used.