Diagnosing and Managing Equine Cushing’s Disease

Pituitary pars intermedia dysfunction (PPID), or equine Cushing’s disease, remains challenging to diagnose. And even if veterinarians do select the correct test, myriad uncontrollable factors can impact test results, leaving one often confused as to whether of not the horse in question actually has PPID or not.

“According to the latest recommendations of the PPID Working Group*, only two tests are recommended for diagnosis of the disease, depending on its severity,” said Kathleen Crandell, Ph.D., a Kentucky Equine Research (KER) nutritionist.

For horses with clinical signs consistent with equine Cushing’s disease (e.g., excessive hair growth, recurrent laminitis and skin infections, muscle wasting, and excessive thirst and urination) experts recommend a simple blood test to measure adrenocorticotropic hormone (ACTH), which is often elevated in horses with PPID.

If a horse shows early signs of PPID (e.g., decreased athletic performance, lethargy, delayed shedding, regional fat deposition) or if the ACTH test returns equivocal, the PPID Working Group recommends the thyrotropin-releasing hormone (TRH) test. This test involves administering TRH intravenously and assessing the response of ACTH. If ACTH levels elevate excessively following TRH administration, a diagnosis of equine Cushing’s can be made.

“Time of day, time of year or season, stress, exercise, and even whether a horse was fed or fasted prior to a test can impact the results of ACTH testing,” Crandell shared.

To determine if similar factors also impact the TRH-response test, one research team recruited 16 apparently healthy, PPID-free Thoroughbreds and Standardbreds. Horses were offered one of four diets—control, starch-rich, fiber-rich, and sugar-rich—that all provided the same number of calories.

While breed did not impact ACTH results, age and season were identified as potential factors affecting outcome. In addition, the study data showed that horses fed a starch-rich diet could be incorrectly diagnosed with PPID. In sum, a need for improved testing was clearly revealed.

“In addition to testing and treatment guidelines, the PPID Working Group also made recommendations regarding general wellness, including the need for fresh water in horses suffering from excessive thirst, dietary changes, weight management, and exercise,” noted Crandell.

Such recommendations include maintaining an appropriate body condition score through routine assessment, weight loss when appropriate, and regular exercise, so long as the horse is not suffering a current bout of laminitis.

Feed selection should be based upon body condition score and insulin dysregulation, according to the researchers. Many horses may be diagnosed with more than one endocrine disorder.

Questions about developing a diet to avoid obesity, insulin dysregulation, laminitis, and potentially PPID? Ask a KER nutrition advisor now!

*PPID Working Group; Frank, N., F. Andrews, A. Durham, et al. Recommendations for the Diagnosis and Treatment of Pituitary Pars Intermedia Dysfunction (PPID). Revised June 2017.

**Jacob, S.I., R.J. Geor, P.S.D. Weber, et al. 2017. Effect of dietary carbohydrates and time of year on ACTH and cortisol concentrations in adult and aged horses. Domestic Animal Endocrinology. 63:15-22.