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Equine Cushing’s Disease
(PPID – Pituitary Pars Intermedia Dysfunction)
Also known as Pituitary Pars Intermedia Dysfunction or PPID, Cushing's Disease is a dysfunction of the pituitary gland. This disease is most common in older horses, and though all breeds are susceptible, ponies and Morgan horses have an increased incidence of the disease. Since it is sometimes associated with insulin resistance, Cushing's can be confused with another condition called Equine Metabolic Syndrome.
Clinical signs of equine Cushing’s are varied but can include the following:
• A long, thick and curly hair coat that does not shed well in the spring
• Loss of muscle, especially along the top line
• Weight loss
• Increased water consumption and urination
• Horses with Cushing’s can be at increased risk for laminitis/founder
Diagnosis - TRH Stimulation Test
A diagnosis of equine Cushing’s is based on the horse’s history, clinical signs, and laboratory blood testing. In the past a single blood sample was drawn to test for baseline ACTH levels. The hormone ACTH is produced in abnormally high levels in the majority of horses with Cushing’s. Using the baseline ACTH measurement alone, some of these horses (particularly those with in the early stages of Cushing's disease) tested negative despite a strong clinical suspicion of Cushing’s. Now a more sensitive method is available to test for equine Cushing’s disease. This new method is called the TRH Stimulation test. The TRH stimulation test requires that a small dose of TRH (thyrotropin releasing hormone) be administered intravenously. A blood sample is drawn 10 minutes later. This test is considered positive for equine Cushing’s if there is an abnormally high level of ACTH in the blood sample. This method of testing helps to decrease the number of false negative test results. ACTH testing is less accurate during fall and winter due to natural seasonal variations in the amount of circulating ACTH. In our practice area, this time period extends from early October to late December. For this reason we try to perform the TRH stimulation test during the spring and summer months.
Equine Cushing’s is treated with a medication known as pergolide. The brand name for pergolide is Prascend. It is available in a tablet form that is given once daily orally. Prascend has few side effects and is considered generally very safe, but some horses will have a decreased appetite when the medication is started. Periodic retesting to monitor the horse’s response to treatment is recommended, as it may be necessary to adjust the dose of pergolide for each horse.
Supportive therapy for these horses includes clipping abnormally long coats in the summer to prevent overheating, close monitoring for signs of laminitis, and careful diet and weight management.