Cushing’s disease, more specifically known as Pituitary Pars Intermedia Dysfunction (PPID), is a very common condition seen in middle aged to older horses of all breeds. Ponies, however have a higher likelihood of contracting this disease. Horses suffering from Cushing’s disease will have several classic clinical signs such as: excessive urination, failure to shed their hair coat (hirsutism), weight loss despite high quality diet, difficulty maintaining weight, higher incidences of laminitis, and overheating due to decreased sweating. Cushing’s disease can also cause a predisposition to secondary infections and delayed or difficulty healing wounds.
What is the cause of Cushing’s disease? This disease is due to the development of a hormone secreting tumor located in the pars intermedia of the pituitary gland. This very important organ is located near the base of the brain and is responsible for the release of many vital hormones that help regulate bodily functions. The pars intermedia is a specific portion of the pituitary gland that is responsible for the release of ACTH. This hormone controls the production of cortisol within the body. Cortisol is classically known as the “stress” hormone. Despite its negative connotation, cortisol plays a very important role in the body by allowing it to appropriately respond to stressful situations. The tumor that forms in the pars intermedia causes an abnormally high production of ACTH, leading to constantly elevated levels of cortisol circulating in the body.
Horses naturally have a seasonal variability in the level of resting cortisol they have, with late summer into fall typically being the time of natural elevations. Due to this variability, previous testing for this disease has typically been done during the spring. However, with new research, better understanding of natural variability, and the development of new testing modalities we are now able to test horses at any time! Here at Conley and Koontz Equine Hospital we commonly perform a TRH stimulation test. This is a blood test that measures ACTH levels before and following stimulation of the pituitary gland with TRH (administered via IV injection).
Since Cushing’s disease is due to a hormone producing tumor, it is unfortunately not a curable disease. However, there is good news! With the use of medication we are able to provide long term control of ACTH and cortisol, and provide a good quality of life to many patients! The most common medication used today is pergolide, also known as Prascend. Further management of horses suffering from Cushing’s includes low carbohydrate diets and avoiding the use of steroid hormones such as dexamethasone due to predisposition to laminitic events. Although some compounded forms of the medication may be seen out there, these formulations are typically unstable and do not provide true and accurate dosages in a powdered or liquid formula.
If you are concerned about Cushing’s disease in your horse, please contact Conley and Koontz Equine Hospital for additional information and testing!
Dr. Jordan Flewellyn