What is it?

This is an endocrine disease occurring in 20% of older horses (including donkeys and ponies.) The pituitary gland is located at the base of a horse’s brain and is responsible for producing hormones that control essential reproductive and metabolic functions. The pars intermedia is the middle lobe of the pituitary gland. Cushing's disease results when dopamine secreting neurons that extend from the hypothalamus to the pars intermedia undergo oxidative degradation. Normally these neurons secrete dopamine which inhibits the pars intermedia. When oxidative damage occurs the pars intermedia is no longer inhibited which results in hyperplasia and increased hormone production which overtime can lead to neoplasia (abnormal growth leading to tumor development). Pituitary tumors in horses do not spread to other parts of the body, and only in rare cases do they get large enough to cause neurologic signs due to compression on adjacent structures in the brain. The main hormone increased in this disease is adrenocorticotropic hormone (ACTH). An increase in this hormone leads to the clinical signs discussed below.

CLinical Signs

An increased ACTH secondary to this pituitary tumor can cause a wide variety of clinical signs. Common clinical signs include delayed shedding, muscle wasting, dampened immune function, abnormal sweating, and increased thirst and urination. Horses with Cushing’s disease that are left untreated can have chronic issues such as dental disease, sinus infections, skin infections and can have an overall poor quality of life. Oftentimes horses with PPID also have abnormal insulin regulation which can leave them at a higher risk for developing laminitis. 

What Is It?

  • Endocrine Disease

  • Inhibited Middle Lobe of Pituitary Gland (the Pars Intermedia)

  • Caused by Damage to Dopamine Secreting Neurons

  • Also known as Equine Cushing’s Disease

Occurring in 20% of Older Horses including Donkeys and Ponies

PPID Positive Horse with Dr. Rachel Roemer

Diagnostics

Screening for PPID includes pulling blood to determine if ACTH levels are above the normal reference range. Seasonal rises in ACTH levels occur from mid-July to mid-November so it is important to interpret results with caution when testing during this timeframe.  In early disease horses may have resting ACTH levels that are within the normal reference range. In these cases the recommendation is to perform a thyrotropin releasing hormone response (TRH) test. A baseline ACTH level is pulled and the horse is given TRH intravenously and a second sample is obtained 10 minutes later. The baseline ACTH levels are then compared to levels post TRH injection to determine if the horse has PPID.

 

 

Prognosis

The prognosis for these horses with Cushing’s disease is good and owners typically notice improvements in quality of life once this disease is well regulated on Prascend.