Pituitary pars intermedia dysfunction (PPID), also known as equine Cushing’s disease, is an endocrine disorder that affects horses, ponies, and donkeys. Equids affected by PPID have a disorder of the pituitary gland resulting in excessive secretion of certain hormones. These hormones, called pro-opiomelanocortin, and hormonally active derivatives like adrenocorticotropic hormone (ACTH) have multiple effects throughout the body. Cushing’s disease is most common in middle-aged to geriatric horses (over 15 years of age) and has been reported rarely in younger animals.
Clinical signs
There are many possible symptoms associated with Cushing’s disease. The most seen signs include a long, unhealthy hair coat that does not shed normally (called hypertrichosis), weight loss and muscle wasting, laminitis, excessive drinking and urination, and delayed wound healing or recurrent infections such as hoof abscesses. There is overlap in symptoms with equine metabolic syndrome, including chronic laminitis, and some horses may be affected by both endocrine disorders simultaneously. Successful management of these disorders requires appropriate diagnosis with your veterinarian.
Diagnosis
To diagnose a horse with PPID, initial testing includes a blood draw to measure ACTH, a hormonal derivative excreted in high amounts in horses with Cushing’s disease. If the results are in an equivocal range, your veterinarian may recommend a secondary test called a TRH-stimulation test. Your veterinarian will draw a baseline blood sample, then administer TRH intravenously. This causes increased secretion of ACTH in all horses, but in horses affected by Cushing’s disease it causes an extreme elevation. A second blood draw is completed 10 minutes after administering the TRH, and ACTH is measured in each sample.
It is important to note that there is a normal, seasonal elevation of ACTH in the fall/early winter, which can make it difficult to definitively diagnose PPID during this time. Additionally, ACTH will be increased by times of stress (such as travel, pain, or other disease). Following initiation of treatment of PPID, baseline ACTH levels return to normal levels.
Treatment
Following a diagnosis of PPID, there is a treatment option available to control clinical signs. Because there is abnormal growth of the pituitary gland in horses with Cushing’s disease, treatment is lifelong but can minimize or eliminate symptoms of the disease. The most common treatment method is an oral medication given once daily called pergolide (Prascend). The dosage is based on the horse’s size, severity of disease, and symptomatic response to the treatment. The most common side effect of this medication is a decreased appetite.
There is current, ongoing research at CSU testing an injectable medication called cabergoline that’s administered intramuscularly every two weeks, showing promise to control PPID in horses that may not be amenable to oral medications. While these medications are the most effective treatment option for Cushing’s disease, there are many herbal supplements, usually containing chaste tree berry, that can help support medical treatment of PPID in horses.
If you suspect your horse might have Cushing’s disease, contact Equine Field Service to schedule testing and receive a diagnosis.