Cushing’s disease, more accurately called pituitary pars intermedia dysfunction (PPID), is an endocrine disorder that affects about 20% of senior horses, ponies, and donkeys. The condition can cause serious health complications if not treated appropriately, and affected horses must be monitored regularly, so their treatment regimen can be adjusted as needed. Because our Leatherstocking Veterinary Services team loves our senior horse population, we are providing information about this problematic disease so you will know if your horse is affected. 

What is Cushing’s disease in horses?

The pituitary gland is a small, hormone-producing organ at the base of the brain that normally regulates metabolic and reproductive functions and helps maintain blood pressure and electrolyte balance. In horses with PPID, the pituitary gland section (i.e., the pars intermedia) enlarges, causing the gland to overproduce certain hormones. These hormones are known as the proopiomelanocortin-derived peptides (POMC) and include melanocyte stimulating hormone (MSH), corticotropin-like intermediate lobe peptide (CLIP), beta-endorphin, and adrenocorticotropic hormone (ACTH). In most species, Cushing’s disease involves only one hormone system, which is why the condition in horses is more accurately called PPID. 

How does Cushing’s disease affect horses?

Most horses diagnosed with PPID are 15 years or older, although horses as young as 7 have been affected. The resultant hormonal imbalances cause a range of clinical abnormalities, including:

  • Delayed shedding — Many affected horses don’t shed out normally.
  • Wooly hair coat — One of the most common signs is a long, curly hair coat, especially as the condition progresses.  
  • Excessive thirst — PPID horses commonly drink excessive amounts of water and produce large urine volumes. This condition is called polydipsia/polyuria.
  • Sweating abnormalities — Affected horses may sweat excessively or be unable to sweat. In the latter case, the horse is at increased risk for hyperthermia, which can be life-threatening.
  • Abnormal body conformation — PPID horses may exhibit muscle wasting and have abnormal fat deposits on their neck, above their eyes, and at their tail head, and some horses develop a pot belly. 
  • Chronic infections — Many affected horses are plagued by issues such as dental disease, skin infections, diarrhea, respiratory diseases, sinus infections, and foot abscesses. They also tend to carry higher parasite loads and their wounds heal more slowly.
  • Lethargy — PPID horses may seem lethargic or be exercise intolerant.
  • Insulin dysregulation — Many affected horses also have abnormal glucose and fat metabolism, which is associated with equine metabolic syndrome (EMS). Horses can be affected by EMS and PPID simultaneously.
  • Laminitis — One of the most concerning PPID effects is laminitis, which causes weakening in the hoof tissues, and allows the coffin bone to rotate. Affected horses experience significant pain, and the condition can be severe enough to warrant euthanasia. 

How is Cushing’s disease diagnosed in horses?

While horses with a wooly hair coat are PPID poster children, not every horse with the disease exhibits this sign. The only way to know if your horse has PPID is through screening tests, which should begin when your horse is in their early teens to help ensure the condition is detected before health complications occur. PPID tests include:

  • Baseline ACTH test — This test measures your horse’s ACTH plasma level. Your horse doesn’t require fasting, but they should not be fed grain for at least 12 hours prior to having their blood drawn. Normal ACTH levels vary depending on the season, and can also fluctuate with stress, illness, exercise and, in some cases, diet. Performing the test when your horse is in their home environment when they are healthy and not stressed is recommended. This test may not detect horses in the early disease stages.
  • Thyrotropin releasing hormone (TRH) stimulation test — The TRH stim test, which is more sensitive for PPID, involves collecting a baseline ACTH sample, administering TRH intravenously, and then collecting a second ACTH sample 10 minutes later. 

How is Cushing’s disease treated in horses?

PPID has no cure. Treatment focuses on reducing or preventing clinical signs, and must be continued throughout the horse’s life. Management strategies include:

  • Medications — A Food and Drug Administration (FDA)-approved drug is licensed to treat PPID. Once treatment begins, testing is necessary to ensure the correct dose.
  • Dietary management — If your horse is overweight, a safe weight loss strategy that limits or avoids grazing, especially on lush grass, and excludes grains and cereal based foods, which are high in non-structural carbohydrates, is necessary.
  • Exercise — Exercise can help control weight and improve insulin sensitivity in horses,  but you must consider your horse’s fitness level and laminitic status when developing their exercise regimen.
  • Clipping — Horses whose long hair coat doesn’t shed out appropriately should be body clipped in the summer to prevent overheating. 
  • Monitoring — ACTH levels should be regularly measured to monitor your horse’s progress and ensure their medication dose is accurate.

PPID is a problematic condition that can significantly affect your horse’s health and wellbeing. Contact our Leatherstocking Veterinary Services team if you would like your horse screened for this concerning disease.