PPID in Horses (Equine Cushings Disease)

Introduction 

Equine Cushing’s disease is one of the most common diseases of horses greater than 15 years of age. The clinical signs are associated with abnormally elevated hormone concentrations in the blood and along with other hormones, cortisol, plays an important role in this disease. This syndrome is better defined as Equine Pituitary Pars Intermedia Dysfunction (PPID) because it reflects the location within the brain that is abnormal. In humans, Cushing’s disease affects a different location within the pituitary gland, thus the reason why veterinarians prefer to call this disease PPID.

The pituitary gland is located at the base of the brain. In horses with PPID, the middle lobe of the pituitary gland (pars intermedia) becomes enlarged over time and results in over production of hormones. The growth of the middle lobe of the pituitary gland can compress the adjacent structures in the pituitary and hypothalamus resulting in loss of their function. The enlargement of the pituitary gland is often referred to as a pituitary adenoma. 

Clinical signs of PPID can vary depending upon the stage of disease.  The average age of horses diagnosed with PPID is 20 years, with over 85 percent of the horses being greater than 15 years of age.  Although most common in aged horses, PPID has been diagnosed in horses as young as seven years of age. All breeds of horses can develop PPID, however ponies and Morgan horses have a higher incidence of the disease. The most classic symptom of PPID is a long curly hair coat that does not shed properly. This condition is referred to as hirsuitism. 

Other symptoms that have been associated with PPID include excessive drinking and urination (polyuria/polydipsia), laminitis, lethargy, excessive sweating, muscle mass loss, repeated infections such as sole abscesses, tooth root infections, and sinusitis; infertility and bulging eyes that are a result of redistribution of supraorbital fat.  

Horses with PPID may also be insulin resistant. What is insulin resistance? Insulin is a hormone produced from the pancreas in response to elevated blood glucose that occurs after a meal. In the normal horse, insulin stimulates the uptake of glucose by tissues.  Insulin resistance is the failure of tissues to respond appropriately to insulin. The pancreas will continue to secrete insulin to compensate for decrease in tissue effectiveness; thus the resting serum insulin levels will be high in horses with moderate to severe insulin resistance. Insulin resistance can be confirmed by measuring insulin in the blood.  Insulin resistance can contribute to muscle loss, abnormal fat accumulation, laminitis, excessive water consumption, frequent urination and increased susceptibility to disease. 

Diagnosis 

The best indication of PPID is the clinical sign of hirsuitism in the aged horse. The sensitivity of this clinical sign is better at predicting an abnormal pituitary than any diagnostic test available.  

Horses that have subtle signs of PPID need to be tested to confirm the disease. Of the tests available, no single test is 100 percent accurate. The most commonly used tests in the field are the dexamethasone suppression test and the measurement of resting plasma ACTH concentration. A single blood measurement of ACTH concentration is advocated by some as a confirmatory test for PPID. The best time to test for PPID in horses in Australia is during March/April.

In summary, an older horse with hirsutism is likely to have PPID. Early in the course of the disease the long curly hair coat may not be present. If the horse does not have hirsutism, a repeated test at four to six month intervals may be necessary to confirm the disease. 

Therapy 

Treatment seldom achieves complete resolution or remission of disease. The most important reason to treat is to improve the quality of life. Infertility may be temporarily overcome by treatment of older broodmares with confirmed PPID. 

Pergolide is the drug of choice for treatment of PPID.  The dose can be adjusted based on response to therapy or repetitive testing. Cyproheptadine may be marginally effective at controlling clinical signs and is therefore not used as frequently. 

Supportive care is also important for enhancement of the quality of life of horses with PPID. High quality preventative medicine such as regular deworming, dental care and consistent farrier work has been advocated. In the warm months, clipping of unnecessary hair will keep the horse more comfortable. 

Some horses with PPID have been documented to be insulin resistant and require a special diet. Insulin sensitivity can be improved by reducing body fat and avoiding feeds high in starch and sugar. Horses or ponies that have both insulin resistance and PPID should be fed hay that is less than 12 percent in non-structural carbohydrates. If the hay cannot be tested, soaking it for 30 minutes in cold water will lower the sugar content prior to feeding. Pasture access should be limited to one to two hours a day in addition to elimination of all treats such as sugar cubes. The affected horses can be fed a commercially prepared low-starch diet. If they are thin, the addition of fat in the form of rice bran or corn oil can be supplemented for added calories. 

Not all horses with PPID are insulin resistant. This would be an additional consideration in a complete diagnostic evaluation especially if the horse or pony has laminitis. 

Recognition of the clinical signs and early diagnoses are the first steps in identification of horses with PPID. Treatment and implementation of excellent wellness practices are the key features to extending the life of a horse with PPID.  

References 

Asplin KE, Sillence MN, Pollitt CC et al. Induction of laminitis by prolonged hyperinsulinaemia in clinically normal ponies.  Vet J 2007;174: 530-535. 

Donaldson MT, McDonnell, Schanbacher BJ et al. Variation in plasma ACTH concentration and dexamethasone suppression test results with season, age and sex in healthy ponies and horses. J Vet Int Med 2005; 19:217-222. 

Dybdal NO, McFarlane D. Equine Pituitary Pars Intermedia dysfunction. Large Animal Internal Medicine 4^th edition, 2009;1340-1344. 

Frank N. Managing Equine Metabolic Syndrome. Compend Contin Educ Prac Vet 2008; 348-355.

McFarlane D.  Diagnosing Pituitary Pars Intermedia Dysfunction. Compend Contin Educ Prac Vet 2007; 208-213. 

Dana Zimmel, DVM, DACVIM, DABVP is a faculty member of the University of Florida College of Veterinary Medicine. 

Dianne McFarlane, DVM, PhD, DACVIM is a faculty member of Oklahoma State University College of Veterinary Medicine.