Equine Cushings in Horses

Equine Hyperadrenocorticism or PPID

Cushing’s disease in horses is caused by excessive hormones in the body. This excess is brought on by a malfunction of the pituitary gland. The pituitary gland is found at the base of the brain and is in charge of regulating all the hormones in the body. When the gland becomes enlarged or develops a non-cancerous growth (benign adenoma), stress hormones circulate in higher levels, causing multiple effects on the body.

This condition is more commonly seen in older horses, with an average age of 19 years. Close to 10% of older animals may be affected by Cushing’s disease. The disease may also be found in horses as young as seven years. No particular breed of horse is more susceptible to the disease, nor is any particular sex afflicted more than the other. Ponies have been shown to be more commonly diagnosed than horses.

Symptoms and Types

As stress hormones begin to circulate in increasing levels, the blood sugar levels in the body begin to climb and the immune system begins to fail. This leads to multiple other symptoms including:

• Increased thirst (polydipsia) and urination (polyuria)
• Increased amounts of curly hair growth (hirsutism)
• Decreased shedding with dull haircoat and flaky skin
• Muscle wasting (atrophy) along the topline
• Potbelly development and swayback appearance
• Lethargy and depression
• Founder (laminitis)
• Increased infections (lung and dental disease)
• Slow wound healing
• Fertility problems in mares

Cushing’s disease in horses is known by several names. Depending on the way in which the pituitary gland is affected, it can develop due to a malfunction of part of the gland itself (pituitary pars intermedia dysfunction (PPID)) or brought on by a non-cancerous growth of the gland (adenoma). There is a form of metabolic syndrome known as peripheral Cushing’s syndrome that causes symptoms similar to Cushing’s disease in horses. It may play a part in bringing on the disease in those animals that do develop the condition.

Causes

The pituitary gland is responsible for producing important hormones. Adrenocorticotropic hormone (ACTH) is the main one that prompts the adrenal glands (found near the kidneys) to produce excess steroids (cortisol and cortisone). Under normal conditions these steroids help the body reduce inflammation and regulate levels of glucose in the bloodstream.

If the pituitary gland malfunctions, an increased level of ACTH circulates, leading to higher levels of steroids in the blood. These steroids will cause stored energy (protein and fat) to be pulled from body tissues, raising blood glucose levels and breaking down connective tissue. Over time, this will lead to a potbellied appearance and weaken the body as the disease progresses. Damage to the immune system leads to increased infections all over the body and a failure to thrive.

Diagnosis

Your veterinarian will perform basic blood tests to determine blood sugar levels, including a complete blood count (CBC) and chemistry panel. A urine analysis will be conducted. Hormone-specific blood tests will test for cortisol levels. In a test called the dexamethasone suppression test, a baseline level is taken first, then a steroid (dexamethasone) is given to see if the body will suppress ACTH production. A second blood sample is taken later to establish cortisol levels. In a horse with Cushing’s disease, levels will be elevated.

The ACTH stimulation test can also demonstrate cortisol levels. Baseline levels are taken and then ACTH is given. Levels of cortisol are tested several hours later. Horses with Cushing’s will show a major elevation of cortisol. Conducting a dexamethasone suppression test along with the administration of a hormone (thyrotropin releasing hormone (TRH)) may show a spike in levels of cortisol in the blood of animals afflicted by Cushing’s disease. This type of test may be more accurate in detecting horses earlier in the disease process.

Treatment

If Cushing’s disease is caught early enough, treatment may help the animal feel better for several years. Horses with advanced disease, and its associated infections, may prove difficult to treat. The only treatment currently available is in the form of medication. The pituitary gland is inaccessible due to its location near the brain and this makes it inoperable.

The two medications used most frequently to treat Cushing’s in horses are pergolide mesylate and cyproheptadine. Treatment is typically given orally once a day. Pergolide was originally used to treat Parkinson’s disease in humans. It helps to decrease hormone levels in the circulation. Pergolide does have some drawbacks in that it can worsen the chronic foundering (laminitis) seen in horses with Cushing’s.

Both of these medications should be formulated by a compounding pharmacy in order to obtain the correct dosage and the formulation that is preferred. The shelf life of pergolide is largely determined by its form. The most stable formulation is a powdered form coated (encapsulated) which helps prevent breakdown by air and humidity. Pergolide is less stable when mixed with water, and must be kept refrigerated.

Cyproheptadine treatment begins at a low dose and is gradually increased until symptoms improve. Owners can monitor the amount of urine excreted and the thirst level seen in the horse. As the horse reduces its intake of water and its frequency of urination, the dosage can be held at that level. After some time and improvement, the dose of cyproheptadine can be reduced until it reaches a maintenance level.

The majority of horses with Cushing’s disease need to continue taking medication for life to control the symptoms. After treatment has been given for a couple of months, your veterinarian may wish to retest for hormone levels to see if the medication dosage should be modified. If improvement is significant, the dosage may be reduced and retesting will take place in another six weeks. If the dosage is stabilized, testing will be done less frequently until the minimum dose can be established for the animal.

Living and Management

Horses with Cushing’s require a proper diet, dental care and deworming regularly as well as special attention to their hoof health due to the potential for developing laminitis. Dietary considerations for horses with Cushing’s should include reducing the amount of energy in the form of carbohydrates (concentrates and grains) provided, increasing fiber in a balanced diet along with antioxidants to maintain a good body condition. The hope is that this modified diet will help prevent obesity and the onset of laminitis in these animals.

Other considerations will help keep the horse comfortable, such as:

• Avoid stressing the animal
• Provide water and feed on a regular schedule in the same location
• Clip excessive hair in warm weather and keep the animal warm in colder weather
• Groom hair and skin to prevent infection
• Give regular immunizations

Prevention

There is no evidence available showing that Cushing’s disease in horses can be prevented.

Support and Resources

Beech J. Diseases of the pituitary gland. In: Colahan PT, Mayhew IG and Merritt AM, editors. Equine medicine and surgery. 5th ed. St. Louis, MO: Mosby; 1999. p. 1951-1956.
Donaldson MT, Jorgensen AJR and Beech J. Evaluation of suspected pituitary pars intermedia dysfunction in horses with laminitis. JAVMA 2004;224:1123-1127.
Donaldson MT, LaMonte BH, Morressey P, Smith G and Beech J. Treatment with pergolide or cyproheptadine of pituitary pars intermedia dysfunction (equine cushing's disease). J Vet Int Med 2002;16:742-746.
Dybdal NO, Hargreaves KM, Madigan JE, Gribble DH, Kennedy PC and Stabenfeldt GH. Diagnostic testing for pituitary pars intermedia dysfunction in horses. JAVMA 1994;204:627-632.
McGowan CM. Diagnostic and treatment protocols for equine cushing's syndrome. In Practice 2003;25:596-592.
McGowan CM. Diagnosis and treatment of equine cushing's syndrome. The Veterinarian 2005.
Perkins GA, Lamb S, Erb HN, Schanbacher B, Nydam DV and Divers TJ. Plasma adrenocorticotropin (ACTH) concentrations and clinical response in horses treated for equine cushing's disease with cyproheptadine or pergolide. Eq Vet Jour 2002;34:679-685.
Schott HC. Pituitary pars intermedia dysfunction: challenges of diagnosis and treatment. Proceedings: Amer Assoc Eq Pract 2006;52:60-73.

source: PetMD