Cushing's Disease in Horses (Equine Cushing's Disease)

In 1912, Dr.Harvey Cushing discovered this disease and hence the name. Cushing's disease can be found in dogs, horses and even humans. Equine Cushing's disease refers to Cushing's disease in horses. These days this disease is also referred as "Pituitary Pars Intermedia Dysfunction."

Causes of Cushing's disease in Horses
Pituitary gland, located close to base of brain, is a major organ of endocrine (hormone) system. Hypothalamus is a gland located near the pituitary gland. The synergy of hypothalamus and the pituitary gland help regulate the body's system. Cortisol is a steroid hormone secreted by adrenal glands. The hypothalamus and pituitary gland influence the production of cortisol. In Cushing's disease this cortisol is produced in an excessive manner, resulting in different symptoms.

Presence of a benign tumor in the pituitary gland is considered as main cause for Cushing's disease. The tumor induces excessive secretion of cortisol. The growth of the tumor causes the breakdown of the neurons in hypothalamus, thus resulting in malfunction of hypothalamus and hence the pituitary gland.

Symptoms of Cushing's disease in Horses
The symptoms of Cushing's disease in horses can be noticed easily, they are

• Excessive thirst and urination: A horse suffering from Cushing's disease drinks 20 gallons of water, whereas a normal horse drinks 5-8 gallons.
• Horse coat: The horse coat does not shed, it becomes wavy, hard and heavy.
• Change in appearance: Loss in weight, dull eyes, hay belly.
• Increased glucose level: Blood and urine contain increased level of glucose.
• Infections: Because of improper functioning of immune system, the horse suffers from various infections
• Laminitis: Older horses show symptoms of laminitis.
• Neurological problems may arise due to increase in the size of the pituitary gland.

Diagnosis of Cushing's disease in Horses
A veterinarian will perform following tests in case he finds the symptoms of Cushing's disease.

• Blood test: This test may bring out results like increased blood sugar level (120 mg/dl to 300 mg/dl) and blood fats, anemia, etc.
• Urinalysis: It may reveal glucosuria and ketonuria (very high level of glucose and ketones in urine).
• DST- Dexamethasone Suppression Test : There are two steps involved in this test. First the blood sample is taken to know the level of cortisol. Then dexamethasone is injected. The dexamethasone suppresses the adrenal gland to secrete less cortisol. The blood sample is taken again after 24 hours of injection of dexamethasone. Both samples are compared to check the change in level of cortisol. In normal horses, there is a marked reduction in level of cortisol whereas horses with Cushing's disease show very less reduction in the cortisol level. This test is most commonly used by veterinarian to detect Cushing's disease. Recently a new test is carried out which involves combination of DST with TRH (thyrotropin releasing hormone) stimulation test.
• ACTH stimulation test: It's another method of testing cortisol level. One unit of ACTH is injected per kg of body weight. Horses affected by cushing's disease show a four times rise in cortisol, while normal horse shows only doubled rise. This test is not that reliable when compared to others.

Treatment of Cushing's disease in Horses
Cushing's disease in horses cannot be cured. If detected at an early stage it possible to give the horse a proper treatment. However, the treatment has to be continued life long. Various drugs are used to treat these horses. They help to reduce the cortisol level. These drugs are expensive. Following are some methods to help a horse suffering from Cushing's disease, live a comfortable life:

• The coat should be trimmed in summer, since the horse is unable to shed it.
• If the horse has skin infections, use of antibacterial gel is recommended.
• The blood test may help in deciding the diet of the horse so as to avoid increase in sugar level.
• Proper dental care and vaccinations.

Trials were conducted to give a herbal treatment to such horses. Haste berry (Vitex agnus castus) was found effective when used in early stages of the disease. The horses who were given this herbal treatment showed improvement in a short span of 3 weeks.

Cushing's disease in horses can be easily noticed and it is a treatable disease. Though this disease cannot be cured, the horse suffering from this disease can lead a normal life if it is given a proper treatment.