Cushing's disease occurs in various animals (e.g. dogs, horses, people), although the symptoms in horses are different than the symptoms in other animals (e.g. in dogs it results in hair loss whereas in horses it results in greater hair growth). When the disease occurs in horses, it is known as Equine Cushings Disease (or ECD).
ECD is an illness caused by excessive hormone production. The pituitary gland produces too much hormone, which over-stimulates the adrenal glands, which in turn produce too much steroids. It is the excessive quantity of steroids which causes the symptoms associated with ECD. The over-production of the pituitary can be caused various changes, such as the growth of benign tumors, enlargement of the pituitary due to age-related changes, or a decline in the regulatory chemical dopamine due to age-related changes in the brain. All of these factors tend to be age related, so the disease is much more common in older horses (over 15 years) although it has been found in horses as young as seven years.
The nature and cause of the disease itself was first explained by Harvey Cushing, an American brain surgeon, in 1921. Consequently, the disease is named after him.
Symptoms and Diagnosis
The symptoms of cushings disease vary somewhat from horse to horse, with the more common symptoms including:
- Changes to the coat. It may become longer and curly. It may not be shed in Spring. In some cases the color becomes lighter.
- Increased sweating. This is often due to the coat become longer and thicker, and retention of the winter coat into summer.
- Weight loss despite increased appetite and food consumption. Changes to the body shape, with loss of muscle in back and neck, and development of pendulous abdomen.
- General depression, loss of coat shine, horse looks ill. Development of diabetes and a resulting increase in water consumption. This can be difficult to detect with pastured horses, but with stabled horses one may notice an abnormal amount of urine (or soiled bedding) in its box.
- Development of laminitis. The most common cause of normal laminitis is carbohydrate overload due to eating spring grass, whereas ECD-caused laminitis is more frequent in autumn and winter since ECD is worse when the days are shorter (this is because the pineal gland is light-sensitive and as daylight decreases it stimulates the pituitary gland).
- Immune system declines, leaving the horse more prone to infections and slower to heal from external injuries.
- The depressions above the eyes fill in. These depressions are most evident when a horse is chewing, but gradually fill in with fat in ECD horses.
Any of these symptoms can be caused by a variety of illnesses other than ECD. Consequently, blood and urine tests are normally used to confirm that the cause is ECD (e.g. by measuring hormone levels and hormone response levels) and to rule out other possible diseases. Studies have shown that many horses with ECD are either not diagnosed or incorrectly diagnosed (the symptoms are incorrectly attributed to a different disease), with the result that they do not receive appropriate treatment.
ECD cannot be cured. However, the hormone levels can be reduced to more normal levels through medication, slowing the progress of the disease. As some of the medications can have serious side effects (e.g. liver disease), one needs to monitor for any such side effects and change the medication accordingly.
As cushings disease can result in the development of secondary diseases (e.g. laminitis, diabetes), the horse needs to be closely monitored for any symptoms of these, with appropriate and prompt treatment.
Management of the symptoms can make the horse more comfortable and increase its quality of life. For example, if the horse cannot shed its winter fur and consequently over-heats in summer, the coat should be clipped.
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