Equine Cushing's and Insulin Resistance in Horses

Cushing's Disease is caused by a tumor in the pituitary gland. The pituitary gland, located just under the brain, is responsible for producing several hormones. Adrenocorticotropic hormone (ACTH) is one of the hormones produced by the pituitary gland. The tumor causes the pituitary gland to produce excessive amounts of ACTH. In response to excessive amounts of ACTH the adrenal gland then produces sustained, elevated levels of cortisol. The cause of the pituitary tumor development has not yet been isolated.

Symptoms of Cushing's include:

- Long, curly coat that may not shed
- Greatly increased water consumption and urination
- High susceptibility to infections (due to compromised immune system)
- Mares may fail to cycle, have enlargement of the mammary glands, or milk production
- Geldings may have a fatty sheath and heavy sheath secretions

Treatment of Cushing's disease:

- Diet is of utmost importance; diet for a Cushing's horse should emphasize fiber and fat
- Strict attention to dental care is very important in Cushing's horses. Often the diet of the Cushing's horse is strictly hay, meaning the horse will not be using his incisors to nip grass. Regular dental care to make sure the incisors do not become elongated is very important.
- Medication: There are several options for this including
A)Pergolide - This is a drug which helps correct chemical imbalances by boosting the activity of dopamine
B)Vitex (chasteberry herb) - This is an herb that acts in a manner similar to Pergolide. To date there has not been as much clinical experience with this herb.
C)Cyprohetadine - This is drug which suppresses serotonin levels in an attempt to help correct chemical imbalances in the brain.

Insulin Resistance is the term used to describe the condition when body cells do not respond appropriately to insulin. The function of insulin is to activate cells to take in glucose (sugar in the blood). When the cells do not respond to insulin they are termed resistant, hence Insulin Resistance. When the insulin fails to activate the cells to take in glucose, the pancreas responds by producing more insulin than normal to get the glucose into the cells.

Insulin Resistance is similar to early Type II diabetes in people. However, people with Type II diabetes will have elevated insulin and glucose levels, while in horses we see only elevated insulin levels and do not see elevated glucose levels except in extreme cases.

The reason we often see Insulin Resistance in horses with Cushing's disease is due to the elevated cortisol levels caused by the Cushing's disease interfering with the normal functioning of insulin. Thus, many Cushing's horses are also Insulin Resistant, but this is not always the case.

Symptoms of Insulin Resistance Include:

- Easy weight gain
- Abnormal fat deposits including a cresty neck and fat at the tail base
- Laminitis
- Low energy levels
- Significant increase in water intake and urination

Treatment of Insulin Resistance:

- Control of starch and sugar levels in the diet is very important
- often all grain-based feeds need to be eliminated
- Hay should be tested to determine nonstructural carbohydrate levels (NSC). Generally levels need to be below 10% for severe cases and below 15% for any IR horse
- Mineral levels in hay should be analyzed to allow for appropriate vitamin/mineral supplementation to meet the horse's needs

An excellent resource to learn more about Equine Cushing's Disease, Insulin Resistance and other maladies normally associated with Cushing's is the Equine Cushing's Yahoo Group. This group is very active and a wealth of information. The group can be found here.

Written by Melissa Webb
This article is not meant to be used for treatment or diagnosis of any disease. Please consult with your veterinarian for comprehensive treatment and diagnois explanations.