The Painting Pony - About Cushings Disease

Equine Cushings Disease

Equine Cushing’s Disease is one of the most common endocrine disorders affecting the older horse, however, there is still alot that is unknown about the disease. The clinical signs seen with Cushing’s disease are related to the effects of the abnormal secretion of hormones and neurotransmitters by a tumor of the pituitary gland, located within the brain. The abnormal gland over-secretes a variety of hormone molecules that have widespread effects on all body systems. The gland also becomes insensitive to regulatory feedback loops, which normally control hormone levels within physiologic ranges. Depending on the location and size of the tumor, horses may have a more severe or milder case of the disease.

Horses and ponies of all breeds and both sexes are affected by the disease. The average age of onset of clinical signs is typically the mid to late teens. Although Equine Cushing's Disease was thought to be only a disease affecting senior horses, horses as young as seven years of age have been positively diagnosed with the disease. Most commonly, the disease affects horses at aged 15 years old and up.

Symptoms of Cushing's Disease include laminitis that often seems to occur for no reason. Horses with Cushing's Disease may have laminitis while on pasture or during the Fall & Winter. Other symptoms that may be noticed include the following: excessive water intake coupled with excessive urination, being slow to shed out a winter coat, the development of a long, shaggy haircoat that does not shed out in the summer, abnormal sweating, muscle-wasting (especially along the topline) with the development of a pot-bellied appearance, increased susceptibility to infections and chronic laminitis, appetite may be increased even though loss of weight may be apparent or Cushing's horses may develop obesity. Cushing’s disease shares many clinical signs with Insulin Resistance, making a complete evaluation by your veterinarian critical for proper treatment. Cushing's Disease and Insulin Resistance are often seen together making diagnosis confusing. Cushing's Disease and Insulin Resistance are two separate metabollic conditions requiring two separate treatments; Cushing's Disease is treated with medication, Insulin Resistance is treated with a strict low sugar/low starch diet.

The above pictures are of the great John Henry who suffered from Cushings Disease for many years. In these pictures
it is easy to see the shaggy coat that is most often seen in Cushings horses. A lose of muscle tone is also evident

Diagnosing Cushing's can be done by a few different tests. A veterinarian may visually inspect a horse for the outward clinical signs of the disease and may then go on to test the horse by doing bloodwork. The different tests that may be run to test for Cushings are as follows:

- Blood ACTH: Used to diagnose classical Cushing's disease, where ACTH will be greatly elevated. Higher than normal levels may also be found in stressed horses, and possibly in aged horses, but not to the same degree. Normal seasonal changes may also throw-off test results by causing a false positive in the fall when ACTH rises to prepare for the changing seasons. However, clinicians reviewing ACTH results during the Seasonal Rise (August to December) should combine results with clinical symptoms in choosing a treatment plan. ACTH is also useful in ongoing monitoring of the disease and response to drug treatment.

- Cortisol Rhythm Test: Screening test for Cushings disease. May be both false positive and false negative results. False positives can be as high as 35%. Involves sampling twice on one day (AM and PM) and is generally not recommended for diagnosis.

- Dexamethasone Suppression Test: Screening test for Cushings disease. Involves injection of the corticosteroid drug dexamethasone. This test is often avoided as it may cause or worsen laminitis. Involves collection of samples over 8 to 12 hour period.

Diagnosis of Insulin Resistance (IR) is done by drawing blood for NON-Fasting Insulin & Glucose testing. Results for this test often show both insulin & glucose in the normal range while some IR horses show elevated insulin. The glucose:insulin (GI) ratio is basically a very simple concept. This ratio/number indicates how many “units” of insulin are being secreted per “unit” of glucose. The smaller the number, the less sensitive the cells are to insulin. The resulting calculated G:I ratio correlates extremely well in horses. Divide the glucose (in mg/dl) by the insulin (in uIU/mL) to achieve the G:I Ratio. The higher the number, the more insulin sensitive/less insulin resistant, the horse is. GI ratio 4.5 or less – uncompensated IR, correlates with a high laminitis risk and poor IR control. Ratio between 4.5 and 10 – compensated IR. Ratio 10:1 or higher with a normal blood glucose* - normal.

Treatment for Cushing’s Disease is all about management and careful detail to how your horse is feeling on a regular basis. Small warning signs may be all an owner has to tell them something is not right. Specific medical management of the disease involves drugs directed at altering the balance of neurotransmitters in the brain to help control the abnormal hormone secretion by the pituitary gland. The gold standard, Pergolide, a drug used in people to treat Parkinson's Disease has been shown to be very effective in controlling symptoms of Cushing's Disease offering these horses a very good quality of life.
There is also an herbal remedy, Chaste Berry that has been shown to help make improvements in Cushing's horses as well although often only short term before Pergolide is needed. Improvement in the clinical signs and bloodwork takes several weeks to begin to occur. There is also a wide range of effective dosages for the drugs, meaning that it may take some time to determine the dose that is most effective for your horse, dependent mainly on how progressed the disease was at time of diagnosis. Treatment with Pergolide can range from 1 mg daily to 5 mg daily in advanced cases.

Supportive care for your horse is essential to helping him or her stay as healthy as possible. Because a horse with Cushing’s Disease is more susceptible to infections of any type, basic preventative health care and quality nutrition is even more important now. Greater attention to health care is important due to the increased susceptibility to infections and regular worming is important as the level of parasites may be increased with the immune suppression.

**References: http://pets.groups.yahoo.com/group/EquineCushings/

** 10% of Buddy's Profits from the sales of his paintings and gifts from the Painting Pony Gift Shop are donated to research on Equine Cushings Disease that is ongoing at the University of Tennessee.