Cushing’s Disease Threatens the Health of the Older Horse

Last Updated on February 21, 2022 by Allison Price

Horses live longer, it is undisputed. The marketplace is proof of this. Feed companies create and produce specialized feeds for older horses. Each one is praised as having a high level of energy-rich, easily digestible ingredients. Booksellers have a wide selection of texts that are specifically designed for caring for older horses. They also include information about what to expect as the horses age.

Many retirement farms have been established over the past decade to provide horsemen with a safe place for their older horses when there is not enough land. While the benefits of having a pony or horse around for many years is innumerable, it does come with its drawbacks. Older horses are more susceptible to the many diseases that can often plague them. Particularly, Cushing’s disease has been steadily increasing in the past few years.

Cushing’s Disease

The Endocrine System

The endocrine network is a collection of organs that are responsible for hormone production and distribution. The regulation of metabolism is one of the many responsibilities that hormones have. The body is considered to be in hormonal equilibrium if all endocrine functions are performed without fail. Cushing’s disease is a form of hormonal imbalance that can be quickly disrupted by endocrine processes.

Because it oversees body-wide endocrinology, the pituitary gland has been called “the master gland”. A pituitary tumor, which is usually an adenoma or abnormal growth, can cause the production of too many hormones.

Adenoma refers to any benign tumor that arises from glandular tissue. An adenoma is a benign tumor that arises from the pituitary. This causes excessive production of adrenocorticotrophic hormonal hormone (ACTH). ACTH excretion causes adrenal glands to produce excessive cortisol. This hormone can have negative effects on many body systems.

Presence of an adenoma and accompanying hormone fluctuations often lead to equine hyperadrenocorticism, more commonly called Cushing’s disease or simply Cushing’s. This condition is most common in middle-aged horses or elderly horses. It can affect horses aged between seven and ten years. Researchers believe that the condition is more common in ponies than it is in horses. Due to ponies’ tendency for laminitis and unusually dense hair, clinical misdiagnosis could be more common.

Telltale Signs

Horsemen notice a problem with aged horses as soon as their wooly winter coats stop falling off in the late spring or early summer. Cushing’s disease is characterized by excessive hair growth (hirsutism) and inability to shed winter coats. Cushing’s disease is often characterized by a long, thick coat that causes excessive sweating, even in mild conditions. The hair may also change from a dark brown or black color to an orange-brown. The general dullening of the horse’s coat can also happen, regardless of how well he is fed or groomed. Cushingoid horses can experience subtle changes in their coats, such as an abnormally thick wintercoat or the retention of guard hairs underneath the chin and on top of the neck, long after the winter coat is shed.

Horses may also experience weight changes. Cushing’s syndrome can lead to weight loss or gain in horses. However, it is more common for horses to become overweight. A common sign in Cushing’s syndrome horses is weight loss. The neck may show signs of decreased muscle tone, such as a thin, knife-like appearance and a high topline that may lead to a saggy and pot-bellied appearance. Cushing’s syndrome can also be characterized by the loss of fat. An unusual accumulation of fat around the eyes, crest and over the croup may occur. Horses have hollows located just above their eyes. These indentations are common in cushingoid horses. They often contain fat.

Horses may have increased thirst and appetite, which can lead to an unusual amount of urine production. Horse owners may find themselves in a situation where they have to deal with water buckets that are bone dry. Horses that are healthy and have been fed a balanced diet of all-forage in a moderate environment will typically drink between five to eight gallons per day. Cushing’s disease horses, however, consume much more. There have been instances of horses drinking over 20 gallons per day. Stalls can become difficult to clean due to the increased water intake and subsequent increased urine production.

Secondary complications, laminitis or diabetes mellitus, are more severe than the signs that indicate the disease. A study of 21 horses affected by the disease revealed that 24% of them were laminitic and 38% showed signs of diabetes mellitus. While the exact cause of laminitis remains unknown, it is believed that high cortisol levels may lead to constriction in blood vessels located in the hoof capsule. Another school of thought is insulin dysfunction. Insulin acts as a vasodilator. This is a substance that increases blood carrying structures like veins, arteries and capillaries. Insulin resistance can lead to a horse suffering from stricture in blood vessels, which may cause insufficient flow to peripheral structures like the hoof. Recurrent episodes with laminitis can lead to solar abscesses, displacement or rotation of coffin bones, and even permanent injury.

Diabetes mellitus, a chronic condition caused by low insulin levels or resistance to insulin use by target organs, is a chronic disease. Diabetes mellitus can cause increased thirst, increased urine production, and delayed wound healing. Horses are more susceptible to infection if they have high levels of cortisol.


Cushing’s disease can be difficult to diagnose. Although simple blood tests may not reveal the symptoms of Cushing’s disease, horses with Cushing’s disease don’t always show these symptoms. Additionally, only blood tests will provide a diagnosis. Rare adrenal gland tumors can cause hormonal surges that are similar to those seen in cushingoid horses.

Dexamethasone suppression testing (DST) is the most reliable way to diagnose Cushing’s disease. A blood sample is used to determine the baseline cortisol level. Dexamethasone is then administered to the horse. This powerful steroid stops hormone production. A second blood sample is taken twenty-four hours later to compare the horse’s response to dexamethasone. Normal horses experience a marked decrease in cortisol levels after dexamethasone administration. Cushing’s horses are often unresponsive to dexamethasone. Cortisol levels can increase or remain the same due to inability of pituitary slow down hormone production. One risk of the dexamethasone suppression testing is resultant laminitis.

The thyrotropinreleasinghormone (TRH) stimulation test can be used to stimulate horses that are susceptible to laminitis. TRH triggers the release of many hormones, including cortisol. Intravenous administrations of TRH can cause cortisol levels to rise in horses with Cushing’s disease. Once, ACTH serum concentration was thought to be a reliable way to diagnose the disease. However, ACTH test results were not always accurate when compared with DST and TRH results.


Cushing’s Disease cannot be cured. There are ways to manage the symptoms. Pergolide mesylate, also known as pergolide, and Cyproheptadine are the most common drugs. Pergolide reduces ACTH secretion by the pituitary, which in turn decreases cortisol production. Research at the University of Michigan has shown that pergolide’s efficacy is superior to that of cyproheptadine.

Researchers created The Michigan Cushing’s Project to gather 77 horses that were confirmed to have Cushing’s disease. They used clinical signs, the TRH stimulation test results and DST results. The average age of the test population was 22.8, with a range between 12-34 years. The baseline endocrine test results and the results after a 6- to 12-month treatment period were compared. Horses were then divided into three groups: treatment using cyproheptadine or pergolide.

Horses treated with pergolide showed the most clinical improvement, but a few horses responded positively to cyproheptadine. None of the horses in the no-treatment group showed any improvement, as expected. Cyproheptadine was not associated with any side effects.

However, horses who were given pergolide experienced a decrease in appetite after the first week. Pergolide improvement has been observed in field trials within three to four weeks. Usually, the first signs of improvement are shedness and an increase in energy. Pergolide treatment has one major drawback. Pergolide treatment costs an average of $60 per month.

The effects of trilostane in horses with cushingoid horns was recently examined by the Royal Veterinary College of the United Kingdom. Trilostane prevents certain hormones from being produced by the adrenal glands. The drug was administered once daily to 20 horses with Cushing’s syndrome. After one month, 11 horses with Cushing’s disease experienced excessive drinking and frequent urination. 19 horses suffering from lethargy showed more interest in their surroundings, but were not more energetic.

After trilostane, 14 of 17 horses with laminitis were stable or showed improvement in their hoof conditions. Six horses showed improvement in lameness, and they were able to stop taking nonsteroidal anti-inflammatory medication. Three horses had acute laminitis and rotational changes to their coffin bones prior to receiving trilostane. These laminitic symptoms didn’t respond to trilostane. Trilostane is still under investigation for safety and long-term efficacy.

Dietary Considerations

Cushing’s disease can be treated with judicious feeding management. High starch diets such as those that contain sweet foods can worsen diabetes mellitus and increase the likelihood of developing laminitis. Why? Why? Elevated levels can cause cortisol secretion to be altered, which can negatively affect metabolism and lead to conditions like Cushing’s syndrome or laminitis. Low-starch diets are recommended.

In the treatment of insulin resistance in horses aged over 60, supplementation with chromium has been shown to reverse hyperinsulemia. A few researchers think insulin resistance could cause laminitis. Horses may be less likely to develop laminitis, which is one of the most severe complications of Cushing’s disease. Horses with Cushing’s disease are often prescribed Metaboleeze by their veterinarians.

Vitamin E and vitamin C supplementation can also help to recharge the immune system. Vitamin C levels have been shown to decrease in older horses so supplementation may prove beneficial. Vitamin C may be expensive to give daily. Some veterinarians recommend that vitamin C is only administered to horses with an infection or wound.

Other Management Tips

Cushing’s disease is a condition that horses suffering from it should be treated with special care. Due to the potential for sole abscesses or laminitis, hoof care must be taken seriously. It is essential to trim your hoof regularly and have it shooed at least once every four to six weeks. All measures should be taken to decrease the chance of developing laminitis, including gradual changes to diet and limited exposures to high-carbohydrate spring pastures. Regular deworming is essential for veterinary care. Cushingoid horses may have lower immunity to parasite damage due to a loss of intestinal wall integrity. An experienced dentist should inspect the horse at least once a year for any dental issues. As there may not be enough tooth to emerge from the gum line, dentists should avoid aggressively floating the teeth of elderly horses. A veterinarian should treat any abscesses that are detected by the dentist. A qualified professional should treat any type of infection immediately.

Horses with Cushing’s disease need to have their bodies clipped in order to be more comfortable during warm weather. Horses that can’t escape the heat of summer’s midday sun need to be clipped. It is important to groom your horse well. Cushing’s disease horses may be more vulnerable to skin problems during the summer months because of their unnaturally long hair coats. Owners need to be vigilant for signs of rainrot and dew poisoning in their horses.

Cushing’s disease does not mean that a horse is doomed to die. Although there is no cure for Cushing’s disease, there are many management practices that can help keep affected horses in good health for many more years.

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!