Understanding Equine Cushing’s Disease

Equine Cushing’s disease: many horse owners have heard of it or may have even had a horse with it. Buzz words such as cortisol, stress, and shaggy hair coat are often used when talking about this condition, but sometimes the details get confusing. Let’s get back to basics and take a look at this relatively common disease in mature horses.

What is Cushing’s Disease? With less dopamine (shown here) in the bloodstream as a horse ages, the pituitary gland is free to release more of certain hormones, including ACTH. ACTH tells the adrenal gland to secrete the stress hormone cortisol, which can lead to the classic signs of equine Cushing’s disease.

What is Cushing’s Disease?

Equine Cushing’s disease is a chronic, progressive endocrine disease seen in middle-aged to senior horses. More accurately labeled pituitary pars intermedia dysfunction, or PPID, this is a disease of the pituitary gland.

To better understand how this disease works, let’s take a step back and talk about this vital gland that is sometimes referred to as the “master gland” because of its importance in the body’s daily functions.

The pituitary gland lies at the base of the brain and dangles from a stalk right below the hypothalamus. The pituitary gland is about the size of a pea, and has three distinct parts: the anterior (front) pituitary, the posterior (back) pituitary and the pars intermedia (the middle).

All three parts have distinct jobs. Specific hormones originate from each portion and travel to other parts of the body, regulating things like reproduction, growth, kidney function and general body homeostasis (internal stability).

Each part of the pituitary gland is kept in check by the hypothalamus sitting above it. A problem arises when the checks and balances stop working. This leads to an unregulated pituitary gland secreting too much of its own hormones; this is what happens with equine Cushing’s disease.

Historically, it was believed that a benign tumor in the pars intermedia caused excessive hormone production. But now researchers think equine Cushing’s disease is primarily caused by faulty regulation within the hypothalamus, which sends another hormone—dopamine—to the pars intermedia.

Dopamine inhibits the production of hormones from the pars intermedia. However, as a horse ages, levels of dopamine in the blood decrease. Researchers aren’t clear why this occurs.

With less dopamine around, the pars intermedia isn’t as inhibited. It instead grows larger and releases too many hormones that influence thirst, appetite, temperature regulation and the body’s stress response.

One example of a hormone that is elevated in Cushing’s disease is adrenocorticotropic hormone (ACTH). ACTH travels to the adrenal gland and tells it to secrete the stress hormone cortisol.

Cushing’s disease also occurs in dogs and humans, but the equine version is slightly different based on the types of hormones that are elevated. This difference is why veterinarians prefer the name PPID to distinguish the equine disease from Cushing’s in other species.

Symptoms

An abnormally high level of stress hormones in the bloodstream cause what are known as the classic signs of Cushing’s in horses: a potbellied appearance coupled with muscle wasting; a long, shaggy hair coat that doesn’t shed out in the spring; and increased thirst and subsequent urination.

This constant exposure to stress hormones is detrimental to a horse’s overall health. Chronically high levels cause the horse to become immune compromised, leaving him at risk of bacterial and viral infections, such as chronic hoof abscesses or pneumonia. These hormones also make a horse more susceptible to laminitis.

Diagnostics

In many equine Cushing’s cases, a veterinarian can make a diagnosis based on clinical signs alone. A shaggy hair coat that doesn’t shed is considered a hallmark sign of this disease, making many cases relatively easy to identify. However, if clinical signs are mild or unclear, diagnostics should be conducted.

Unfortunately, blood tests for equine Cushing’s disease can be finicky.

DEXAMETHASONE SUPPRESSION TEST: This common blood test measures the horse’s physiologic ability to respond to external “stress” as mimicked via an injection of steroid.

It is conducted in two phases. First, the vet draws blood, then gives the horse an intramuscular injection of the steroid dexamethasone. The next day, the vet draws a second blood sample. In normal horses, dexamethasone should lower the cortisol level in the blood. In Cushing’s horses, the horse’s resulting cortisol levels should theoretically be higher.

However, the dexamethasone suppression test has the following drawbacks: It requires two veterinary visits, and an injection of dexamethasone could induce laminitis in a horse that already has excessive levels of stress hormones in his bloodstream.

Results are sometimes inconclusive because a normal horse’s cortisol levels vary widely depending on the horse’s own stress response and natural waxing and waning of the hormone, including during certain seasons. For these reasons, some vets prefer other diagnostic methods.

ACTH TESTING: This test measures blood levels of ACTH, which are typically higher in a Cushing’s horse compared to a normal horse. This test requires one veterinary visit.

However, ACTH is a delicate hormone that can degrade quickly, and ACTH results are not always accurate, which can lead to false negatives.

THYROTROPIN-RELEASING HORMONE (TRH) STIMULATION TESTING: The veterinarian takes a blood sample to measure baseline ACTH and then administers TRH. After 10 minutes, another blood sample is taken to measure ACTH again.

A positive result shows an exaggerated increase in ACTH in the second blood sample. This test can also produce ambiguous results, especially depending on the time of year (see “Three Things to Know,” pg. 48).

For these reasons, some veterinarians prefer to run more than one test to corroborate results. Or a vet may recommend retesting in a few months to see if results have changed over time.

Caring for Cushing’s

There is no cure for equine Cushing’s disease. However, pergolide (trade name Prascend), an FDA-approved medication, can manage the disease. Given orally once daily, it works to decrease the amount of ACTH in the blood.

Follow-up physical examinations or testing after a few months can evaluate the horse’s response to treatment. Some horses require an increase in dose over time or every autumn.

In addition to pergolide, other management practices are important. Because many Cushing’s horses are also insulin resistant, sweet feed and grain high in soluble carbohydrates should be avoided. Instead, offer lots of roughage and feed that’s higher in fat and fiber.

Maintaining a healthy body weight is essential to avoid other complications caused by obesity. Additionally, consistent high-quality preventative care, such as regular tooth floating and hoof care, are important for staving off secondary complications.

This article on equine Cushing’s disease appeared in the March 2020 issue of Horse Illustrated magazine. Click here to subscribe!