Cushing’s Disease in Horses

Horse walking in the yard

When you picture a horse, what do you see? You probably imagine a sleek coat and mane, a graceful trot, and a sturdy creature—one that could buck you off its back if only it wanted to.

So what’s going on when your horse has a strangely long and wavy coat, is lethargic, and appears to be losing weight unexpectedly?

In addition to several others, those characteristics are marked features of what’s often called equine Cushing’s disease, or more accurately, pituitary pars intermedia dysfunction (PPID). Cushing’s disease in horses is unfortunately common. However, it’s also increasingly easy to treat and manage for a long, full life with your valiant steed. 

The important thing is that you know how to identify, treat, and monitor it. 

Cushing’s Disease in Horses Symptoms 

PPID doesn’t usually present itself until later in a horse’s life. For that reason, you should pay careful attention to these Cushing’s disease symptoms in horses as they reach age 15 and onwards:1

  • Noticeable changes in their coat – Most horses with PPID can’t shed their coats properly in the summer. As a result, they’ll have long, sometimes thin, curls and waves with flaky skin underneath. You may also notice excessive sweating from the thick coat or hardly any sweating at all.
  • Excess hydration and urination – If your horse seems abnormally thirsty or regularly wets the stall, this could be a sign of polydipsia and polyuria, respectively. Overdrinking is classified as anything over 25 or 30 liters in a day.
  • Extreme foot pain and swelling (Laminitis) – Inflammation and disrupted blood flow can lead to weakness and damage in the laminae, the or tissues that hold the hoof wall and pedal bone together. This displacement, rotation, and even separation are extremely painful. Lameness in horses and strange weight shifts are therefore signs of Cushing’s.2
  • Slow, lethargic movements – This isn’t always immediately apparent as many horses naturally become less active with age. However, it’s imperative to monitor changes in energy, performance, and apathy as these early behavioral signs will be critical in prompt diagnosis.
  • Muscle atrophy and weight loss – Early on, you might notice a loss of topline muscle. As the disease progresses, symptoms include looseness of tendons and ligaments and both topline and bottom-line muscle atrophy.3 This atrophy will likely manifest as a dipped back or rounded abdomen. Despite the associated weight loss, your horse may also exhibit a ravenous appetite.
  • Eye complications – Cushing’s can result in bulging eyes because of the redistribution of fat around the eye socket. It can also result in blindness, which stems from the compression within the optic nerves and brain. 

While it’s true that Cushing’s is a late-stage disease, you don’t necessarily have to wait until one of these symptoms manifests to start looking into your horse’s health.

Other Early Warning Signs 

Technically, Cushing’s disease can happen to any horse, but it’s much more likely to appear in horses that share these risk factors or demographic features:

  • Insulin resistance – While the two aren’t always related, insulin resistance is believed to be a risk factor for Cushing’s. This is usually evidenced by high blood insulin levels and indicates an increased likeliness that PPID will develop later on.4
  • Specific breeds – The reason is still unknown, but research has identified that PPID is perhaps more prevalent in ponies, Arabians, and Morgans.5 If you own one of these breeds, it doesn’t hurt to be extra vigilant, especially as your horse ages.
  • Advanced age – More than 85% of horses with diagnosed Cushing’s are over 15 years old1, and one large-scale study found that 21.2% of surveyed aged horses had the disease, yet only 1.6% of owners knew about it.6 

Some owners wrongly assume that a swayed back, weight loss, and even long, wavy hair are natural signs of aging. In reality, these are much more likely to be examples of undiagnosed Cushing’s. Pay careful attention to behavioral and physical changes so that you can spot the onset and seek veterinary care accordingly before your horse is at the stage of painful laminitis and blindness.

What’s Actually Happening to My Horse?

Young weanling horse lying down in stall with sunbeams shining through cracks

These symptoms can be scary when you first notice them, especially when they seem random and unconnected. But all of these individual visible factors stem from the same initial problem: dysfunction in the pituitary gland, which triggers hormonal balances. 

Let’s break this down step by step to demystify the disease a little:7

  • With age, the neurons in the hypothalamus break down and produce less dopamine. Dopamine is essential in regulating hormone release from the pituitary gland. 
  • Without this much-needed dopamine to regulate the process, the pituitary gland becomes enlarged. Previously, experts thought PPID resulted from tumor growth, but the consensus around it changed with research.
  • In tandem, the pituitary gland releases an unnatural influx of hormones, including adrenocorticotropic hormone (ACTH). 
  • In response to the ACTH, the adrenal gland releases an abundance of cortisol, the body’s stress hormone.
  • The excess cortisol production is likely the cause of many physical symptoms associated with Cushing’s. There may also be a combination of hormones involved, each responsible for a different set of symptoms. 

What’s still unclear is the exact cause of the initial dopamine degeneration. One leading theory is oxidative stress, which appears to occur in both humans and animals alike. Free radicals, both from internal metabolic processes and external factors like UV rays, pollutants, and cigarette smoke, can damage the body’s cells, including the horse’s dopamine neurons.

What Can I Expect From the Diagnosis Process?

Despite the importance of prompt detection and diagnosis, there isn’t a sure-fire way to identify Cushing’s disease in its early stages. 

Still, there are several standard diagnostic tests that a vet may use on your horse:8

  • Endogenous ACTH Testing – A plasma sample at any time of day can determine resting ACTH concentration. The diagnostic threshold is usually ten picomoles per liter (pmol/L); however, between July and November, plasma levels must reach 22 pmol/L9 because of the increased hormonal fluctuations at this time of year.7 Vets will usually use this test in conjunction with:
  • Insulin Testing – The same plasma can be used to determine baseline insulin levels. However, for optimal results, you should take the sample in the morning before feeding. Testing insulin and glucose levels will help identify insulin resistance, a common risk factor of Cushing’s.
  • TRH-Response: ACTH Testing – In horses with PPID, thyrotropin-releasing hormones (TRH) will overly stimulate the ACTH-producing cells and trigger an increase in production. To see if this is the case, you can collect a baseline blood sample, inject a dose of TRH, then order another blood sample to check for a spike in ACTH.
  • Dexamethasone-Suppression Test (DST): Cortisol Testing – As the gold standard of Cushing’s diagnoses, this overnight test similarly requires a baseline blood sample and another sample 18 to 20 hours later, following a dexamethasone injection. Cortisol levels will drop in healthy horses but stay the same in horses with PPID.7

If your horse is showing symptoms but returns a negative ACTH test, this might just be a sign that their hormone levels aren’t yet high enough to meet the diagnostic threshold. Unfortunately, an early negative doesn’t mean they don’t have Cushing’s. Instead, you should re-test every six months or a year to stay ahead of the curve. 

Once your horse receives a diagnosis, you can proceed with creating a treatment and maintenance plan.

What Can I Do For My Horse With Cushing’s Disease?

side view of farmer holding bucket and feeding horse in stable

Unfortunately, we still don’t have a cure for Cushing’s disease. However, many horses live happy lives for years to come, based on the average lifespan of a horse, on a combination of medications and lifestyle adjustments. 

The most common drug for equine Cushing’s disease is pergolide mesylate, a dopamine agonist. If you think back to the causes of PPID, you’ll recall that the root problem is the sudden decrease in dopamine levels, which impacts hormone secre