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Towards the end of winter, some horses—especially the older ones—often appear quite thin. Their topline looks angular, and you can feel the ribs through their thick winter coat. Additionally, they may have difficulty shedding their coat compared to younger herd members.

But is this indicative of Cushing’s disease?

If you contact the vet on a warm day and request a Cushing’s diagnosis, you will often receive a positive result.

Taking a blood sample from a horse
A high ACTH level does not necessarily mean that the horse has Cushing’s disease. © Adobe Stock / charlymorlock

What does this mean for me and my horse?

The diagnosis for Cushing’s disease involves conducting the so-called ACTH test. This test measures the levels of adenocorticotropic hormone (ACTH) secreted by the pituitary gland. If a horse has a pituitary adenoma (a benign tumor on the pituitary gland), it will continuously release elevated amounts of ACTH, disrupting the entire hormonal system.

However, a high ACTH level doesn’t necessarily confirm a diagnosis of Cushing’s disease.

This is because ACTH is a hormone that is released whenever the animal experiences stress. For instance, a thick winter coat in warm sunlight can cause thermal stress for the horse, leading to increased ACTH levels. Similarly, conditions like laminitis or osteoarthritis can cause pain, resulting in elevated ACTH levels as well. Therefore, the problem with this diagnosis lies in making the incorrect reverse conclusion.

Sweating horse with saddle and bridle
A thick winter coat in warm sunlight can cause thermal stress for the horse, because it’s too warm. © Adobe Stock / Debra Lawrence

A horse with “true” Cushing’s has an elevated ACTH level. However, an elevated level does not necessarily indicate that the horse has Cushing’s disease. To illustrate, when it rains, the road is wet. But just because the road is wet doesn’t mean it has rained.

In fact, true Cushing’s, or Pituitary Pars Intermediate Dysfunction (PPID) as it is now medically termed, remains a very rare disease primarily seen in very old horses.

What about Pseudo-Cushing’s?

However, the more common scenario is what is referred to as pseudo-Cushing’s (or sometimes known as “peripheral Cushing’s”). Horses with this condition exhibit symptoms similar to those of Cushing’s candidates.

This is because the symptoms associated with Cushing’s (or PPID) are not caused by ACTH, but rather by a dysregulation of the adrenal gland. This dysregulation leads to the production of glucocorticoids, specifically the body’s own cortisone. These are released in large quantities in horses experiencing stress. Consequently, the stress hormone ACTH triggers an increased release of glucocorticoids, resulting in visible symptoms.

Among other functions, the glucocorticoids are tasked with breaking down any available protein to provide the body with sugar energy. The thick, continually growing coat that fails to shed (hirsutism) is a direct consequence of this process, as is the propensity for conditions such as laminitis and brittle tendons, progressive muscle loss, and reduced performance willingness.

White pony with thick coat
Not every horse with a thick coat has Cushing’s disease. © Adobe Stock / Christian Musat

However, dysregulation of the adrenal glands can be caused by various factors beyond a pituitary adenoma.

These factors include thyroid gland dysfunction, elevated selenium levels, chronic stress, among others. Therefore, it’s crucial to examine the circumstances surrounding a horse suspected of having Cushing’s disease. Often, errors in feeding or management over the years have contributed to the horse becoming increasingly metabolically imbalanced, leading to the manifestation of severe symptoms.

Therapy should prioritize addressing these underlying metabolic issues. Restoring balance to the metabolism can result in the disappearance of symptoms in many horses without the need for medication.

Medication, such as Prascend, only suppresses symptoms; it does not address the root cause

While in horses with true Cushing’s, characterized by a pituitary adenoma, medication may provide some relief by suppressing symptoms for a few more months, in cases where symptoms are induced by metabolic imbalances, the introduction of medication often exacerbates the problem by adding to the existing metabolic load.

What to do with the emaciated senior horses?

Firstly, it’s important to note that during the shedding period, the presence of a thick winter coat doesn’t necessarily indicate Cushing’s disease. Horses, especially those kept without rugs or in robust environments, may naturally develop a thicker coat, particularly seniors and foals who spend much time outside. This adaptation helps conserve energy during winter.

As long as the shedding process begins with the onset of warmer weather, concerns about Cushing’s disease may be premature. Additionally, weight loss in older horses over winter is common. Due to dental issues, they may struggle to grind hay efficiently despite being provided with ad libitum access, resulting in an energy deficit.

One solution is to supplement their diet with soaked hay pellets along with sainfoin pellets, both during the transition from winter to spring and throughout the following winter months. Early access to carefully managed grazing can also be beneficial. Older horses often find it easier to chew grass, which provides essential nutrients compared to hay. However, caution is advised if the horse is prone to laminitis, commonly associated with metabolic disorders.

For older horses, it’s advisable to enter the grazing season slightly overweight at the end of summer, as they tend to lose weight rapidly during winter or due to illness.

If uncertainty remains regarding whether the horse is a “true” Cushing’s candidate, may have “pseudo-Cushing’s,” or possibly suffers from unrelated health issues like muscle atrophy or laminitis associated with insulin resistance, EMS, pseudo-EMS, or cryptopyrroluria, consulting a knowledgeable metabolic therapist is recommended.