Cushing's in Horses Disease Explained

Cushing's in horses is something that has become more prevalent as horses have continued to live longer lives thanks to nutrition and veterinary medicine. The disease can be very serious to horses, particularly because it is most commonly seen in geriatric horses.

Cushing’s in Horses in a nutshell

Pituitary Pars Intermedia Dysfunction (PPID), most commonly known as Equine Cushing’s Disease, is a benign tumour of the pituitary gland located at the base of the brain. It is named after the neurosurgeon Harvey Cushing who first described it. The tumour affects the horse’s hormonal regulation which results in chronically increased cortisol levels that negatively affect the body in a number of ways. All equines can be affected with the Cushing’s disease but ponies and breeds such as Morgans and Quarter horses seem to be at greater risk.

Symptoms of Cushing’s in horses

Horses with Cushing’s Disease can exhibit a variety of symptoms, with an excessively long and curly hair coat that fails to shed in the summer being the most recognisable one. Other symptoms include:

• Weight loss due to loss of active back muscle, seen as a swayback and potbelly
• Excessive body fat
• Development of insulin resistance which can lead to laminitis
• Polydipsia/polyuria (excessive thirst/urination)
• Blindness
• Weakened immune system; delayed wound healing, chronic infections
• Behavioural changes such as lethargy
• Reproductive disorders

Mechanisms behind Equine Cushing’s Disease

In order to understand the mechanisms behind Equine Cushing’s Disease we must first understand the hormonal regulation of cortisol as the excessive production of cortisol has many harmful effects on the horse. It is important to note that the hormone cortisol regulates body temperature, appetite, blood pressure, muscle tone and helps the body respond to stress. All these important processes are disturbed if cortisol levels are imbalanced (not within the normal range) and this is exactly what happens in horse’s with Cushing’s Disease as the tumour disturbs the delicate balance of cortisol levels.

How is cortisol produced? – Cortisol is produced through the communication of three glands including the hypothalamus, the pituitary and the adrenal glands. The pituitary gland is in close physical and functional association with the hypothalamus (region of the brain) and the adrenal glands are located near the horse’s kidneys. See figure 1. When the hypothalamus senses low cortisol levels, it sends a messenger (hormone CRT) to the pituitary gland to produce a messenger (hormone ACTH) which stimulates the adrenal glands to produce cortisol. When cortisol levels in the blood rise to normal the production of CRT and ACTH (messengers) stops, which in turn stops the production of cortisol. This regulation system maintains cortisol levels within a normal range. See figure 2. How does the tumour affect cortisol levels? – Cushing's in horses begins when cells (tumour) in the pituitary become overgrown, which results in an enlarged production of messengers, ACTH hormones, being sent to the adrenal glands, which increases the production of the stress hormone cortisol. Given the excess of cortisol, there is a hormonal imbalance which may lead to weight loss, weakened immune system, the typical long, curly coat, and so forth.

Diagnosis

A variety of tests are available to diagnose Cushing's in horses. These include a blood test to identify hyperglycaemia (high blood sugar levels) and a urinalysis to detect abnormally elevated levels of glucose and ketones in the urine. More specific hormone-related tests might be performed to confirm the diagnosis of Cushing’s disease following a repeat blood test to establish the consistency of high blood sugar. Two conditions that can cause misdiagnoses due to similar symptoms are hypothyroidism and insulin resistance..

Treating Cushing's in horses

There are three ways to treat Cushing's in horses once symptoms are present. All three can be used once a diagnosis is given and some measures can be taken to help prevent Cushing's.

Medical therapy - Treatment can be very successful if the symptoms are caught early, basically returning the horse to normal health for several years. Unlike in humans, removal of the tumour by surgery is not a possible treatment in horses since the pituitary gland is inaccessible. The most common treatment is hormonal replacement therapy, where a drug which works to reduce cortisol production is administered. The most common drug used today is Pergolide. It is important to note that treatment does not cure the tumour but controls its’ growth and treats the symptoms.

Management - Management is particularly important for horses with Cushing’s Disease due to the body’s inability to regulate many processes and the reduced immunity. This includes routine hoof care, deworming, vaccinations and dentistry. Horses with excessive hair growth should be clipped in the warmer months to increase comfort and better assess their body condition. Due to the lack of being able to control their own body temperature, it is important to provide a shelter and/or rug in the cooler months of the year.

Nutrition - Due to the predisposition of horses with Cushing’s to develop nutritional disorders such as laminitis and insulin resistance, nutritional management is very important. Affected horses may therefore have high blood sugar levels and a good quality diet that is low in starches and sugars should be fed to avoid obesity and exacerbating any of the above mentioned conditions. Cereal grains should be avoided and the starch and sugar content should be no more than 15% of the horse’s overall diet. The sugar and starch content of forages tends to be low in legume forages such as lucerne hay whilst high in cereal forages such as oaten and wheaten hays. Horses should not be put on lush pastures and should be turned out between 8 PM and 10 AM as sugar levels are likely to be at their lowest late at night through early morning. HYGAIN® ZERO® and HYGAIN® BALANCED® are low starch feeds that are great options for horses with Cushing’s Disease.