Understanding Cushing’s disease

This ailment can cause laminitis and infertility. Outward signs are coarse hair and excessive thirst. Kim Dyson explains.

Understanding Cushing’s disease
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Cushing’s disease (hyper-adrenocorticism) is often thought of as a disease that afflicts only older horses. However, it has been known to occur in horses as young as eight years old. A horse with Cushing’s disease can easily be recognised by its coarse, wavy coat. It can sometimes appear pot-bellied and show loss of muscle on the topline.

Another symptom is excessive thirst and urination. A healthy horse will drink between 22l and 36l/day; a horse suffering from Cushing’s disease will drink almost double that. Horses and ponies with Cushing’s are more susceptible to other diseases as their immune systems have been compromised. Some are more prone to laminitis, infections, and slow-healing wounds. Mares tend to be infertile and somewhat lethargic.

Cushing’s disease is caused by a tumour on the pituitary gland. The latter, at the base of the brain, regulates the endocrine system. As the tumour grows, it puts pressure on the hypothalamus, the portion of the brain regulating body temperature. This is thought to be the primary cause of the coarse, wavy hair coat. Biochemical alterations include increased endogenous cortisol, insulin resistance, an elevated adrenocorticotrophic hormone level and a decreased thyroid hormone level.

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A number of tests have been developed over the years to diagnose the disease. Treatment relies on a form of hormonal replacement therapy. The most effective drug is pergolide mesylate, a Parkinson’s disease medication that stimulates production of dopamine, which controls the affected part of the pituitary gland.

Horses are initially assessed every two to four weeks to ensure that the dose is strong enough to start hair shedding, muscle and weight gain and improvement in demeanour. The drug does not have any effect on the size of the tumour nor does it stop it from growing. Horses with mild Cushing’s disease may be returned to good health for a few years, but ultimately euthanasia is the kindest option.


Pergolide mesylate should be given at full dosage in summer and autumn and at half dosage in winter and spring. If your vet agrees, add 4g levothyroxine sodium every day. This will help maintain thyroid function. Affected horses are highly susceptible to large worm burdens, infections, weight loss and laminitis. They need regular deworming, teeth checks, and shoeing every six to eight weeks. Their vaccinations should be kept strictly up to date and their diet must be closely monitored.

Feeding Speedibeet can be beneficial as it helps the immune system. It also provides easily digested energy, unlike grain, which is incompletely digested in the small intestine and can cause laminitis if too much reaches the large intestine. Good quality lucerne chaff and hay, provided it is not too lush, can supply the protein required.

High quality supplements are also important. Food-source vitamin mineral supplements include apple cider vinegar, carrots and oranges. Essential fatty acids are needed to help make the cell wall more sensitive to insulin. Omega 3 fatty acids are especially deficient in many equine diets. Flax and hemp oil provide omega 3 fatty acids that are palatable to the equine, although fish oils may have an even better fatty acid profile.

It is important to stress that a horse suffering from Cushing’s disease can lead a contented and active life. There are reports of ponies returning to the show ring, being pain-free and successfully breeding again.