Western Counties Equine Clinic Ltd

Client Fact sheet – Cushing’s disease.

What is Cushing’s disease?

Cushing’s disease in horses is a syndrome of signs caused by over activity of part of the pituitary gland in the brain. The pituitary gland is responsible for many of the automatic functions of the body including cortisol and insulin production. In some horses this may be due to a benign brain tumour but these are very slow growing and very rarely cause problems except the “Cushing’s” symptoms.

Some experts consider Cushing’s in horses is likely to be a normal aging process. It is certainly not the case that all cases will be fatal or even problematic, in fact many horses may show no worse signs then coat changes. Laminitis is the only commonly seen medical problem seen associated with Cushing’s disease and the most likely reason this disease requires treatment.

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What are the signs of Cushing’s disease?

Cushing’s disease is primarily a disease of older horses and is rare in horses under the age of 15years old.

The signs of Cushing’s vary and not every affected horse will have all of them but often include:

Hirsutism: changes in hair growth – this can vary from lack of shedding of winter coat to growth of a thick woolly coat.

Laminitis : an older horse suddenly developing laminitis when it has never had it before can be an indication of Cushing’s disease. Unfortunately laminitis that is not responsive to treatment or recurs is often the main problem caused by Cushing’s disease.

Increased Drinking : This is often seen with Cushing’s disease but may not be obvious, especially with horses that are turned out. Many people describe noticing that their horse’s bed is wetter before noticing their horse is drinking more.

Lethargy: Horses with Cushing’s disease often seem to have less energy but as they are often older this can go unnoticed

Fat Redistribution : Often a generally potbellied appearance with loss of top line. Also affected horses often show a bulging in the fat pads above the eyes.

Excessive Sweating : especially seen in response to pain (i.e. laminitis) or following sedation

How can we test for Cushing’s disease?

The most usual test we use is the dexamethasone suppression test, which involves two blood tests around 20 hours apart. However as this test involves giving the horse an injection of steroids (which has been associated with causing laminitis) we may decide not to use this test in a horse already suffering from laminitis.

In these cases we may perform an ACTH stimulation test or a simple blood test for serum insulin, steroids and glucose which while not diagnostic can be very suggestive.

Unfortunately no test for Cushing’s disease is 100% accurate and false positives are recognised especially in the autumn months.

How can we treat Cushing’s disease?

There are three main treatment options:

1)  No drugs! – Many horses may show no signs other then drinking more or having a hairy coat and these cases they can be managed without any medication at all. In many cases the only appropriate treatment required may be providing adequate supplies of water and clipping.

2)  Pergolide - this drug helps to regulate the function of the abnormal pituitary gland and can be effective against the signs of Cushing’s disease. However the dose rate required and effectiveness varies between individual horses.

3) Trilostane – this drug affects the production of steroid from the adrenal gland