Understanding Cushing's Disease

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You are here: Home / Articles / Healthcare / vet and healthcare

Jane Lindop MRCVS

2 September, 2005

New research shows that many more horses suffer from Cushing's Disease than was initially thought, although not all exhibit the classic symptoms of a long curly coat

A long, curly coat that fails to shed normally is the classical clinical sign of Equine Cushing's Disease (ECD). However, it is now thought that many more horses and ponies have the condition to some degree, even if they don't show the most obvious signs.

Recent research has shown that five out of 165 Thoroughbreds, aged 17-25 years, and three out of 100 Dutch Warmbloods, aged between 2-28 years, that showed no visible signs of the disease, were shown to have Cushing's at post-mortem.

All breeds and types may be affected, but ponies appear to be at greatest risk. Cushing's is usually seen in horses and ponies over the age of 15 years, and cases are often in their 20s or 30s. Mares and geldings are equally affected.

The signs of Cushing's are often vague and variable, and may easily be confused with other diseases or simply put down to "old age". Weight loss and lethargy are commonly observed, despite a normal or occasionally increased appetite. Loss of muscle mass, particularly over the saddle area and rump, is caused by protein breakdown.

Some horses and ponies may develop the appearance of a "pot belly" due to weakening and stretching of the abdominal muscles. Fat may be deposited along the crest of the neck, above the tail and above and behind the eyes. Sweating is a common sign, particularly in areas where the coat is long. Affected horses may drink and urinate more than usual, although this can be very hard to assess, especially in field-kept animals.

Affected horses may appear more docile and tolerant of pain than others. Recurrent infections (especially dental and respiratory) are common due to suppression of the immune system. In addition, wound healing may be delayed and simple things such as mouth ulcers can become a major problem. Sometimes, there are other potential clues, like the coat that looks as if it has been permed or otherwise inexplicable bouts of laminitis.

Sometimes the link is not recognised, but laminitis is probably the most serious complication of Cushing's and may be difficult to treat. In fact, more than 50% of horses with Cushing's are estimated to suffer from laminitis and associated foot abscesses. If your horse or pony keeps suffering from pus in the foot, it might be worth checking to see if there is an underlying cause.

Despite its name, ECD is different from Cushing's Disease in humans and dogs. In horses, there is a problem with part of the pituitary, a small gland at the base of the brain. One of the functions of the pituitary is to control production of natural steroids by the adrenal gland. Equine Cushing's is caused by either hyperplasia (enlargement) or adenoma formation (benign growth) in part of the pituitary gland.

For reasons that are currently unknown, aged horses and ponies affected by Cushing's have lowered production of dopamine, a neurotransmitter released within the brain. This results in increased levels of cortisol, the body's natural steroid, which is to blame for most of the problems associated with the disease.

Confirming Cushing's can be challenging and no test is 100% accurate. A diagnosis can often be made based on the history and the delayed shedding of a long, uneven coat, as well as other clinical signs. In less clear-cut cases, it may be very difficult to reach a diagnosis and your vet will be able to advise on the most suitable test.

Cushing's cannot be cured, but effective treatment can improve the horse or pony's quality of life.

Management tips

The most important aspect in the treatment of Cushing's is excellent general health care, combined with a variety of management changes to reduce the impact of clinical signs.
Horses with thick coats will sweat and become uncomfortable and distressed in warm weather and will benefit from regular clipping, particularly in the summer.
Dental care is very important to help prevent infections and maximise the absorption of nutrients from the diet. Dental problems leading to painful chewing and quidding will reduce appetite and further exacerbate weight loss.
Careful assessment of diet is necessary, and giving a pelleted feed designed for older or veteran horses may be helpful. But at the same time it is important to minimise the risk of laminitis: good professional advice may be needed to provide a decent balanced diet.
Regular use of a weigh tape is useful to monitor weight, and the diet should be altered accordingly.
Routine foot care is important to help prevent recurrent abscesses and laminitis.
Due to suppression of the immune system, prompt treatment of infection is recommended.
Vaccinations, regular worming and treatment for ectoparasites (lice, etc) are more important than usual and should be kept up to date.

If management measures are not enough to keep an affected horse or pony comfortable, there are medical treatments that can help, but they are expensive and must be given for the rest of the horse's life. All four different medications currently in use are used in man for other diseases, but can also benefit horses. They are:

Pergolide (increases dopamine production)
Bromocriptine (increases dopamine production)
Trilostane (inhibits steroid synthesis)
Cyproheptadine (inhibits ACTH production)

An improvement in the coat and a reduction in drinking and urination are usually seen within three weeks of starting medication. Treatment does not completely halt the progression of the disease but it can lessen the clinical signs and improve the animal's quality of life. Advanced cases are less likely to respond to treatment.

The disease cannot be cured and often develops slowly over a period of several years. Treatment may extend or improve the horse's quality of life, but medication is expensive and once started must be maintained. Many affected horses and ponies are able to live comfortably with good management for at least a couple of years. But ultimately, recurrent bouts of laminitis or infection are likely to necessitate euthanasia.

This veterinary feature was first published in Horse & Hound (11 August, '05)