The symptoms of Cushing’s disease can often be dismissed as signs of old age. Our love of horses, good management and modern medicine has led to an increasing population of older horses and ponies. And, as many horses live longer, equine Cushing’s disease is one condition now more commonly recognised.
Named after Harvey Cushing, an American brain surgeon, the most common cause of the disease is a benign tumour or over-activity of the pituitary gland at the base of the brain. This produces extra chemical messengers which confuse the rest of the body, particularly the adrenal glands. In rare cases, the same condition can be caused by a disease of the adrenal glands themselves and is called Cushing’s syndrome.
Cushing’s disease seems to be more common in aged ponies than horses, although there is no apparent breed predisposition. It can also affect donkeys.
Frequently, the early stages of Cushing’s are overlooked or blamed on old age. The signs are many and varied. The most classic is an excessive thirst (properly called polydipsia); affected animals will drink three or four times as much water as normal. Unfortunately, this can easily go unnoticed if the horse or pony is out at grass with a self-filling water trough.
Other indications include:
- excessive urination (known as polyuria)
- weight loss and muscle wastage, particularly loss of top line, i.e. a dipped back appearance
- sway backed or pot-bellied look
- abnormal curly, long coat
- coat colour may change and become paler
- persistent sweating
- increased appetite, but no obvious weight gain
- persistent infections often involving the skin, feet or respiratory system
- lethargy and dullness
- mares may produce milk, even if they have not had a foal.
Solving the clues:
If all these signs are shown, it will be obvious that your animal is affected. However, the changes are gradual, so that if you see your horse or pony every day, it is easy to think it is just becoming older, rather than developing any illness.
Many subtle signs can be related, including temperament or appearance changes, particularly around the eyes, which may have a “pop-eyed” appearance.
An obvious clue is abnormal hair growth. This shows up as a heavy, coarse, coat, which is often unusually curly as if the horse has had a perm. This thick coat may not be shed, even in summer. Affected ponies are often too hot and, because of this, they will often sweat for no obvious reason. As a result, the coat becomes damp and smelly and skin infections result. Many Cushing’s cases will feel much better if they are clipped.
Other cases may be more serious. If you have an old horse or pony with chronic foot problems, recurrent bouts of laminitis or foot abscesses, it may be worth considering Cushing’s as the underlying cause.
Cushing’s cases may also have reduced immunity and an increased chance of developing infections. Once they have an infection, it may take longer to clear up and they are more likely to need antibiotics for complaints that healthier horses can eliminate on their own. Wounds take longer to heal and simple things such as mouth ulcers can be a major problem.
If you are worried, speak to your vet, who may be able to diagnose Cushing’s from the clinical signs. Blood and urine tests are often required to confirm the diagnosis and rule out other problems, such as kidney disease or diabetes. Be vigilant as the most important thing is to be aware. You can control the disease as much as possible with good management measures, such as clipping, careful feeding and foot care. There is no cure. Surgery is not an option, particularly as many cases are more than 20 years old, but certain drugs can be used successfully to control the underlying pituitary gland problem.
The most commonly used drugs are compounds designed for treatment of other brain disorders in humans. Fortunately, they have been found to work on equine Cushing’s disease as well. They are given in the feed and act by modifying the messenger chemicals secreted from the abnormally functioning pituitary gland.
These drugs include pergolide, which currently seems to be the most effective, but costs the most. An alternative is cyproheptadine, which is effective in about 35% of cases. Although fairly expensive, these drugs can work well.
Once treatment is started, a rapid improvement is often detectable. In many cases, the age of the horse and simple economics may mean that drug treatment is not justifiable. Talk to your vet about what is best for your horse.
CASE HISTORY: Perseverance pays off
By Carole Mortimer
Treatment usually used for humans puts Aspen back on his feet. Aspen, a Thoroughbred/Irish Draught, was bought 23 years ago by art teacher Sue Williams and, for many years, they competed locally. As he grew older, Aspen became prone to slight periods of laminitis, but, in September 1996, he suffered a bout that Sue found particularly difficult to clear up.
Sue says: “Despite a carefully controlled diet, I presumed it was grass-induced.” In August the following year, Aspen again contracted laminitis, this time complicated by foot abscesses. “I started to read everything there was to read about laminitis,” she says, and again, through careful management, managed to overcome the problems.
Last year, Aspen, now aged 28, once again showed critical signs of the disease. “I almost began to believe it was time to let him go, but I was reluctant as he is a horse with such a strong personality that I knew he wasn’t ready,” says Sue.
Sue’s farrier, Mick Joliffe, suggested taking Aspen to Richard Jones at the Bell equine veterinary clinic. At the clinic, Cushing’s disease was suggested as the source of the problems. One of the vets looking after the case, Ingunn Risnes, says: “Cushing’s disease is more common in older ponies. Horses are not as susceptible, but Aspen was not a typical laminitis candidate, particularly as he was not overweight and he also had some difficulty shedding his coat, which is a common clinical sign with Cushing’s cases.”
Aspen had his feet X-rayed and blood tests were taken. Corrective farriery was decided on and heart-bar shoes were put on. Blood tests revealed Cushing’s disease and the vets decided to give Aspen pergolide – a “dopaminergic agonist” that works on the pituitary gland – more commonly used for humans, but selectively used on horses.
Ingunn says: “Cushing’s disease can cause varying clinical signs, from barely perceptible to serious illness. Careful management through correct farriery, diet and clipping come first in our approach. In a lot of cases, that is all that is necessary. Treating with drugs is expensive and we have to consider what is best for that particular case.”
A month later, Aspen was walking out and was more comfortable with careful foot support, good nursing and appropriate medication. However, unfortunately for Sue and her horse, the side-effects of the drug meant that Aspen’s appetite was suppressed for a few months.
“He ate his hard food, but was picky with hay. This, plus management to prevent any recurrence of laminitis, contrived to make him a little thin,” says Sue.
A year later, Aspen is able to be hacked out happily and is still fond of a gallop round his field when allowed. “The drug is expensive, but I wanted to keep Aspen, as he seemed so full of life, and he still enjoys a good shy while hacking,” says Sue. This year, she hopes to forestall any recurrence of the problem, and she, the vets and her farrier will continue to watch Aspen carefully.