Cushing’s disease: diagnosis, treatment and prognosis *H&H VIP*

Ten years ago a mention of equine Cushing’s disease would draw blank looks from the majority of owners. The condition is well known today, but to many it still conjures up an image of an ancient and shaggy-coated pony. In fact, Cushing’s is equally common in horses and ponies and is considerably more widespread than many owners realise.

Any breed, from Shire to Shetland, can be affected, and it isn’t limited to the very old, either. Up to 30% of horses over the age of 15 have the condition, yet it can occur in horses of 10 or even younger. Vets now recognise that Cushing’s disease is less an illness and more of an age-related deterioration in a part of the brain, a fact that means that no horse is immune. The age of onset and the severity may vary, but if your horse lives long enough he will eventually get Cushing’s.

Subtle clues

The classic signs of Cushing’s include a long, curly coat that isn’t properly shed; a dipped back and pot belly; sweating; fat bulges above the eyes and increased drinking.

But the one sign that is the most significant is laminitis. Cushing’s disease greatly increases the laminitis risk, and can frequently be the underlying cause of an attack in an animal who is not showing any other signs of the condition.

For this reason, any laminitic episode in an older horse or pony should alert you to the need to test. Some horses with Cushing’s may show very little except a general lack of energy and enthusiasm.

Testing for Cushing’s is not difficult — a blood test is taken and the sample is then separated and chilled for sending to the laboratory.

The difference between normal and abnormal levels of adrenocorticotropic hormone (ACTH), which is measured, varies during the year. It is greatest between August and October, making this the most accurate time of the year for testing.

A positive result will confirm Cushing’s. Occasionally, however, the result may be normal yet a strong suspicion of Cushing’s remains. In these cases a second, more sensitive test can be carried out, which involves injecting a small amount of a hormone and taking blood samples before and after.

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Effective — but pricey

While no owner wants their horse to test positive for Cushing’s, the good news for animals that do is that there is a simple and effective treatment.

A drug called pergolide — marketed under the brand name Prascend — is given daily in tablet form. Improvement is usually seen within a few weeks; as hormone levels begin to return to normal, any curly coat is shed, laminitis resolves and energy levels increase. The premature ageing triggered by Cushing’s goes into reverse and the horse may start to look and behave as if he is several years younger.

After a month on treatment the blood test should be repeated to check that the levels of ACTH are back under control. The horse is then retested approximately every six months — your vet will advise depending on the initial levels and the degree of clinical signs.

The only significant side effect of the treatment is a temporary loss of appetite by some horses. This usually returns in a few days, but the initial dose may need to be halved.

A simple, once-a-day treatment that really works? Nothing with horses is ever that easy, so there has to be a catch somewhere.

In this case, it is the cost. Each tablet will set you back just over £1 — not a huge sum, perhaps, until you realise that a horse with Cushing’s will be on treatment for the rest of his life and that the dose usually needs to increase over time. Ultimately horses can be on three or more tablets a day.

Vets are extremely keen to see cheaper pergolide made available, because at present some horses and ponies are put to sleep by conscientious owners who cannot afford the ongoing cost of treatment.

The manufacturers of Prascend do offer free tests at certain times of year, but this is only for new cases. Herbal remedies for Cushing’s, such as chasteberry, have not shown any lowering of hormone levels, so save your money for the proper treatment.

When to treat?

Tests can sometimes reveal levels of ACTH that are more than 10 times the normal. Clearly, these cases need immediate treatment. However, Cushing’s is a gradual and progressive condition, so horses in the early stages of developing the disease may show just a small rise in levels of the hormone.

Where there are no significant clinical signs it may be decided — given the cost of pergolide — that treatment does not need to commence straight away. Such cases should be monitored and retested, and it is important for owners to be aware of the increased laminitis risk and manage them accordingly. Cushing’s also suppresses the immune system, so untreated horses are more prone to infections and to worms.

Each case needs to be judged as an individual. As a rough guide, however, treatment is advisable if ACTH levels are up by more than a third.

The important thing for owners to remember is that while Cushing’s disease can be controlled, it cannot be cured. Nonetheless, with treatment and careful management, affected horses can continue to enjoy a good quality of life for many years.

Ref: Horse & Hound; 3 December 2015

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