The ENDOCRINE disorder known as Cushing's disease - named after the American neurosurgeon Harvey Cushing - was something of a novelty in horses when I began working at Glasgow University's veterinary school in the late 1980s.

Then, we occasionally came across elderly ponies with a 'pot-belly' who had dense hair coats that failed to shed in the summer. But the clinical significance of Cushing's was not clear; it was difficult to diagnose in life and treatment was viewed as either unnecessary or ineffective.

Fast forward 25 years and we now see cases of equine Cushing's disease on an almost daily basis. It has become a prominent cause of illness in older horses and ponies, albeit one that is easily diagnosed and responds well to treatment.

A recent survey showed equine Cushing's disease affects more than 20% of animals aged over 15 years in the UK. And an American study found that there was a 12-fold increase in admissions for Cushing's to an equine hospital in the 12 year period from 1993 to 2004.

Whereas we once saw clinical signs of Cushing's as being of little consequence, we now recognise that it renders patients susceptible to bacterial infections, makes them infertile and, on rare occasions, can cause seizures.

The most serious problem is, however, laminitis. Surveys in the UK and Australia have shown that between 13% and 33% of horses and ponies with Cushing's disease have laminitis.

Its importance was also highlighted by the fact that mroe than 90% of animals presenting with laminitis to a Finnish equine hospital had either Cushing's disease or the related condition equine metabolic syndrome.

Equine Cushing's disease is caused by malfunctioning of the pituitary gland and specifically of a portion called the pars intermedia. It affects horses as well as ponies and, while it is most common in older animals, horses as young as seven can be affected.

Academics refer to the disease as pituitary pars intermedia dysfunction or PPID. This results in excessive production of adrenocorticotropic hormone (ACTH) that, in turn, stimulates the adrenal glands to secrete increases amounts of steroid hormones.

Early indicators of equine Cushing's include abnormal hair-shedding, often with an unusually curly coat, weight loss and wasting of top-line muscles giving a 'pot belly' appearance.

Bulging fat pads above the eyes are also recognised, although not as commonly as often suggested.

Other signs include lethargy, fading or changing of hair colour, excessive sweating, and increased drinking and urination.

Susceptibility to infection may be manifested by recurrent foot abscesses and troublesome skin conditions.

Laminitis can sometimes be unusual in that a single front foot or both back feet are affected instead of the two front feet as typically seen in pasture-associated laminitis.

Affects horses and ponies can be frustrating to treat, with recurrent or relapsing laminitis that does not respond to remedial farriery and standard pain killers/anti-inflammatories.

Measurement of blood concentrations of ACTH is the most speedy, cost-effective and accurate means of diagnosis equine Cushing's disease.

A single blood sample is sent to the laboratory on ice, with results usually available the next day, at a cost of around £30.00.

Concentrations of ACTH do naturally vary throughout the year - being highest in the autumn and lowest in the spring - and the laboratory takes this into account when reporting results.

Coexisting severe illness or pain can also increase ACTH levels and potentially complicate diagnosis.

These concerns aside, studies have shown that measuring ACTH is a highly sensitive and specific test for equine Cushing's disease and, in only a few cases, are other tests needed to confirm diagnosis.

It is not clear exactly how Cushing's disease triggers laminitis. One popular theory, with scientific support, is that high levels of cortisol cause resistance to the action of insulin. This is also believed to be the mechanism by which equine metabolic syndrome also causes laminitis.

Consequently it may be beneficial to measure blood insulin and glucose levels, ideally after giving a specific amount of sugar orally, in order to better assess the risk of laminitis in both equine Cushing's disease and metabolic syndrome.

Treatment of choice for equine Cushing's disease is the drug pergolide, which is licensed for horses and marketed in the UK as Prascend (Boehringer Ingleheim).

This drug was first developed to treat Parkinson's disease in humans.

Prascend comes as 1mg strength tablets - retailing at around £1 each - and horses typically start on one tablet once a day. This may be reduced to 0.5mg or half a tablet in smaller ponies.

The correlation between levels of ACTH and severity of clinical signs is not exact and so dosage is initially based on body weight rather than ACTH results.

Generic human forms of pergolide are also available but, under our medicines legislation, cannot be prescribed for horses. It used to be that generic pergolide was cheaper than Prascend but this is no longer the case.

Effectiveness of treatment is assessed after one to three months of treatment - by repeating measurement of ACTH - and pergolide dosage altered accordingly. Majority of cases respond within one month and around 85% have normal ACTH levels by three months.

Normalisation of ACTH levels is accompanied by improvement in one or more clinical signs, including livelier general attitude and demeanour, increased activity, shedding of hair coat, and a decrease in water consumption if intake was previously increased.

As laminitis improves there generally both resolution of pain and an end to previous episodes or relapses. Horses and ponies may then return to work and compete at the same level as prior to diagnosis of the condition.Testing should be repeated two to four times a year once ACTH levels are normal and clinical signs under control.

Affected horses and ponies will require life-long treatment with pergolide.

Occasionally it can cause a decrease in appetite or worsening of lethargy and inactivity. In these instances the dose should be reduced.

Other, unlicensed drugs have been used to treat equine Cushing's disease - notably bromocryptine and cyproheptadine - but without the success observed with pergolide.

It is striking how much our understanding of equine Cushing's disease has changed since I co-authored a paper describing 17 cases back in 1992.

While we now appreciate its clinical significance, advances in diagnosis and treatment fortunately mean that affected horses and ponies can continue to lead long and normal lives.