Cushing’s disease is increasingly being recognized as a cause of ill health in horses, ponies and donkeys. It is one of the commonest diseases of older animals.
Named after Harvey Cushing, an American neurosurgeon who first described the condition in humans in 1912, Cushing’s disease results from excessive production of the hormone cortisol.
In horses, this is usually caused by an over-active pituitary gland, which stimulates cortisol synthesis through the chemical messenger adrenocorticotrophic hormone (abbreviated to ACTH).
Signs of Cushing’s commonly include an abnormally long, heavy and curly hair coat, laminitis, excessive drinking and urination, and weight loss. In the early stages these signs may simply be blamed on old age, especially when they are gradual in onset.
One of the earliest indicators of the coat changes – often referred to as hirsutism – is that a horse, pony or donkey fails to shed its winter coat. The coat can also change color, becoming paler, and there may be persistent, patchy sweating resulting in a damp and smelly coat.
The weight loss associated with Cushing’s is the result of muscle wastage, especially along the top line, and this can lead to a dipped back with a pot-bellied appearance. Affected animals usually continue to have good appetites, and owners often attribute the change in body condition to old age.
One of the effects of cortisol is to block the action of the hormone insulin and this can cause what is called secondary diabetes, resulting in excessive urination (polyuria) and increased thirst (polydipsia). This may not be obvious in horses and ponies at grass, as well as those stabled with automatic water drinkers. An unusually wet bed, requiring frequent replacement, may indicate polyuria.
Laminitis is a second consequence of the resistance to insulin induced by high cortisol levels, and this is arguably the most problematic of the Cushing’s symptoms. This is because the laminitis is often unusual in both its manifestation and response to treatment.
The laminitis might worsen despite box rest, remedial farriery and avoidance of grass and high starch feeds, or there may be little or no response to standard pain-killers and anti-inflammatories.
High cortisol levels also have the effect of suppressing the immune system, and animals with Cushing’s can suffer from persistent or recurrent infections of the skin and respiratory tract, conjunctivitis and foot abscesses.
Wounds can take much than normal longer to heal. Affected animals typically require antibiotics in instances where healthy horses would naturally eliminate the infection, or need much longer treatment with more powerful antibiotics.
A laboratory test that measures blood levels of ACTH – the chemical that is over-produced by the pituitary gland and is consistently elevated in horses, ponies and donkeys with Cushing’s disease can help in diagnosing the disease.
This test is simple, requiring a single blood sample, economical and provides an accurate diagnosis. Care has to be taken to send the sample on ice, by next day delivery, since ACTH is relatively unstable at room temperature.
The preferred treatment is the drug pergolide, which inhibits the production of ACTH by the pituitary gland and is usually effective in returning ACTH levels to those seen in normal horses and ponies.
In many cases, pergolide stabilizes or improves the laminitis, so that previously unrelenting pain is controllable with standard painkillers or there is no need for anti-inflammatories and the animal is protected against further episodes of laminitis.
Treatment may also help in animals with recurrent infections but will not necessarily reverse some of the other signs of Cushing’s, including hirsutism and muscle wastage. An alternative to pergolide is cyproheptadine, but the results of clinical trials suggest that it is less effective.
Several herbal feed supplements claim to be helpful in treating equine Cushing’s disease. For the most part, these claims made for such products have not proven in related studies, so they should be viewed with caution.