CUSHING’S disease, more correctly referred to as pituitary pars intermedia dysfunction (PPID), is the most common hormonal condition affecting horses, particularly, but not exclusively, older animals.

It results from excessive activity of a part of the pituitary gland, a structure located at the base of the brain. This gland secretes a hormone called ACTH which in affected horses results in excessive production of cortisol and other hormones. These are the substances that cause the signs of the disease we see.

Signs of Cushing’s Disease

The signs of this illness are usually slow and gradual in their onset. The classic signs of a long shaggy coat and laminitis are common but not seen in every case.

Other indications that a horse may be suffering from Cushing’s disease include a tendency to drink more, and therefore urinate more, abnormal fat deposition that is often most noticeable above the eyes, loss of muscle, a weakened immune system resulting in an increased likelihood of infection, and a general lethargy.

Of all these signs, laminitis is the one with the greatest potential to cause problems for the horse. This can vary from a mild discomfort that will resolve with medical treatment, to severe and debilitating disease with the horse unable to walk and, in the most severe cases, can result in euthanasia.

Diagnosis

A diagnosis of Cushing’s disease can be made on clinical signs alone. Blood tests can confirm the condition. Several blood tests can be used but the most common is measurement of ACTH. As discussed above the signs of Cushing’s disease are a result of overproduction of this hormone. By measuring blood levels and comparing them with that of horses without the disease the diagnosis can be made. This test is most sensitive in the autumn.

Treatment

Cushing’s disease is a condition that requires lifelong treatment as there is no known way of resolving the excessive activity of the pituitary gland. The disease is therefore managed by reducing the production of ACTH. The treatment of choice is a drug called pergolide.

This drug mimics the effect of the neurotransmitter dopamine which results in decreased ACTH production. Pergolide, often known by the trade name Prascend, comes in the form of a small tablet which is given orally, usually once a day. The dose is fine tuned by repeating blood tests to measure ACTH levels and monitoring clinical signs displayed by the horse. An improvement in clinical signs typically takes around 2 months to be seen, although this varies from case to case.

Pergolide treatment is generally reasonably successful, resulting in resolution of the clinical signs with side effects thankfully very rare. The greatest obstacle to treatment is usually getting the horse to eat the tablet, though this can normally be overcome by feeding it in a small treat.

Management changes can also be of great benefit to horses with Cushing’s disease. Weight loss, dietary modification and an altered exercise regime can all help as can the prompt treatment of signs of infection.

Various dietary supplements have been proposed to help with the management of equine Cushing’s disease, including chasteberry, magnesium and vanadium. At this time there is no published data to back the use of these, although one study has found that chromium supplementation was not beneficial.

Prognosis

In the majority of cases, prompt diagnosis and appropriate medication will allow horses to continue for many years with no ill-effects from this illness. In cases that have suffered recurrent laminitis the prognosis is more guarded although treatment can still be of great benefit.

In conclusion, equine Cushing’s disease is common and can have devastating effects, but with the appropriate treatment is very manageable.