Horse Disease Focus - Cushing's Disease

Cushing's disease (which affects several animal species including humans) is a disease of the pituitary gland. It is associated with excessive levels of a hormone called cortisol. The cause is either a tumor on the gland or enlargement of the gland.

Cushing's disease is more common in ponies than horses and is often (but not always) a disease of old age. Most affected animals are over fifteen years old, with the average age at diagnosis being twenty. However, it does sometimes occur in younger animals.

Symptoms of Cushing's include sudden and unexplained laminitis that normally occurs in the fall. A classic symptom is an excessively long and often curly coat, with the coat not being shed properly in the spring. Some horses may not shed at all. (There are horses that are naturally curly, but those tend to shed out in the spring more than other horses, not less).

The bony topline and heavy belly that are usually seen as being signs of 'old age' are often symptoms of Cushing's. Other symptoms include lethargy, muscle wasting, excessive drinking and urination, excessive or insufficient sweating and a darkening and thickening of the skin. Horses with Cushing's are particularly susceptible to other infections and to parasites and may develop allergies they did not have before. Cushing's is also associated with infertility in both mares and stallions.

There is no cure for Cushing's disease.

The best current treatment is a drug called pergolide mesylate. Once the regime is established and a dosage finalized, symptoms are normally significantly reduced. However, the horse will have to remain on pergolide for the rest of its life. (Some horses only require it in the fall and winter). Pergolide is usually given using an oral syringe. Infertility may or may not be corrected (due to the age of most mares that get Cushing's, they are often simply retired as broodmares).

Trials are also in process using a herb called Vitex or chasteberry as an alternative to pergolide, with promising results. Another drug, trilostane, is used in the UK and also being investigated further.

All horses with Cushing's disease should be checked for insulin resistance, as the two conditions are sometimes associated.

Cushing's horses also require a special diet. The best diet is similar to that given to horses that are prone to laminitis. Grass hay and grass hay pellets are the best feed, with alfalfa and high energy grain contraindicated. Pellets are better for older horses that have dental problems. Rice bran is good to keep weight on. All treats should be avoided.

Most cases of Cushing's are associated with old age and cannot be readily prevented.

Cushing's in younger horses is disproportionately seen in show horses that have very active careers. This may indicate a connection with stress, but could also be a result of excessive feeding of high sugar grains to "pep a horse up." Feeding no more grain than is needed by that particular horse, therefore, is a good idea. It is also possible that it is linked to insufficient turnout.

In most cases, though, watching your older horse for possible symptoms is all you can do.