As summer sets in most horses have shed their winter coats. But some older horses are still clinging to their winter coat. Some have not shed it at all.
Failure to shed the winter coat is a good indication that a horse has Cushing’s disease.
Cushing’s disease is being recognised more and more frequently in older horses and ponies. The condition is named because of its similarities to the human disease of the same name. But there are significant differences.
Equine Cushing’s disease is associated with excessive cortisol production by the adrenal gland. In almost all cases, it is caused by increased activity in the intermediate lobe of the pituitary gland, which sits at the base of the brain. The underlying problem seems to lie with nerves that should limit the activity of the affected part of the gland.
In normal horses, ACTH, a hormone from the pituitary gland, stimulates the release of cortisol from the adrenal glands. The blood cortisol level is controlled by a complex “feed back” mechanism. Basically, an increase in blood cortisol inhibits further ACTH release, which in turn causes the cortisol level to fall.
In horses with Cushing’s disease the abnormal pituitary gland produces ACTH, and other related molecules, some of which increase the “potency” of ACTH. In addition, the abnormal gland does not respond to the normal feedback controls. Most of the signs seen with Cushing’s disease are due to the increased cortisol activity.
Often the first sign that a horse is affected with Cushing’s disease is hirsutism, the development of an excessively long and often curly coat. Shedding of the coat that normally occurs in summer either does not occur at all or is greatly reduced.
Other signs include excessive sweating, weight loss, poor performance, increased drinking and passing increased amounts of urine. Almost all affected animals go on to develop laminitis eventually. Because the laminitis in these cases is the result of internal problems rather than a momentary dietary indiscretion, it is often more difficult to treat than laminitis due to dietary causes.
Various tests have been used to help to confirm the diagnosis. Unfortunately the tests can sometimes be misleading. In most cases they are not necessary. There is really only one condition that causes older horses not to shed their hair in summer – and that is Cushing’s disease.
So, what can be done for horses with Cushing’s disease? Vets have used a variety of medications, originally intended for human use, to treat Cushing’s disease in horses. The most effective seems to be a drug called pergolide. If the treatment is successful, as well as showing a general improvement in condition, a horse may start to shed its coat again.
Some people have found that herbal mixtures containing chaste berry extracts have helped, but others disagree.
Clipping the excessive coat will make the horse more comfortable, and may reduce the risk of skin infections. Regular corrective trimming of the feet is likely to be necessary once laminitis develops.
The onset of Cushing’s disease need not necessarily mean a horse’s days are numbered. But once you spot the telltale signs have a word with your vet to discuss the options that are available for managing the condition.