Although Cushing's disease in horses has been recognized for more than 70 years, the disease has often been misunderstood. However with improved diagnostic and treatment options, horses with Cushing's disease can enjoy a better quality life and live longer than previously thought possible.
Older horses - Prone to Cushing's disease
With improved diagnostic and treatment options, horses with Cushing's disease can enjoy a better quality life and live longer than previously thought possible.
Over the years, the disease has had a few name changes, with equine Cushing’s disease the most commonly accepted. But labels such as “equine ‘Cushing’s-like’ disease,” “equine Cushing’s syndrome,” “equine pituitary pars intermedia adenoma,” and the most currently accepted name in the veterinary community, “equine pars intermedia dysfunction,” have all been used to describe the condition.
Equine Cushing's disease is one of the most common diseases in older horses. It is a disorder of the pituitary gland that results in hormonal imbalances, causing a variety of clinical signs.
In Cushing's disease in horses, the pituitary gland produces excessive amounts of pro-opiomelanocortin (POMC) along with several hormonally-active derivations including ACTH, but the levels of cortisol are usually normal or below normal.
In most cases of the disease in equines, the pituitary gland is enlarged and overactive as a result of faulty regulation by the hypothalamus.
The disease tends to occur in middle-aged and geriatric horses, with an average age of approximately 20 years at the time of diagnosis. PPID has been diagnosed in horses as young as seven years and all breeds of horses can develop the disease. Ponies and Morgan horses have a higher incidence of the disease than other equines.
Without treatment, symptoms tend to worsen over time and many horses are euthanized as a consequence of laminitis, recurrent foot abscesses, or complications related to bacterial infections.
New research is leading to a lot of answered questions and development of new questions for this disease. It’s now known that specific nerve cells in the brain secrete dopamine. In normal horses these cells inhibit an overactive pituitary gland and are present in large numbers.
Horses with Cushing’s disease have dopamine-producing cells with decreased antioxidation capacity that are more susceptible to dying. But the question remains as to why. What is known is that fewer dopamine-producing cells means pituitary gland activity goes unchecked.
Cyproheptadine was the original drug of choice for treating equine Cushing’s disease, but many studies are now showing that its effectiveness is not nearly as good as pergolide. Some reports indicate that cyproheptadine used with pergolide is actually more effective.
One misconception about Cushing’s disease is that affected horses are hypothyroid and fat. They’re not. We don’t know enough about the thyroid gland and its hormones, but currently it’s believed that the thyroid does not play a direct role in equine Cushing’s disease.
In the past, many older horses that were “easy keepers” were diagnosed with Cushing’s disease and put on thyroid medication. Thyroid supplementation hasn’t been proven to help horses with Cushing’s disease. In fact, many horses that are true hypothyroid are not fat at all and are often thin.
In both mildly and severely affected horses, early diagnosis and aggressive treatment of bacterial infections is imperative. Special attention to the horse's feet may help prevent development of laminitis.
While Cushing's disease is not curable by treatments that are currently available, the horse's quality of life can be improved with careful management. Shaving the horse's thick coat when the weather is warm, using effective fly prevention, and keeping the horse groomed can help make the horse comfortable in spite of the disease.
Learn more about Cushing's Disease