What Is Equine Cushing's Disease?

Cushing's disease, also known as PPID (pituitary pars intermedia dysfunction), is a progressive disorder that begins with the dysfunction of the pituitary gland. This gland at the base of the brain sends out a number of hormones, and a horse with Cushing’s disease starts to send out more hormones, causing a number of clinical signs.

What Are the Signs?

The most common clinical sign is a long, curly hair coat that fails to shed during the change from winter to summer. Other signs include: excessive sweating, lethargy, poor athletic performance, infertility, muscle wasting (especially along the top line), abnormal fat distribution (accumulations in the crest of the neck, along the tail head, sheath, and above the eyes), delayed wound healing, increased susceptibility to infections, and increased water consumption with passage of large amounts of urine.

Cushing's tends to occur in middle-aged and older horses, around age 20. Without treatment, signs tend to worsen over time and can be fatal. Clinical signs are easily observed in advanced cases.

What Are the Causes?

Cushing's disease is caused by a hormone-secreting tumor on the pituitary gland at the base of the horse's brain. The tumor causes an over-production of hormones, resulting in the observed clinical signs.

Diagnosis

Diagnosis of early cases or those characterized by few obvious clinical signs can be more difficult. There are two clinical tests available: 1) dexamethasone suppression test, and 2) plasma ACTH measurement test. Consult your veterinarian for the appropriate tests if you suspect your horse has Cushing’s disease.

Feeding and Management

These horses are often insulin resistant and have high blood sugar levels so non-structural carbohydrates (NSC, sugar and starch) need to be minimized. Feeding recommendations are to provide a total diet with less than 15% sugar and starch for most horses with Cushing's disease. Pasture grasses can have high sugar content, especially during the spring and fall seasons. Since laminitis and founder are more common in horses with Cushing's disease, pasture grazing should be severely limited or totally avoided. Regular exercise reduces blood glucose levels, so it will help horses with Cushing’s disease.

This article was written by Dr. Tania Cubitt & Dr. Stephen Duren, Performance Horse Nutrition.