The role of nutritional therapy in the treatment of Equine Cushing's syndrome and laminitis - Equine Cushing's/Laminitis

Alternative Medicine Review, Sept, 2001 by Joyce Harman, Madalyn Ward

What causes Cushing's to occur is not known. Is it just a hyperplasia of the pars intermedia, or is it from loss of dopaminergic control by the hypothalamus? If the former, what is causing the hyperplasia? One theory is that chronic stress affects dopamine secretion by the hypothalamus, causing a loss of control of the intermediate lobe and leading to the development of hyperplasia. (7) The answers to these questions have yet to be determined.

Clinical Signs of Cushing's Syndrome

The most important diagnostic tools for identifying Cushing's as the underlying cause of laminitis are the history and clinical signs. The history needs to be both complete and extensive. A thorough physical examination may reveal some of the less obvious signs, such as poor teeth and reproductive problems. The clinical signs most commonly associated with equine Cushing's syndrome are hirsutism (long hair that does not shed in the summer), (9) refractory laminitis, weight problems (over- or underweight), lethargy/poor performance, polyuria/polydipsia (Pu/Pd), and hyperhydrosis (Table 1). (10) Some horses will exhibit numerous symptoms while others will have very few.

One of the most serious complications of Cushing's syndrome is laminitis, often with no apparent cause derivable from either the history or the examination. (9,11) Commonly, some of the most refractory cases of Cushing's-based laminitis occur in the winter, an otherwise uncommon season for typical cases of laminitis. Even the more common summer laminitis, which would appear to be caused by overeating of grass, can be quite refractory to treatment, especially when other clinical signs of Cushing's are present.

Many horses diagnosed with Cushing's are overweight and are very easy keepers, sometimes unable to eat more than a small amount of hay each day because they gain weight so rapidly. In some cases, a horse that was a previously easy keeper suddenly starts requiring more food to maintain body weight. The overweight horse generally has a cresty neck and fat pads in specific places. (2,9) The fat pads are generally behind the shoulder blades, on each side of the tail, and in the lumbar area. In addition, the fat on its body is often visibly lumpy. Some horses will maintain their fat pads despite obvious weight loss.

Diabetes accompanied by Pu/Pd (9,12) is observed occasionally. (2,5) Pu/Pd can also be caused by compression of the pars nervosa by the enlarging pars intermedia, resulting in a decrease in antidiuretic hormone production. (5)

Frequent infections of the skin or other organs occur, (2,6) probably due to hypercortisolemia and hyperglycemia. (10-12) It is well known that increased levels of cortisol are immunosuppressive, and that diabetics are prone to infections due to the high levels of sugar in their blood.

A sluggish thyroid gland or thyroid dysfunction is common in horses; yet it has been difficult to associate clinical signs with laboratory findings. (13) Some of the conditions previously attributed to thyroid problems, such as muscle soreness, are also part of Cushing's syndrome. (15)