By Dr. Martin Adams
Horses, like people, are living longer. Finding ways to preserve their health and athletic function, and improve their quality of life is important. It requires managing and monitoring the horse's diet and being on the lookout for disease. In this issue, we will take a look at the causes of Cushing's disease, how to recognize it, and a diet recommended for horses with Cushing's disease and horses with metabolic syndrome and insulin resistance.
Equine Cushing's Disease
Cushing's disease, also known as PPID, pituitary pars intermedia dysfunction, is caused by a hormone-secreting tumor of the pituitary gland at the base of the horse's brain. In affected horses, the pituitary gland produces excessive amounts of pro-opiomelanocortin (POMC). This hormone is usually released at low levels, helps the body respond to short periods of physical, emotional or environmental stress. Sustained secretion of excessive POMC seemingly leads to the development of disease.
Cushing's Disease Symptoms
The most common symptom is a long, curly hair coat that fails to shed during the change from winter to summer. Other symptoms 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 twenty. Without treatment, symptoms tend to worsen over time and can be fatal. Symptoms are easily observed in advanced cases. 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.
Managing Cushing's Disease
The disease can be managed with a combination of medication and supportive care. This will be a life-long process as there is no way to reverse or cure this disease. In early stages, medication may not be required and measures such as body clipping to remove excessive hair coat, nutritional management, and attention to teeth, hooves and other preventive care may be sufficient to provide a good quality of life. There are two drugs used to treat Cushing's disease. The drug of choice is pergolide mesylate (Permax®) administered orally on a daily routine. Check with your veterinarian for appropriate treatment.
Feeding Horses with Cushing's Disease
These horses are often insulin resistant and have high blood sugar levels so non-structured carbohydrates (NSC) need to be avoided. Feeds low in soluble carbohydrates (sugar and starch or NSC) are recommended. Feeding recommendations are to provide a total diet with less than 20% NSC for most horses with Cushing's disease. Some horses and ponies may need a dietary NSC level of less than 10% to avoid excessive complications.
Horse feeds with a low NSC level include Triple Crown Low Starch (15.0% NSC), Triple Crown Lite (15.9%), Triple Crown Senior (15.7% NSC) and Triple Crown Safe Starch Forage (8.8% NSC). Most hays have NSC levels of 10% to 15% and can be fed along with an appropriate Triple Crown feed to maintain a dietary NSC content under 20%. For horses and ponies needing a dietary NSC level under 10%, Triple Crown Safe Starch Forage is a complete, chopped forage with mineral and vitamin supplementation that can be fed as the sole diet. Avoid small-grain hays and ryegrass, fescue and bromegrass as their NSC content can be 20% or greater. Overweight horses are also more likely to have laminitis and founder, so manage the horse's feeding program to prevent obesity and maintain a body condition score of no more than 5.0.
No More Sweet Grass
Pasture grasses can have a high NSC content, especially during the spring and fall seasons, and the risk of colic and laminitis is greater when horses are on pasture. Since laminitis and founder are more common in horses with Cushing's disease, pasture grazing should be severely limited or totally avoided. For more information go to www.equi-analytical.com and read the article "Carbohydrates in Equine Nutrition" and "Feeding the Horse with Cushing's Disease" at www.equussource.com.
In addition to a diet, there are nutritional supplements recommended for the management of equine Cushing's disease. Additional dietary magnesium and chromium may help reduce insulin resistance. Regular exercise reduces blood glucose levels in insulin-resistant people, so it should also help horses.
Martin W. Adams, Ph.D., is Nutritionist and Sales Manager, Horse Feed Sales, Southern States.