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Role of Diet in Cushing’s in Horses

Cushing’s is a disease of the endocrine system. It occurs in horses, dogs and humans and is caused by an abnormal growth of the pituitary gland, which is located at the base of the brain. This causes the adrenal gland to produce excessive amounts of the stress hormone cortisol. Cortisol is required to regulate blood pressure and cardiovascular function as well as control the body's use of proteins, carbohydrates, and fats. The continuous and excessive production of cortisol is harmful.  Cushing’s Syndrome was considered a disease associated with old age and appears in mainly geriatric and senile horses.  However it is now appearing in teenage horses and ponies.

Symptoms of Cushing’s.
The effects are usually seen as a pot bellied appearance, sway back, shaggy coat and lameness.  Another visible sign is the abnormal distribution of fat, with a cresty neck and fat deposits over the back and tailhead. Additional symptoms include increased appetite, thirst and urination, chronic laminitis and lethargy.  Cushing’s is a lifelong disease.

Complications of Cushing’s. 

  1. Immunosuppression. Simply suppression of the immune system that could cause your horse to be prone to other diseases especially skin diseases like rain scald.
  2. Insulin resistance. Cushings and Insulin Resistance (IR) are often confused. Many Cushing’s horses can also be IR, however not all Cushing’s horses are IR, and not all IR horses have Cushing’s.
  3. Laminitis. High levels of circulating blood insulin caused by IR can cause laminitis, which may explain why horses affected by Cushing’s disease are especially prone to laminitis.

What can you do?   Cortisol is a stress hormone, and you should endeavour to reduce as many stress related factors as possible. 

Two drugs, Pergolide and Cyproheptadine are used routinely to treat Equine Cushing’s Disease.  These treatment options should be discussed with your veterinarian.  Each treatment is lifelong.  There is no quick fix.

General Management
Establish a management program that will help to reduce stress.  This can include selecting feeds that don’t cause IR, reduce stress from other horses, clip the horse in warm weather and using rugs when cold.  Reducing stress and good husbandry are important. There is no single factor causing Cushings. 

Dietary Management
The Cushing’s horses may lose some of their ability to control the levels of glucose and insulin in their blood and are susceptible to laminitis. Therefore high NSC (sugar/starch) feeds that cause a significant increase in blood glucose and insulin concentrations post-feeding; and grain based feeds that cause hindgut disturbances that can lead to laminitis, should be avoided.

Feeding low NSC (<12%) feeds is recommended to avoid exacerbating the effects of glucose overload on insulin resistance, and laminitis. Lush pastures should be avoided. Pulse feeding should also be avoided, as it can cause insulin and glucose spikes. Feeding roughage in hay nets is one way of preventing pulse feeding. Select feeds that do not cause insulin and glucose spikes (please refer to previous article on Slow Foods for Horses).

The feeding of fats and high fat feeds must be carefully managed. Select oils that are stable, do not go rancid, are not modified seed oils, and are readily metabolised (see last month’s article on Understanding Oils). Oils can be a valuable addition to the diet of any Cushing’s horse as they provide a supply of energy free of  NSCs. Oils containing MCT are readily digested, and provide antimicrobial effects that promote gut health.

Does diet cause Cushing’s.
The horse as we know it has been around for over 3 million years.  Why is Cushing’s now such a recent disease in horses?  Horses are living longer, and so understandably Cushing’s as a disease of ageing is expected.  However, we should be concerned at the appearance of Cushing’s in younger horses.  The horse has not had enough evolutionary time to suddenly develop Cushing’s, so something must have changed.  This series of articles highlights the metabolic chaos being caused in our horses by feeding feeds they were not designed to eat.  High NSC feeds can cause Insulin Resistance, which in turn causes elevated levels of the stress hormone cortisol.  Not all Cushing’s horses are IR and not all IR horses have Cushing’s, however there are many horses suffering from both.

Food for Thought.
Given the major metabolic effects that high NSC diets have on glucose and insulin production in some horses, it could be possible that high NSC diets cause some horses to become IR, increase the stress hormone cortisol, and in turn predispose them to Cushing’s later in life by damaging the pituitary. Yes this is a hypothesis, however given the metabolic chaos being caused in some horses by feeding high levels of NSC, we can’t discount the possibility that high NSC diets early in life may be predisposing some of our horses to Cushing’s.  We choose what our horses are eating...maybe we are making the wrong choice.