Equine Cushing’s disease, scientifically referred to as pituitary pars intermedia dysfunction (PPID), is typically an age-related neurodegenerative disorder. Many horses will develop Cushing’s as they get older, but the symptoms come on so gradually, the disease often goes unrecognized. Usually around the age of 15 the risk becomes greater; however, younger and younger horses are developing PPID, which is a disturbing trend.
PPID is a progressive disorder, meaning it gets progressively worse over time. Cushing’s is not fatal in and of itself, but rather from the conditions it can lead to if left untreated, such as infections, muscle wasting, colic, and of course, laminitis. There is no cure, only treatment. However, by reducing oxidative stress, it is possible to slow down its progression.
The pituitary gland is at the forefront of the disorder.
This gland is suspended from the hypothalamus at the base of the brain. One of three lobes in the gland, the pars intermedia is responsible for equine Cushing’s disease; it produces adrenocorticotropic hormone (ACTH), the regulation of which is at the heart of the Cushing’s problem itself.
When a healthy, non-cushingoid horse experiences some type of stress (e.g., related to intense exercise, pain, or an empty stomach), the hypothalamus releases corticotropin releasing hormone (CRH). CRH signals the pituitary gland (pars intermedia) to release ACTH, which then stimulates the adrenal gland to secrete the stress-hormone known as cortisol. Cortisol is involved in releasing glucose out of glycogen stores to provide energy.
To bring cortisol levels back to normal, cortisol will stimulate certain neurons in the hypothalamus to produce the neurotransmitter, dopamine. Dopamine, in turn, tells the pituitary gland to stop secreting ACTH, which then causes cortisol production from the adrenal gland to subside.
However, with oxidative stress over a period of time, those dopamine-releasing neurons become fewer and fewer in number. The hypothalamus is no longer able to produce enough dopamine; without adequate dopamine to signal it to stop, the pituitary gland continues to pump out ACTH. An increase in ACTH then leads to an increase in cortisol levels, which in turn causes the pituitary gland to hypertrophy or enlarge.
What is oxidative stress?
Elevated insulin, excess body fat, mental and physical stress, as well as environmental toxins can all lead to oxidative stress, resulting in the creation of a large number of proinflammatory molecules known as free radicals. These free radicals are volatile molecules that can destroy every kind of tissue. In the specific case of Cushing’s, progressively higher levels of oxidative stress damage the dopamine-releasing neurons until they can no longer function properly. Instead of producing dopamine (that would turn off the pituitary gland’s secretion of ACTH), dopamine secretion is diminished. Consequently ACTH secretion increases, telling the adrenal gland to produce more cortisol. And then the vicious cycle continues: Cortisol pulls glucose out of glycogen stores as well as breaks down muscle for amino acids (which also get converted to glucose). Elevated glucose stimulates the pancreas to release insulin. Insulin promotes body fat storage, which can lead to insulin resistance. Excess body fat releases inflammatory molecules, leading to free radical formation, damaging the dopamine-releasing neurons. Over time, this condition progressively worsens.
The key to slowing down this progression is reducing oxidative stress.
While most cases of PPID eventually require pharmaceutical treatment with pergolide, or its trademarked version Prascend, other management approaches should always be implemented as a first line of defense. Reducing oxidative stress can slow down the destruction of dopamine-promoting neurons and therefore diminish the progression of Cushing’s disease. This can be accomplished by reducing inflammation-producing body fat (if the horse is overweight), modification of mental, physical, and environmental stressors, and feeding an anti-inflammatory diet.
Restricting forage is incredibly stressful. And yet, this is the way many horse owners attempt to help their horses lose weight. The cascading negative effects of stress exacerbate this endocrine disorder in an already compromised horse into one with a higher risk for laminitis. Please read the article, “Bring Back the Horse’s Instincts” for more information, as well as books in the Spotlight on Equine Nutrition Series for a detailed, easy-to-do approach to weight loss through free-choice forage feeding.
Don’t neglect exercise. It not only burns calories, but exercise makes cells more receptive to insulin, allowing the horse’s body to burn fat. The blood insulin level declines, thereby reducing inflammation and the risk of laminitis. Exercise also helps protect muscle mass (which the cushingoid horse is losing). Finally, it makes your horse more sensitive (less resistant) to leptin, a hormone that is supposed to tell your horse to stop eating.
Decreasing inflammation through the diet.
You’re probably already paying close attention to the non-structural carbohydrates (NSC) in the diet. That’s important to help reduce insulin secretion. But, here are a few pointers to reduce inflammation:
- Antioxidants neutralize damaging free radicals that can diminish dopamine-releasing neurons. Some examples include vitamins E and C, as well as lipoic acid, grapeseed extract, boswellia, and curcumin.
- Omega 3s reduce inflammation whereas too many omega 6 fatty acids increase it. Ground flaxseeds or Chia seeds are particularly high in the essential omega 3 fatty acid, alpha linolenic acid (ALA). Fish oils contain omega 3s as well, and are worthwhile in extreme inflammatory states.
- Protein quality makes a difference in the horse’s ability to produce and repair tissue as well as maintain healthy endocrine and immune systems. To achieve this, a variety of complementary protein sources must be included in the diet. Strive toward feeding several types of grasses and consider adding some whole foods such as ground flaxseeds, split peas, copra meal, whey protein isolate, hemp seeds, and Chia seeds.
The Bottom Line
PPID becomes progressively worse over time as dopamine-releasing neurons in the brain are damaged by oxidative stress. Through attention to damaging aspects of your horse’s lifestyle, diet, and environment, this neurodegenerative disease can be significantly slowed down.
Helpful resources on PPID—Equine Cushing’s Disease
1) Spotlight on Equine Nutrition Series: http://gettyequinenutrition.biz/TeleSeminars/TeleseminarBooks/SpotlightonEquineNutritionTeleseminarSeries.htm
2) Free-Choice Forage Feeding – Beyond the Basics: http://gettyequinenutrition.biz/library/freechoiceforagefeedingbeyondthebasics.htm
3) Bring Back the Horse’s Instincts: http://gettyequinenutrition.biz/library/bringbackthehorsesinstincts.htm
4) Restricting Forage is Incredibly Stressful: http://gettyequinenutrition.biz/library/restrictingforageisincrediblystressful.htm
5) Eating! Help Insulin Resistance become Insulin Sensitivity: http://gettyequinenutrition.biz/library/eatinghelpirbecomeis.htm
6) Supplements that are helpful for cushingoid horses can be found at: http://horsesupplements.gettyequinenutrition.biz/supplementsinsulinresistanceandcushings.html
Juliet M. Getty, PhD, is an internationally respected, independent equine nutritionist who believes that optimizing horse health comes from understanding how the horse’s physiology and instincts determine the correct feeding and nutrition practices. Buy Getty’s comprehensive resource book Feed Your Horse Like a Horse at her website www.gettyequinenutrition.com, and have it inscribed by the author. Or buy it at Amazon (www.Amazon.com), Barnes and Noble (www.barnesandnoble.com) or Books A Million (www.booksamillion.com). The seven separate volumes in Getty’s topic-centered Spotlight on Equine Nutrition series are available at her website, where Getty offers special package pricing, and also at Amazon in print and Kindle versions.
Getty provides a world of useful information for the horseperson at www.gettyequinenutrition.com. Sign up for her informative, free monthly newsletter, Forage for Thought; browse her library of reference articles; search her nutrition forum; and purchase recordings of her educational teleseminars. And for the growing community of horse owners and managers who allow their horses free choice forage feeding, Dr. Getty has set up a special forum as a place for support, celebrations, congratulations, and idea sharing. Share your experiences at jmgetty.blogspot.com. Reach Getty directly at firstname.lastname@example.org.