Feeding horses with pituitary pars intermedia dysfunction (PPID), previously referred to as Equine Cushing’s disease, can be a challenge.

Horses affected by PPID are typically older and may have other health issues including equine metabolic syndrome (EMS). This condition can involve hyperinsulinemia, recurring laminitis, and abnormal fat deposits. [1]

PPID can also result in abnormally long and curly coat, failure to shed, muscle wasting, muscle weakness, depression and an underweight or overweight body condition. [1] Determining a suitable diet for a PPID-affected horse will depend on their body condition and if insulin resistance is present.

Your veterinarian will diagnose PPID and hyperinsulinemia/insulin resistance based on blood tests and determine an appropriate treatment plan, including whether medication is needed to control symptoms.

Follow this guide to learn how to feed a horse with PPID and consult with an equine nutritionist to ensure your PPID-affected horse is receiving a balanced diet. Ongoing consultation may be required to maintain a healthy body condition.

What is PPID in Horses?

PPID is a slowly progressive endocrine disorder that involves excessive production of multiple hormones from the intermediate lobe of the pituitary, including adrenocorticotrophic hormone (ACTH).

In horses, ACTH is involved in maintaining metabolic homeostasis, regulating immune responses, and supporting normal adrenal gland function. ACTH also plays a critical role in regulating stress responses by modulating cortisol production.

PPID-affected horses can have mild to advanced forms of the disease and differing ages of onset, symptoms, and responses to treatment. However, the disease primarily affects older horses over the age of 15. [2]

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Pituitary Gland

Adrenocorticotrophic hormone is produced by the pituitary gland located at the base of the brain. [2] The equine pituitary gland consists of three lobes including the pars distalis, pars intermedia, and pars nervosa.

PPID affects the pars intermedia part of the pituitary gland, resulting in excess hormone production.

The release of hormones from the pituitary gland is controlled by a part of the brain called the hypothalamus.

In healthy horses, the hypothalamus releases the neurotransmitter dopamine to inhibit the production of hormones by the pars intermedia. [4] Dopamine binds to receptors on the surface of cells to stop the secretion of hormones from the pars intermedia of the pituitary gland.

Pathophysiology

In PPID-affected horses, there is degeneration of the dopamine-producing neurons in the hypothalamus. This results in excess hormone production by the pituitary gland.

One identified cause for the degeneration of dopamine-producing neurons is oxidative stress in the hypothalamus, but researchers continue to investigate why this happens. [3]

In horses with PPID, high levels of circulating ACTH trigger the adrenal gland to increase production of the stress hormone cortisol. Higher cortisol levels promote insulin resistance. [5]

Low dopamine levels permit the pituitary gland to increase in size as the cells of the pars intermedia divide and enlarge. Benign tumors may subsequently develop in this part of the gland. [6]

The expansion of the pars intermedia can compress the other lobes of the pituitary and adjacent structures in the brainstem. As these structures become compressed, a loss of function may occur and result in various clinical symptoms.

Prevalence

According to epidemiologic studies, an estimated 20% of horses over the age of 20 are affected by PPID, and 30% of those over 30 are affected. [7]

Aging is the single major risk factor for PPID, and the condition most often occurs in senior horses. However, horses as young as seven years of age have been diagnosed with PPID. Breed and sex are not risk factors for the disease.

Clinical Signs

The clinical signs of PPID are often under-recognized. [8] PPID can cause a range of symptoms depending on the severity of the condition. [9]

Early symptoms of PPID include:

As PPID progresses, horses can develop additional symptoms, including:

  • Weight loss
  • Failure to shed
  • Abnormal sweating
  • Skeletal muscle atrophy
  • Infertility
  • Neurological problems
  • Increased thirst and urination year-round
  • Loose teeth
  • Increased risk of dental, skin, sinus, and parasitic infections due to immune suppression

Treatment

There is no cure for PPID, although drug treatment can help to control and slow the progression of the disease.

The only licensed drug for treating PPID is Pergolide mesylate. This drug is sold under the brand name, Prascend, as the only FDA-approved pergolide formulation for horses.

By acting as a dopamine replacement, Pergolide regulates the release of hormones, such as ACTH, from the pituitary pars intermedia. Once hormone levels are regulated, PPID symptoms improved in over 75% of cases. [10]

10 Tips for Feeding Horses With PPID

Although PPID cannot be managed by diet alone, optimal nutrition will support the overall health of horses with this condition.

All horses and ponies need a forage-based diet with adequate energy, protein, vitamins and minerals to meet their NRC nutrient requirements.

Overweight horses with PPID need fewer calories to promote weight loss. Underweight horses with this condition may require concentrates and additional nutrients to maintain weight and muscle mass.

If your PPID-affected horse is insulin resistant, they also need a diet low in ESC (simple sugars) and starch to prevent increased insulin levels and laminitis risk.

Follow these 10 steps to determine how and what to feed your PPID horse:

1) Evaluate Body Condition

Before changing your horse’s diet, work with your veterinarian or an equine nutritionist to determine your horse’s body condition.

Body condition score (BCS) in horses is evaluated on the 9-point Henneke scale, where a score of 5 is considered ideal. [33]

A body condition score of 4 or lower is considered underweight. At a score of 4, a faint outline of the ribs is visible and a dip between the withers and neck may be apparent. Horses below a 4 on the BCS scale should be assessed by a veterinarian to identify medical conditions contributing to low weight.

A body condition score of 6 or higher is considered overweight or obese. These horses will have general fat accumulation around their body, with rib cover and a tailhead that is spongy to the touch.

Overweight or obese horses may also have abnormal regional fat deposits which are signs of insulin resistance, including on the neck (cresty neck), mammary glands, and sheath.

Once you determine your horse’s baseline body condition score, you can decide whether to feed a diet to support weight maintenance, weight gain or weight loss. Regularly track your horse’s BCS so you can make adjustments to your feeding program, if necessary.

2) Determine Insulin Levels

Monitoring your horse’s insulin status is important for assessing laminitis risk and determining what to feed your horse.

PPID horses are frequently affected by insulin resistance, which refers to cells failing to respond to normal levels of insulin. Horses with severe IR can become diabetic (have elevated blood sugar).

Research shows that horses with hyperinsulinemia that lose excess weight have an improved insulin response. [11]

Contact your veterinarian to perform a glucose/insulin tolerance test to determine your horse’s insulin sensitivity. This test will help you make informed decisions about what to feed your horse.

3) Control Intake of simple sugars and starch

PPID horses need to be fed a diet low in sugars and starches, known as hydrolyzable carbohydrates (HC).

Rations that contain excess simple sugars and starch fed to horses with metabolic dysfunction, can lead to weight gain and higher insulin levels.

Grain, fruits, vegetables, and sugary treats should be removed from the diet of horses with PPID because they are usually high in simple sugars.

Only choose commercial feeds and concentrates with less than 10% HC on a dry matter basis. Some feed labels indicate NSC values on their guaranteed analysis, but many do not.

Note that NSC (non-structural carbohydrates) is the combination of simple sugars, starch and fructans. Fructan is a plant storage carbohydrate which does not increase insulin levels in the horse.

Work with a nutritionist if you are unsure about which products are safe for PPID horses.

Forage Selection for PPID Horses

You should also analyze your hay to determine its ESC and starch content. Your hay analysis report will tell you the percent of starch, ethanol-soluble and water-soluble carbohydrates in the forage. The ethanol-soluble carbohydrates and starch values can be added together to determine the safety of the hay and should be under 10%.

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Mature grass hays such as timothy or orchard grass are most suitable for PPID horses that are overweight. They typically have less energy content than legume hays, such as alfalfa.

Soaking hay can reduce ESC content, but may also lead to other nutrient losses and may not reliably lower the ESC + starch value below 10%. [12]

Pasture grass can be very high in carbohydrates due to its sugar content, particularly in the spring. Overweight horses with EMS or PPID may require restricted or no access to pasture until they reach a normal body weight.

The use of a grazing muzzle or dry lot may be needed to reduce grass intake while encouraging exercise during turnout.

4) Meet Energy Requirements

Your horse’s dietary energy requirement is determined by their body weight, activity level, physiological status and body condition.

If your PPID horse is at a healthy body condition, your goal is to feed enough energy to maintain their current weight without over-supplying calories.

If your PPID horse is over-conditioned, your goal is to safely restrict their energy intake. This will help reduce their body weight and improve insulin regulation.

If your PPID horse is underweight, you need to increase their caloric intake to promote healthy weight gain while choosing safe energy sources that will not contribute to digestive, metabolic or behavioral issues.

Feeding Horses with Ideal Body Condition

Horses with an ideal body condition should be fed a forage-based diet to meet energy requirements and maintain their current weight.

Provided your horse has good dentition, most horses can meet their digestible energy requirement with free-choice access to appropriately selected forage. Some horses in heavy work may need other feeds to provide additional energy.

Although your horse may currently have normal insulin function, it is best to minimize intake of starch and sugar to support insulin sensitivity.

Feeding Overweight Horses

PPID horses that are overweight need reduced calorie intake to support weight loss and improved insulin sensitivity.

The first step is to remove excess starches (such as grains) and sugars from your horse’s diet. You may also need to reduce or eliminate high-fat feeds or energy-dense supplements from your horse’s ration.

You can also promote weight loss by selecting mature grass forages, feeding straw, and using a slow feeder hay net to slow down hay intake.

If considering straw, it is highly recommended to analyze the hay and straw. Good, clean straw has almost as many calories as a mature, low ESC + starch hay but lower protein and minerals.

Restricting access to forage to less than 1.5% of body weight is not recommended due to the risk of increased stress, gastric ulcers, digestive issues, hyperlipidemia, stereotypies and more.

Feeding Underweight Horses

Horses that have PPID and insulin dysregulation are not always overweight and in fact, may be underweight. Researchers do not know why weight loss occurs in some PPID horses, but it affects 5 – 88% of horses with this condition. [6]

Nutritional recommendations for underweight horses with PPID focus on adding calories from appropriate sources. This will promote safe weight gain while avoiding making insulin resistance worse.

However, the weight loss is unlikely to reverse until pergolide dosage is adjusted to adequately control the hormonal output from the pituitary gland.

A forage-based diet is best for all horses and ponies, including those that are underweight. Forage intake should be maximized to increase energy supply in the diet.

Choose high-fiber feeds with low HC content to minimize blood sugar spikes that can raise insulin levels. Feeds that are appropriate for adding energy to the diet include:

  • Forage pellets/cubes
  • Specialized commercial feeds low in ESC + starch
  • Beet pulp
  • Soybean hulls
  • Ground flax seed

Oils are a calorie-dense option that can be used in reasonable amounts (e.g. up to 4 oz or 120 ml per day). However, safety of high fat/oil feeding in animals with insulin resistance has not been established. A source of essential omega-3 fatty acids, such as Mad Barn’s w-3 oil, should always be supplied.

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  • Palatable source of Omega-3's

5) Meet Vitamin & Mineral Requirements

Vitamins and minerals are critical micronutrients that your horse needs in adequate levels to support metabolism and enzyme functions. These nutrients also protect against oxidative stress and inflammation. [13][14]

Unfortunately, research shows that most horses have one or more vitamin and mineral deficiencies in their diet. Horses are unlikely to meet their requirements with forage alone. [34]

Key vitamins and minerals needed to support metabolic health include: [15][17][18][20][34]

  • Zinc, Copper & Manganese: Essential for the antioxidant enzyme superoxide dismutase
  • Selenium: A component of several seleno-proteins involved in insulin production and signaling, as well as antioxidant and anti-inflammatory processes
  • Magnesium: Supports insulin signaling
  • Chromium: Supports insulin signaling and exerts anti-inflammatory and antioxidant benefits
  • Vitamin E: A key antioxidant that supports immune function

If your horse has PPID, it is especially important that their diet provides balanced levels of vitamins and minerals to support metabolic health and immune function.

Importance of Vitamins & Minerals

Adequate vitamin and mineral nutrition in PPID horses will help to:

  • Support hoof health and regulate inflammation.
  • Prevent cell damage caused by harmful free radicals, which are products of normal cellular metabolism. Feeding antioxidant nutrients, such as selenium and Vitamin E, may reduce oxidative stress.
  • Support glucose metabolism by ensuring the diet provides sufficient chromium and magnesium. [20][21]
  • Maintain a healthy immune response by providing Vitamin E, which enhances the capacity of immune cells to destroy bacteria. Vitamin E also helps horses recover from illness more quickly. [22]

Mad Barn’s AminoTrace+ is a vitamin and mineral supplement specifically formulated to meet the nutritional needs of horses with metabolic issues, including PPID, by addressing commonly encountered deficiencies and imbalances. This low-HC formula is expertly balanced and contains higher quality organic trace minerals and antioxidant nutrients.

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  • Formulated for IR/Cushing's
  • Hoof growth

6) Feed Probiotics and Yeast

Horses with PPID have impaired immune function and may be more susceptible to dental, skin, sinus, and parasitic infections. [23]

You can support your horse’s immune system by feeding probiotics and prebiotics, and yeast to maintain a healthy microbiome in the hindgut. [16]

Beneficial microbes in the horse’s hindgut play a key role in maintaining immune defenses. These microbes contribute to the production of short-chain fatty acids and vitamins, regulate inflammation, and compete with harmful pathogens, all of which enhance the horse’s immune response and overall health.

Mad Barn Optimum Digestive Health is a comprehensive gut health supplement that provides 80 billion CFUs of yeast and probiotics per serving. Optimum Digestive Health also contains prebiotics, toxin binders, immune nucleotides and intestinal enzymes to support hindgut health and immunity.

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  • Support hindgut development
  • Combats harmful toxins in feed
  • Complete GI tract coverage

7) Address Muscle Loss

Muscle atrophy (a loss of muscle mass) occurs in approximately 50% of horses with PPID, affecting types 2A and 2B muscle fibres. [24] Older horses show signs of muscle wasting more frequently than younger horses. [25]

Muscle loss in PPID horses commonly occurs in the topline muscles near the spine (epaxial muscles). Loss of muscle may cause the appearance of sunken shoulders and prominent withers, spine, and hip bones.

It is unknown if muscle atrophy in PPID horses occurs due to an increase in muscle breakdown or a decrease in muscle protein synthesis. [6][26]

Recent research indicates that plasma amino acid levels differ in horses with PPID compared to healthy horses. [27]

Provide Adequate Protein

The best way to support your PPID horse’s muscle mass is to feed a high-quality protein source to support muscle protein synthesis and repair. Exercise is also important for stimulating muscle growth. [19]

Research shows that supplementing the diet with lysine, threonine and methionine, supports muscle synthesis and can help maintain muscle mass in older horses. However, this has not been studied in horses with PPID. [36]

Feeds and supplements with high protein content and an ideal amino acid profile include:

Mad Barn’s AminoTrace+ is a concentrated source of vitamins and minerals along with a high level of lysine, methionine and threonine to support protein synthesis.

8) Provide Essential Fatty Acids

Omega-3 fatty acids are essential nutrients that must be provided in the equine diet and that regulate inflammation in the body.

Increasing omega-3 fatty acids in your horse’s diet can help control the production of inflammatory molecules in cells. This may improve regulation of inflammatory responses that occurs with conditions such as PPID and in response to cellular damage in endocrinopathic laminitis. [28][29][30]

In particular, the omega-3’s docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) have been found to exhibit health-boosting properties in horses. [28][29][30][35] Associated benefits include:

  • Support for joint health and mobility
  • Healthier-looking hair coats
  • Decreased stress response
  • Improved respiratory health and reduced allergy response
  • Improved insulin sensitivity in horses with equine metabolic syndrome
  • Improved reproductive health

Choose an omega-3 supplement that contains DHA or EPA. Plant-based oils such as camelina oil or flax oil contain ALA but not DHA or EPA.

Mad Barn’s w-3 Oil is enriched with high levels of microalgal DHA along with natural Vitamin E, to help your PPID horse look and feel their best.

9) Consider Feeding Chasteberry

The herb chasteberry (Vitex agnus-cactus) may benefit some horses with early-stage PPID. Although not a suitable replacement for treatment with pergolide, chasteberry supplements may be used as an adjunct therapy to help control early manifestations of PPID.

Research shows that chasteberry may reduce some of the clinical symptoms associated with elevated ACTH levels, including excessive hair growth and abnormal shedding. [32]

However, chasteberry does not lower ACTH levels, whereas Pergolide does. [32] For this reason, chasteberry is not as effective as pergolide for horses with advanced PPID.

Chasteberry and pergolide should not be used together as there is some evidence that Chasteberry interferes with the action of pergolide. [38]

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10) Accommodate Horses with Dental Issues

Special dietary considerations should be made for horses with dental issues.

Some PPID-affected horses have missing teeth and are affected by equine odontoclastic tooth resorption and hypercementosis (EOTRH). This can interfere with chewing feed and grasping blades of grass with the front teeth (incisors). [31]

Horses with dental issues benefit from being fed chopped hay, hay pellets, and/or soaked hay cubes to meet their fiber requirements and support digestive health.

Summary

A balanced diet that supports an optimal weight and promotes insulin sensitivity is critical for horses with PPID. Follow the 10 steps above to ensure your PPID horse is fed an appropriate diet for their needs.

  • Horses with PPID may be over or underweight and may or may not have insulin dysregulation. These factors will influence what diet is ideal for them.
  • PPID-affected horses with insulin resistance require a low-NSC diet to reduce the risk of laminitis.
  • Horses with PPID need adequate levels of vitamins and minerals to support metabolic and immune health.
  • Consult an equine nutritionist to develop a customized feeding plan for your horse.
  • Ensure your horse receives regular veterinary checks to determine if adjustments are needed to their diet or medication. Monitoring insulin sensitivity is important to reduce the risk of laminitis.

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References

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  2. McFarlane, D. Equine Pituitary Pars Intermedia Dysfunction. Veterinary Clinics of North America: Equine Practice. 2011. View Summary
  3. Spelta, C.W. Equine pituitary pars intermedia dysfunction: current perspectives on diagnosis and management. Veterinary medicine. 2015. View Summary
  4. McFarlane D et al. The role of dopaminergic neurodegeneration in equine pituitary pars intermedia dysfunction (equine Cushing’s disease). Proceedings of the 49th Annual Connvention of the American Association of Equine Practitioners. 2003.
  5. Graves, E. Equine Endocrine Diseases: The Basics. AAEP. 2022.
  6. Kirkwood NC. et al. Pituitary Pars Intermedia Dysfunction (PPID) in Horses. Vet Sci. 2022. View Summary
  7. Schott, H. et al. Recommendations for the Diagnosis and Treatment of Pituitary Pars Intermedia Dysfunction. Equine Endocrinology Group. 2021.
  8. Ireland, J.L., et. al. Comparison of owner-reported health problems with veterinary assessment of geriatric horses in the United Kingdom. In: Equine Veterinary Journal. 2011.View Summary
  9. McGowan TW et al. Prevalence, risk factors and clinical signs predictive for equine pituitary pars intermedia dysfunction in aged horses. Equine Vet J. 2013.View Summary
  10. Tatum RC. et al. Efficacy of pergolide for the management of equine pituitary pars intermedia dysfunction: A systematic review. Vet J. 2020 View Summary
  11. Delarocque, J. et al. Weight loss is linearly associated with a reduction of the insulin response to an oral glucose test in Icelandic horses. BMC Veterinary Research. 2020. View Summary
  12. Bochnia, M. et al. Effect of Hay Soaking Duration on Metabolizable Energy, Total and Prececal Digestible Crude Protein and Amino Acids, Non-Starch Carbohydrates, Macronutrients and Trace Elements. J Equine Vet Sci. 2021. View Summary
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  14. Pleasant, R.S. et al. Adiposity, plasma insulin, leptin, lipids, and oxidative stress in mature light breed horses. J Vet Intern Med. 2013. View Summary
  15. Vervuert, I. et al. Effects of chromium yeast supplementation on postprandial glycaemic and insulinaemic responses in insulin-resistant ponies and horses. 2010.
  16. Respondek, F. et al. Dietary supplementation with short-chain fructo-oligosaccharides improves insulin sensitivity in obese horses. J Anim Sci. 2011.View Summary
  17. Winter, JC. et al. Oral supplementation of magnesium aspartate hydrochloride in horses with equine metabolic syndrome. 2016.
  18. Morgan, RA. et al. Treatment of Equine Metabolic Syndrome: a clinical case series.Equine Veterinary Journal. 2014.
  19. Menzies-Gow, NJ. Et al. The effect of exercise on plasma concentrations of inflammatory markers in normal and previously laminitic ponies. Equine Vet J. 2014. View Summary
  20. Spears, J.W. et al. Chromium propionate increases insulin sensitivity in horses following oral and intravenous carbohydrate administration. Journal of Animal Science. 2020. View Summary
  21. Stewart, Allison. Magnesium Disorders in Horses. Vet Clin Equine. 2011.
  22. Petersson, KH et al. The Influence of Vitamin E on Immune Function and Response to Vaccination in Older Horses. J Anim Sci. 2010. View Summary
  23. McFarlane, D. et al. Neutrophil function in healthy aged horses and horses with pituitary dysfunction. Veterinary Immunology and Immunopathology. 2015. View Summary
  24. Banse HE. et al. Markers of muscle atrophy and impact of treatment with pergolide in horses with pituitary pars intermedia dysfunction and muscle atrophy. Domest Anim Endocrinol. 2021. View Summary
  25. Ireland, JL. and McGowan, CM. Epidemiology of pituitary pars intermedia dysfunction: A systematic literature review of clinical presentation, disease prevalence and risk factors. Vet J. 2018. View Summary
  26. Aleman, M. et al. Myopathy in horses with pituitary pars intermedia dysfunction (Cushing’s disease). Neuromuscul Disord. 2006. View Summary
  27. Stoeckle SD. et al. Plasma Amino Acids in Horses Suffering from Pituitary Pars Intermedia Dysfunction. Animals (Basel). 2022. View Summary
  28. Abbott, K.A. et al. DHA-enriched fish oil reduces insulin resistance in overweight and obese adults. Prostaglandins Leukot Essent Fatty Acids. 2020.
  29. Poudyal, H. et al. Omega-3 fatty acids and metabolic syndrome: effects and emerging mechanisms of action. Prog Lipid Res. 2011.
  30. Elzinga SE, Betancourt A, Stewart JC, Altman MH, Barker VD, Muholland M, Bailey S, Brennan KM, Adams AA. Effects of Docosahexaenoic Acid–Rich Microalgae Supplementation on Metabolic and Inflammatory Parameters in Horses With Equine Metabolic Syndrome. Journal of equine veterinary science. 2019. View Summary
  31. Rahmani, V. et al. History, clinical findings and outcome of horses with radiographical signs of equine odontoclastic tooth resorption and hypercementosis. Veterinary Record. 2019. View Summary
  32. Bradaric, Zrinjka et al. Use of the chasteberry preparation Corticosal for the treatment of pituitary pars intermedia dysfunction in horses. Pferdeheilkunde. 2013.
  33. Henneke, D.R. et al. Relationship between condition score, physical measurements and body fat percentage in mares. Equine Vet J. 1983. View Summary
  34. National Research Council. Nutrient Requirements of Horses. National Academies Press. 2007.
  35. Hess, T. and Ross-Jones, T. Omega-3 fatty acid supplementation in horses. R Bras Zootec. 2014.
  36. Galinelli, N.C. et al. Nutritional considerations for the management of equine pituitary pars intermedia dysfunction. Equine Vet Educ. 2021.
  37. Hofberger, S. et al. Suspensory ligament degeneration associated with pituitary pars intermedia dysfunction in horses. Vet J. 2015. View Summary
  38. Bradaric, Z. et al. Use of the chasteberry preparation Corticosal® for the treatment of pituitary pars intermedia dysfunction in horses. Pferdeheilkunde. 2013.