Cushing’s Disease in Horses - PPID Causes, Signs & Treatment

Updated September 25, 2023 | Special thanks to Dr. Amanda Adams of the University of Kentucky’s Gluck Equine Research Center, who focuses on equine immunology in aging, obesity/metabolic syndrome, and stress, for her thoughtful and thorough review of this article.

What is Cushing's Disease in Horses?

Equine Cushing’s Disease is more commonly known as Pituitary Pars Intermedia Dysfunction (PPID). It involves the overproduction of certain pituitary hormones and is the most common endocrine disease of older horses and ponies. PPID occurs in all breeds and both genders, with increasing age as the only proven risk factor.

It is diagnosed in approximately 30% of horses over 30 years of age, 20% of horses over 20 years of age, and 20-25% of horses over 15 years of age [1, 2, 3, 4]. In rare cases, horses as young as 5 years old have been diagnosed with PPID.

Causes of Cushing’s Disease in Horses

Normally the hypothalamus at the base of the brain regulates the activity of the nearby pituitary gland, including the production of certain hormones for use throughout the horse’s body. However, in PPID chronic damage from oxidative stress and other damaging factors to the neurons in the hypothalamus allows the pituitary gland to operate unchecked, resulting in excessive production of these hormones.

The result is subtle, early-stage clinical signs as well as obvious, advanced-stage clinical signs across several body systems. The most recognizable of the advanced signs is “hypertrichosis,” a long, sometimes curly, hair coat that fails to shed.

Ask the Vet Video on Cushing’s Disease

Signs and Symptoms of PPID in Horses

When diagnosing a horse with PPID, veterinarians will consider both the clinical signs and results from diagnostic testing.

First Signs of Cushing’s Disease in Horses

Once a horse displays the shaggy hair coat characteristic of PPID, the disease has already progressed. Therefore, owners should become familiar with the early signs since the sooner a horse is diagnosed and treatment started, the better its chances of responding to medication and management, thereby improving its quality of life.

According to the 2021 Recommendations for the Diagnosis and Treatment of Pituitary Pars Intermedia Dysfunction from the Equine Endocrinology Group (EEG), early and advanced signs of PPID in horses are:

Early Clinical Findings:

• Regional hypertrichosis (excessive hair growth)
• Delayed hair coat shedding
• Loss of topline muscle
• Abnormal sweating (increased or decreased)
• Infertility
• Ligament and tendon inflammation
• Regional fat deposits
• Laminitis
• Changes in attitude or lethargy
• Decreased performance

Advanced Clinical Findings:

• Generalized hypertrichosis (excessive hair growth)
• Loss of seasonal hair coat shedding
• Dipped back (topline muscle atrophy)
• Rounded abdomen (bottom-line muscle atrophy)
• Abnormal sweating (increased or decreased)
• Increased drinking and urination (PU/PD)
• Recurring infections
• Dry eye/recurring corneal ulcers
• Infertility
• Increased mammary gland secretions
• Tendon and suspensory ligament laxity
• Regional fat deposits (e.g., above the eye)
• Laminitis and recurring sole abscesses
• Dull attitude
• Exercise intolerance

Testing for Related Conditions

When testing for PPID, experts also recommend examining the horse for insulin dysregulation (ID), which is defined as any combination of:

• basal (resting) hyperinsulinemia – high levels of insulin in the blood (HI)
• postprandial hyperinsulinemia - response to an oral sugar test or eating feeds
• tissue insulin resistance (IR) – failure of cells to respond to insulin (hepatic and/or peripheral)

Insulin dysregulation is the central endocrine disorder of Equine Metabolic Syndrome (EMS), a condition typically associated with regional fat deposits (obese EMS) but can be detected in lean horses (non-obese EMS). Some horses with PPID can also have ID but may or may not have EMS.

Diagnosing Horses with PPID

PPID can be one of the more challenging diseases for vets to diagnose in horses, especially in the early stages when the clinical signs are subtle. Therefore, when horse owners first notice a difference in the health or behavior of their horse, a more thorough evaluation should take place.

Patient History

When a veterinarian “takes a history” on a patient, it means they are having a conversation about an animal’s previous health and behavior compared to changes that have been noticed recently. This dialogue includes everything from diet, exercise, medications, and supplements to daily habits, attitude, and appearance.

It’s important that the vet speak to the person or people who know the horse best. Being prepared with the horse’s medical records, training journal, and other documents is also very helpful.

Physical Exam

A thorough visual and hands-on assessment of the horse is a key component of the PPID diagnosis, especially in the early stages. The veterinarian may be able to pick up on subtle differences from the last evaluation, such as:

• the beginnings of fat deposits
• subtle changes in coat, such as patches of slightly longer/thicker areas or needing to be clipped more often
• slight loss of muscle over the topline
• tenderness to hoof testers over the soles
• mild pain to palpation of tendons and ligaments in the legs
• other key indicators of the initial stages of Cushing’s disease

Blood Tests for PPID

Based on the history and clinical signs, your veterinarian may recommend one or more blood tests to rule in or rule out PPID, as well as assess insulin status. Some tests are static, requiring only one sample (such as resting ACTH and resting insulin). While others are dynamic, requiring multiple samples to show the “before” and “after” effect of stimulation (such as the TRH Stimulation Test and the Oral Sugar Test).

Tests for PPID and ID can take place on the same day, so results can be considered concurrently. Which tests are recommended depends on a number of factors, including time of year, whether the disease is in early or late stages, etc. The EEG regularly updates testing recommendations based on the latest research, publishing a diagnostic flow chart for vets to use.

Treatment of Equine Cushing’s Disease

According to the EEG’s flow chart, horses with a history and clinical signs of PPID and whose ACTH blood levels are above the normal range for the season in which testing occurred should be put on the prescription medication pergolide. The brand Prascend® is the first and only FDA-approved treatment for the control of the clinical signs of PPID, improving the horse’s quality of life but not necessarily extending their lifespan.

Although pergolide does not cure PPID, it does slow progression and lessen clinical signs, even changes in hair coat and muscling. Starting a horse on prescription medication as soon after diagnosis as possible may also help reduce the risk of serious medical problems such as laminitis, tendon and ligament injury, recurring infections, and other issues.

Some horses will begin to show improvement in clinical signs in as little as one month after starting Prascend, although others may take two or three months to show benefits. The veterinarian will monitor the case closely and advise when it is time to repeat blood tests to see if the medication is also having an effect internally.

Owners should let the vet know if they notice any side effects of the pergolide, such as lack of appetite. Since this is one of the more common adverse reactions, there is an established protocol for getting the horse back on feed and back on medication.

Feeding and Management of PPID Horses

To provide the best overall outcome, horse owners need to work closely with their veterinarian in all aspects of the horse’s life including wellness or preventive care, the feeding program, and the exercise regimen. This is especially true if the horse has also been diagnosed with ID, or has other health conditions, such as laminitis.

Preventive Care

Most horses with Cushing’s Disease are middle aged and likely experiencing other age-related issues, so the effects of PPID may require additional maintenance. Dental care, vaccinations, parasite control, physical exams, and hoof care need to be undertaken with particular care in senior horses.

As systems – such as the immune system – start to decline, these horses can be more prone to tooth and gum disease, infections, parasitism, sole abscesses, and other health problems. This is even more true when the aging horse has PPID. An adjustment to the exercise program, as well as to tack, may also be in order if loss of topline muscle results in issues with saddle fit.

Changes of Seasons

In addition to stepped-up veterinary and farrier care, horse owners themselves will need to be more observant of the Cushing’s horse and prepared to increase the level of care from season to season.

In warm summer months, horses with long and shaggy haircoats may need to be body clipped to remove unshed winter hair. Since shade and moving air can also aid in cooling, horses with access to barns or run-ins may appreciate fans and sun-blocking curtains. Horses naturally cool themselves by sweating (like people do), so supplementing with salt and electrolytes is encouraged to help replace fluids and minerals lost in sweat.

On the other hand, cold weather may call for blankets, shelter, and deeper bedding for horses with Cushing’s disease that struggle to maintain body condition when temperatures drop. Additional calories in the form of extra hay provide the supplemental energy horses need to stay warm when it’s cold, wet, and windy. Since it’s especially hard for a thin horse to thermoregulate, owners may want to score their horse’s body condition every two to four weeks to stay on top of weight loss.

Herd Dynamics

As horse age – and particularly if they lose weight, become unsound, or develop a medical - they may lose their place in the herd pecking order. This could result in limited access to food, water, and shelter, so owners should pay attention to any change in social status and be prepared to adjust the horse’s lifestyle as needed.

Diet for Horses with PPID

For horses that test positive for PPID but not for insulin dysregulation (EMS), no nutritional changes may be necessary if their current diet is based on high-quality forage; complete and balanced; and appropriate for the horse’s age, weight, and any other existing health conditions.

Overweight or underweight horses should have their diets reviewed by an equine veterinarian well-versed in nutrition, or by a certified equine nutritionist, as it can be challenging to keep horses with PPID at their ideal weight.

Diet for Horses with PPID and Insulin Dysregulation

Horses that test positive for Cushing’s as well as some form of ID likely need their diets adjusted. Leaner horses with PPID and ID are the most difficult to manage as it can be a balancing act to prevent weight loss, but still maintain a low NSC to help prevent laminitis.

If needed, owners can provide safe added calories from fat supplements or oils (vegetable or soy). Rice bran— if it is fortified with calcium to offset its naturally high phosphorus content—is a good choice for adding calories back to the diet through fat. Molasses-free beet pulp is a feed source that has a nutritional profile similar to hay and doesn’t cause a spike in blood sugar and provides energy from bacterial fermentation of fiber.

For the overweight horse struggling with both endocrine conditions (Cushing’s and EMS), the EEG provides this advice for trying to reduce body fat:

• Limit calories (use a ration balancer or multi-vitamin instead of grain)
• Feed hay with low (ideally around 10% or below) NSC or non-structural carbohydrates (sugars and starches)
• At least at first, do not allow any access to grass, even with a muzzle
• Consider whether even low NSC treats are appropriate during the weight loss period
• Add controlled exercise if the hooves are stable and the vet approves it

Later, when the weight has come down, bloodwork is improving, and there are no signs of laminitis, some limited pasture and low NSC treats may be acceptable. Options for limiting pasture include using a muzzle, restricting by time, or setting up strip grazing or a track system.

Supplements That May Lend Support

Supplement selection for horse with PPID, ID, or both must be thoughtful and done in consultation with your veterinarian. In addition to the previously mentioned electrolytes for heavy sweaters and a multi-vitamin instead of fortified grain, other considerations that have anecdotal evidence include:

• amino acids (protein) to help rebuild topline muscle
• omega 3 fatty acids and vitamin E to bridge the gap between fresh grass and dried forage
• antioxidants like vitamin E and C to help combat oxidative stress
• the herb chasteberry can help support normal pituitary function to improve coat quality and normal shedding
• chromium and magnesium combined are thought to help with insulin sensitivity

Horse owners can also consider metabolic supplements designed to support normal insulin and glucose levels to help your horse achieve a healthy weight.

Exercise for Horses with Cushing’s

Controlled activity needs to be just as tailored to the individual horse as the preventive care and diet. Otherwise, healthy horses diagnosed with Cushing’s that are responding well to treatment and management are encouraged to continue with their regular exercise program. Horses with a history of laminitis should gradually return to exercise under a veterinarian’s direction.

Riding Horses with PPID

Structured exercise is highly recommended for horses, as long as the feet are stable and there are no other soundness issues. The EEG has specific guidelines for the level, duration, and intensity of workouts. However, most experts agree that all levels of exercise (walk, trot, and canter; ridden and unridden) are likely to be help reduce weight and improve insulin sensitivity in horses with ID.

Summary on PPID

PPID, also known as Equine Cushing’s Disease, involves the pituitary gland. A horse’s best chances for a successful outcome hinge on:

• increased owner awareness of the early and advanced clinical signs of the disease
• veterinarian confirmation through history-taking, physical exam, and blood testing
• a coordinated management plan

While there is an FDA-approved treatment for PPID, it is not a cure. It is also unlikely a horse’s lifespan can be prolonged. However, administering Prascend (pergolide) – along with managing clinical symptoms, appropriate nutrition, and controlled exercise - may slow progression of the disease. With proper wellness care and management, horses with Cushing’s can improve the quality of their lives.

Evidence-Based References

1. Durham A., McGowan C., Fey K., Tamzali Y., Van der Kolk J. Pituitary pars intermedia dysfunction: Diagnosis and treatment. Equine Vet. Educ. 2014;26:216–223.

2. Horn R., Stewart A.J., Jackson K.V., Dryburgh E.L., Medina-Torres C.E., Bertin F.R. Clinical implications of using adrenocorticotropic hormone diagnostic cutoffs or reference intervals to diagnose pituitary pars intermedia dysfunction in mature horses. J. Vet. Intern. Med. 2021;35:560–570.

3. Ireland J., McGowan C. Epidemiology of pituitary pars intermedia dysfunction: A systematic literature review of clinical presentation, disease prevalence and risk factors. Vet. J. 2018;235:22–33.

4. McGowan T.W., Pinchbeck G.P., McGowan C.M. Prevalence, risk factors and clinical signs predictive for equine pituitary pars intermedia dysfunction in aged horses. Equine Vet. J. 2013;45:74–79.