Pituitary Pars Intermedia Dysfunction (PPID), commonly known as Cushing's disease, is a prevalent endocrine disorder affecting older horses and ponies. It stems from the overproduction of certain pituitary hormones due to chronic damage to neurons in the hypothalamus, leading to a cascade of clinical signs. While PPID is primarily managed with medication like pergolide, diet and overall management play a crucial role in supporting these horses, especially when insulin dysregulation (ID) is also present.
Understanding PPID and Its Impact
PPID is diagnosed in a significant percentage of older horses, with prevalence increasing with age. It affects all breeds and genders, making age the primary risk factor. The disease process involves the hypothalamus failing to regulate the pituitary gland, resulting in excessive hormone production. This can lead to a range of symptoms, from subtle early signs to more pronounced clinical manifestations.
Recognizing the Signs and Symptoms
Early detection is key to effectively managing PPID. Veterinarians consider both clinical signs and diagnostic test results when diagnosing the condition.
First Signs of Cushing’s Disease in Horses:
- 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
Once a horse displays the shaggy hair coat characteristic of PPID, the disease has already progressed. 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.
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The Importance of Diagnosis
Diagnosing PPID can be challenging, especially in the early stages. A thorough evaluation by a veterinarian is essential when horse owners notice any changes in their horse's health or behavior. This includes a detailed patient history, visual assessment, and hands-on examination. Blood tests, such as resting ACTH and the TRH Stimulation Test, are crucial for confirming the diagnosis and assessing insulin status.
Dietary Management Strategies for Horses with PPID
While pergolide is the primary treatment for PPID, dietary management is vital for supporting overall health and addressing concurrent conditions like insulin dysregulation.
The Role of Diet in Managing PPID and Insulin Dysregulation
Diet is not directly used to manage PPID, rather, it is managed with medication (pergolide), but ID and equine metabolic syndrome-which some PPID horses also have-are managed by diet and exercise.
Horses with PPID and good Body Condition Score (BCS) without evidence of insulin resistance can essentially be fed as a normal horse to maintain body condition and prevent obesity.
Obese horses with PPID should be fed similarly to horses with equine metabolic syndrome (EMS) to encourage weight loss, improve insulin sensitivity, and reduce the risk of laminitis.
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Pituitary pars intermedia dysfunction is a disease of older horses and many are underweight as a consequence of senescence or dental problems. Increased energy requirements can be provided for by feeding hay, a low-starch and low-sugar (low-NSC) pellet, and 1/2 to 1 cup (120-240 ml) of vegetable oil twice daily.
Pasture intake should be controlled to avoid sudden increases in carbohydrate intake. Feeds that are high in fat (greater than 7%) can also be fed to supplement calories. High-fat diets have the potential to worsen or create insulin resistance. Pagan and coworkers reported that a high-fat (30% of digestible energy supplied as fat) diet significantly impaired glucose tolerance in aged non-obese Thoroughbred geldings compared to a more traditional diet. Moderate carbohydrate intake (31% of digestible energy supplied as NSC) improved glucose clearance during intravenous glucose tolerance tests compared to both a high fat and an all grass hay diet.
Key Dietary Considerations
- Protein: Providing a source of quality protein in the ration can be of particular benefit for individuals that are generally struggling to maintain weight.
- Concentrates: Concentrates should be fed only as needed to maintain desired body condition, preferably body condition score (BCS) 4 to 6 on a scale of 1 to 9.
- Nonstructural Carbohydrates (NSC): Textured or sweet feed containing greater than 3% molasses and 15-20% nonstructural carbohydrates (NSC) should be avoided if there is evidence of insulin resistance.
- Fiber and Fat: Pellets or extruded feeds that are higher in fiber (greater than 10%) and fat ( greater than 5%) can be fed instead of sweet feed to provide additional dietary energy.
Managing Pasture Access
When I think spring management of horses with endocrine disorders, I immediately think pasture and grass and, from there, my concerns go to laminitis. Recent research has shown that ponies with an insulin level above 45 have a 70% chance of developing laminitis. These are typically the individuals that have some kind of insulin dysregulation (ID). Not all horses with PPID also have ID, so my first recommendation is if you do not know whether your horse also falls into the category of being ID, talk to your vet about doing the bloodwork necessary to make the diagnosis.
If she has ID, I would strongly caution against allowing access to pasture. If her insulin values are normal, you can provide careful access to pasture. Consider the use of a grazing muzzle that will allow access but limit total intake. Monitor her weight closely, and body condition score her regularly to identify the onset of fat deposits.
Whenever possible, horses with PPID should be allowed turnout time, as exercise improves insulin sensitivity. Owners should be advised to be aware during times of the year when pasture grasses can be high in NSC because PPID can worsen insulin insensitivity in susceptible horses and result in laminitis.
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Addressing Weight Management Challenges
Underweight Horses: For those horses that are generally struggling to maintain weight, finding and adding a low nonstructural carbohydrate senior feed to their diet is a good choice.
Overweight Horses: 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.
Feed Options
- WellSolve L/S®: This feed is specifically designed for horses with sensitivities to starch and sugar, like insulin dysregulation, laminitis and PPID. It is guaranteed at 11% maximum starch and sugars, contains no grains or molasses, supports the horse’s health with appropriate vitamins, minerals and protein, as well as biotin to support hoof quality.
- Equine Senior® and Equine Senior® Active: Equine Senior® is a complete feed formulated specifically for the aging horse to completely or partially replace the hay or pasture in the horse’s diet when dental problems or missing teeth prevent them from chewing or digesting long-stemmed forage effectively. Equine Senior® Active is appropriate for senior horses who can still chew hay well and is low in starch and sugar. Additionally, it includes quality protein and higher calorie levels to support harder keepers or those still working or competing.
- 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.
- 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.
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.
Comprehensive Management of PPID Horses
Effective management of PPID involves a holistic approach that extends beyond medication and diet.
Preventive Care
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. 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.
Exercise
Whenever possible, horses with PPID should be allowed turnout time, as exercise improves insulin sensitivity. Owners should be advised to be aware during times of the year when pasture grasses can be high in NSC because PPID can worsen insulin insensitivity in susceptible horses and result in laminitis.
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.
Dental Care
I also strongly recommend having a qualified dental specialist examine these horses’ teeth carefully, as I have worked with a number of PPID horses that also have equine odontoclastic tooth resorption and hypercementosis (EORTH).
Parasite Control
Another cause of decreased body condition in this population of horses is internal parasites. Horses with PPID are at greater risk of having higher internal parasite burdens than healthy horses without PPID. Continue to monitor bloodwork over time so you can make diet adjustments as necessary.
Herd Dynamics
As horses age - and particularly if they lose weight, become unsound, or develop a medical - they may lose their place in the herd pecking order. 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.