Nutritional Considerations for Horses with Endocrine Disorders
As a species, horses have a longer average lifespan now than ever before. Due to advances in veterinary and nutritional management, a protracted lifespan often equates to an extended working life. Once their usefulness ebbs and retirement looms, many horses retain value as companions, both to other horses and humans. Because of this, interest in disorders that affect aged horses has grown among horse owners and veterinarians, with keen attention given to endocrine diseases.
Recent research efforts have amplified the knowledge of endocrine disorders, though certain aspects of these diseases remain elusive, such as gold standard diagnostic tests. As such, diagnosis relies on clinical presentation and current diagnostic procedures. Shifts in how horses are managed, including any necessary changes to the diet, can be implemented once a diagnosis is made.
The most common diagnosed endocrine disorders are pituitary pars intermedia dysfunction (PPID) and equine metabolic syndrome (EMS). Thoughtful management strategies, including careful attention to nutrition and exercise requirements, can provide some relief, especially when paired with appropriate medications.
Pituitary Pars Intermedia Dysfunction
Pituitary pars intermedia dysfunction (PPID), also known as Cushing’s disease, is a slowly progressive endocrine disorder that affects many older horses and ponies. The disorder is characterized by pituitary gland abnormalities, including benign tumors, hyperplasia, or hypertrophy. Most animals are over the age of 15 years old when diagnosed, though occasionally younger horses may be affected.
• Breeds affected: All breeds of horses and ponies.
• Clinical signs: long, wavy hair coat, often with delayed shedding; muscle wasting, especially over the topline; increased thirst and urination; increased sweating; behavioral changes, notably lethargy; subfertility; abnormal fat deposits, despite sometimes appearing thin; increased risk for opportunistic infections, likely due to immune system suppression, including chronic hoof abscesses, skin infections, and dental disease; insulin dysregulation; and laminitis.
• R isk factors: only age has been reported as a risk factor.
• Diagnosis: age coupled with the appearance of clinical signs, especially coat changes, are often sufficient for diagnosis, though earlier and milder disease may be more difficult to detect; screening can be achieved through measurement of resting adrenocorticotropic hormone (ACTH), which is frequently elevated in those animals with PPID, and other diagnostic tests, such as the thyrotropin-releasing hormone stimulation test, which generally seems to be more accurate than measurement of resting ACTH alone.
Nutrient requirements for PPID
Nutrient requirements depend largely on whether horses have been diagnosed with insulin dysregulation and laminitis.
• Forage: for horses with clinical signs of PPID but no insulin dysfunction or laminitic episodes, good-quality forage fed at a rate of 1.5-2% of body weight per day; for those with concomitant insulin dysfunction, laminitis can be exacerbated by high levels of nonstructural carbohydrates (NSC) of some pasture grasses and hays, so extreme care must be taken in regard to hay selection (tested to be 12% or less NSC) and pasture access; some horses especially sensitive to carbohydrate intake must be restricted from all pasture intake through the use of a grazing muzzle or drylot and appropriate preserved forage for these horses tends to be mature grass hays.
• Concentrate: for horses with no insulin dysfunction or laminitic episodes that need to gain weight or have difficulty maintaining body condition, a low-starch or senior feed can be given at the rate recommended by the manufacturer and fat supplements can be added to provide additional energy; for horses with insulin dysfunction and laminitis that are in moderate to obese body condition, no commercial feeds or straight grains should be given, though a well-formulated balancer pellet to satisfy protein, vitamin, and mineral requirements should be fed.
• Supplements: horses without access to fresh forage should be fed a natural-source vitamin E such as Nano-E®; body-wide inflammation can be reduced with omega-3 fatty acids, the most bioavailable of which are obtained from marine-derived sources, like EO-3™; in horses with muscle wasting, a high-quality amino acid supplement may counteract this clinical sign; MFM Pellet™ contains the essential branchedchain amino acids important in rebuilding muscle tissue for horses experiencing muscle wasting; a well-formulated vitamin and mineral supplement, like Micro-Max™, can provide essential nutrients to horses on all-forage diets.
Equine Metabolic Syndrome
Equine metabolic syndrome (EMS) is a collection of endocrine and metabolic abnormalities associated with increased susceptibility to laminitis in horses and ponies, including insulin dysregulation and fat redistribution.
• Breeds affected: genetic link to insulin resistance and EMS in American Saddlebreds, Andalusians, Morgans, Tennessee Walking Horses, Paso Finos, Quarter Horses, and ponies of all types. Predilection for EMS relates to members of these breeds thought to be easy keepers. While no evidence directly links EMS with geography or season, an indirect association between EMS and geography with regard to high pasture quality may occur.
• Clinical signs: general obesity as well as increased fat accumulation in specific locations, also called regional adiposity (neck, shoulder bed, tailhead, sheath, mammary gland); insulin dysregulation; predisposition to development of laminitis depending on other influences such as genetic and management factors, including overfeeding.
• Risk factors: genetic predisposition; excessive feeding of carbohydrates, usually over years; sedentary lifestyle with little or no exercise.
• Diagnosis: physical examination and clinical signs, most notably obesity and laminitis, generally point to EMS; diagnostic tests for EMS include endocrine and metabolic status panel, oral sugar challenge test, and the two-step insulin-response test; confirmation of systemic insulin dysregulation, including postprandial hyperinsulinemia.
Nutrient requirements for EMS
Because obesity is a characterizing feature of EMS, nutritional considerations center on managing weight loss and, when horses are sound, instituting an exercise program to improve insulin sensitivity. Unsound horses can be encouraged to move in drylots or turnout areas by thoughtful placement of water, forage, and salt at opposite ends of the enclosure or through the addition of a younger, more playful pasturemate.
• Forage: maximizing calories from forage supports gastrointestinal health and fosters normal foraging behavior; pasture access should be limited or restricted in horses with EMS, though extent of restriction depends on the degree of obesity, as some horses are able to be turned out on pasture with a grazing muzzle, while others must be maintained in a drylot or kept off pasture completely; mature grass hay fed at a rate of 1.25-1.5% of body weight per day and provided in a way that slows consumption (e.g., slow-feed haynet); hay low in NSC (less than 12%) should be selected for horses with insulin resistance; when hay contains marginal levels of NSC, it can be soaked prior to feeding.
• Concentrate: for EMS horses in low or moderate body condition, low-starch pelleted feed can be given at the rate recommended by the manufacturer and fat supplements can be added to provide additional energy; for obese horses, no commercial feeds or straight grains should be given, though a well-formulated balancer pellet to satisfy protein, vitamin, and mineral requirements should be fed.
• Supplements: horses without access to fresh forage should be fed a natural-source vitamin E such as Nano-E®; body-wide inflammation can be reduced with omega-3 fatty acids, the most bioavailable of which are obtained from marine-derived sources, like EO-3™; a well-formulated vitamin and mineral supplement, like Micro-Max™, can provide essential nutrients to horses on allforage diets.
Strategic nutritional management will cure neither PPID nor EMS. In addition to dietary changes and clipping of the coat in hot weather, horses with PPID often show clinical improvement with the help of certain medications, most prominent of which is pergolide. Moreover, adherence to a strict diet and exercise program can stave off laminitis in some horses diagnosed with EMS. Metformin, a medication that increases insulin sensitivity, has been used in horses with insulin dysregulation. Revising a diet to fit the parameters given within these guidelines can be challenging. If necessary, seek the assistance of a qualified equine nutritionist.
Feeding Suggestions for Laminitis-Prone Horses
Horses and ponies with PPID and EMS are prone to laminitis. Nutritional countermeasures may, however, keep susceptible horses from developing laminitis. Dietary management focuses on restricting intake of nonstructural carbohydrates (NSC), namely sugars, starches, and fructans. The consumption of these increases the risk of laminitis through hyperinsulinemia or hindgut disturbances in laminitis-prone horses.
• Diets should be based on appropriate forage, such as grass hay (or hay substitute) with a low NSC content (less than 12% dry matter) fed at a rate of approximately 1.5-2% body weight. Mature hay typically has lower digestible energy and NSC content when compared to less mature grass hay and legumes, such as alfalfa and clover. NSC content also depends on environmental factors during growth and harvesting. Whenever possible, a forage analysis should be performed on hay intended for horses with PPID and EMS. Soaking hay for 30-60 minutes before feeding is thought to leach sugars and fructans, and may be prudent in the absence of a forage analysis.
• Restrict or avoid access to pasture. The NSC content of some forages can escalate to 40% dry matter at certain times of the year. Consumption of pasture can be controlled through the regular use of a well-fitted grazing muzzle; strip-grazing behind other horses or sheep; or mowing the pasture short and removing the clippings. Other grazing considerations include choosing a time of day when NSC are lowest in plants (late at night through early morning), avoiding spring or autumn grazing (before flower development or seeding), and steering clear of stressed grasses, such as those subjected to frost or drought. In situations that call for severe restriction of NSC intake, no grazing should be allowed, though an appropriate forage alternative can be fed in a drylot.
• Feed a low-calorie balancer pellet or an appropriate vitamin and mineral supplement to horses and ponies on an all-forage diet. A balancer pellet usually contains a source of high-quality protein such as soybean meal. For aged horses or those with signs of muscle wasting, a balancer pellet might be more appropriate than a vitamin and mineral supplement.
• Avoid feedstuffs high in NSC, such as straight cereal grains (plain oats, for example) or sweet feeds. Sweet feeds that contain primarily cereal grains and molasses can boost the NSC content to 45-50%.
• For lean horses or horses in work, other feedstuffs may be necessary for maintenance of weight. Concentrates low in starch and sugar content (15-25% NSC) when compared to traditional concentrates (30-50% NSC) may be appropriate in some instances. Energy is usually conferred by fat (vegetable oil, stabilized rice bran) and fermentable fiber (beet pulp, soy hulls) sources in these low-starch feeds. In other situations, unmolassed, soaked beet pulp or hay cubes with or without added vegetable oil can provide calories to horses at risk for laminitis.
• Monitor body weight through regular weighing or body condition scoring. Attention to changes in weight or body condition score can keep horses in an acceptable weight range.
A nutritionist or veterinarian may suggest other management strategies based on the individual horse.
When to Graze: Protecting At-Risk Horses
During photosynthesis plants convert sunlight, water, and carbon dioxide into oxygen and chemical energy in the form of sugar. Plants use sugars to support metabolism, growth, and tissue repair. When photosynthetic production of sugars outpaces the immediate energy requirements of plants, sugars transform into one of two storage carbohydrates, starch or fructan. Warm-season grasses accumulate sugars and starch, but cool-season grasses accumulate sugars, starch, and fructans
Because photosynthesis only occurs in the presence of sunlight, plants produce and store carbohydrates during the day. At nighttime, when photosynthesis is on hold, plants use stored carbohydrates to power physiologic processes. Given this cycle of accumulation and depletion, carbohydrate levels are generally highest from 10:00 a.m. to 11:00 p.m., making the hours between 5:00 a.m and 10:00 a.m. the safest, most practical time for laminitis-prone horses to graze.
Aside from time of day, carbohydrate levels vary widely based on temperature, light intensity, moisture content, soil fertility, and genetic potential of a plant species. Further, stress encourages plants to store starch and fructan rather than using it for growth. Common stressors include drought, frost, temperatures below 40° F (4° C), soil infertility, and poor pasture management, all of which might make grazing risky for laminitis-prone horses, even during hours when grasses are traditionally considered safer.
When setting out to create a safe grazing space for at-risk horses, sensible pasture management comes into play. Regular mowing and grazing is essential as both stimulate growth. Plants tap into their carbohydrate reserves to push new growth, which reduces accumulation of starch and fructan within the plant.
The presence of insulin dysregulation in certain horses diagnosed with PPID indicates EMS and PPID can coexist. According to researchers, testing for PPID using ACTH concentrations or thyrotropin-releasing hormone stimulation test should be considered in horses with insulin dysregulation and/or laminitis, especially in horses over 10 years of age.
Differential Diagnoses
Aside from PPID and EMS, few other endocrinopathies are diagnosed in horses. Differential diagnoses include diabetes mellitus (persistent hyperglycemia resulting from defects in insulin secretion, insulin action, or both), neoplasia-associated hypoglycemia (such as insulinoma, a usually benign pancreatic neoplasm characterized by uncontrolled insulin secretion and consequent hypoglycemia), thyroid disorders (hypothyroidism, hyperthyroidism, and hyperparathyroidism), and adrenal diseases. An experienced veterinary specialist can help sort through the clinical signs and diagnostic tests necessary to achieve an accurate diagnosis.
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D. 2011. Equine pituitary pars intermedia dysfunction. Veterinary Clinics of North America Equine Practice 27:93-113.
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