Feeding horses with pituitary pars intermedia dysfunction (PPID), also known as equine Cushing's disease, can sometimes be difficult because horses affected with PPID are often older, underweight, and may have insulin resistance with or without recurrent laminitis.

To make the best nutritional recommendations for horses with PPID, veterinarians must first consider these questions:

1) Does the horse need to gain or lose weight?

2) Is insulin resistance present?

Regardless of the answers to these questions, horses with PPID should be fed a primarily forage diet that will maintain (or regain) ideal body condition and to avoid obesity, which will worsen insulin resistance.

Not all horses affected with PPID also have insulin resistance. 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. Feeding small (less than 0.5% of body weight) and frequent concentrate meals produces less deviation in insulin and glucose concentrations and is less likely to complicate insulin resistance.

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. 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. Whenever possible, horses with PPID should be allowed turnout time, as exercise improves insulin sensitivity. Forced exercise or work should be reserved for horses that do not have active laminitis and are free of musculoskeletal disorders.

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. Adrenocorticotrophic hormone (ACTH) concentrations have been shown to have seasonal variation and be highest in September. Insulin concentration is positively associated with increased carbohydrates in pasture grass.

Frank and coworkers measured ACTH concentrations over a one-year period in mature to aged horses kept at pasture. Mean ACTH concentration was highest from August through October. Insulin concentration differed over time in horses affected with PPID. Glucose and insulin concentrations peaked in September, which combined with the seasonal peak in ACTH concentration, may contribute to an increased risk of pasture-associated laminitis risk in the fall.

Many horses affected with PPID are healthy and relatively asymptomatic other than hypertrichosis and mild to moderate polyuria and polydipsia. Horses with PPID and good BCS score without evidence of insulin resistance can essentially be fed as a normal horse to maintain body condition and prevent obesity. Senior-type feeds, sweet feed, or oats with hay can be fed if additional dietary energy is needed.

Obese horses with PPID should be fed similar to horses with equine metabolic syndrome (EMS) to encourage weight loss, improve insulin sensitivity, and reduce the risk of laminitis. A forage- or fiber-based diet is ideal for obese horses with PPID to provide gut fill and supply energy in the form of volatile fatty acids.

Pituitary pars intermedia dysfunction is a disease of older horses and many are underweight as a consequence of senescence or dental problems. Nutritional recommendations for horses with PPID that need to gain weight are more difficult because the need for more calories must be balanced against the adverse effects of worsening insulin resistance.

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. Because a high-fat diet may worsen glucose tolerance and moderate carbohydrate intake improves insulin response, it is advisable to use a combination of fats and carbohydrates along with additional forage to promote weight gain in thin horses with PPID.

Reprinted courtesy Kentucky Equine Research.

David Pugh, PhD