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