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Pituitary pars intermedia dysfunction (PPID, formerly known as equine Cushing's disease) is a very common endocrine disease most commonly seen in geriatric horses. The disease is caused by a loss of inhibition to the pars intermedia of the pituitary gland in the horse's brain. This leads to an overproduction of adrenocorticotropic hormone (or ACTH).

The most common clinical sign in horses with PPID is the development of an abnormally long hair coat (hypertrichosis) that often fails to shed out appropriately. It's important to note that while it an abnormal haircoat is very important in PPID horses, not all affected horses develop hypertrichosis; as such, horses with other signs of PPID but a normal haircoat should still be considered candidates for disease.

Other common clinical signs include:

  • Increased thirst (polydipsia);

  • Increased urination (polyuria);

  • Muscle wasting;

  • Difficulty regulating body temperature;

  • Changes in ability to sweat;

  • Reproductive changes; and 

  • Insulin dysregulation (ID).

PPID generally affects geriatric horses (15 years old and up) but can affect younger horses, as well. It can affect horses of any breed.​

The recommended dynamic test for PPID is the thyrotropin-releasing (TRH) hormone stimulation test, in which ACTH levels are analyzed before and 10 minutes after TRH administration.

However, if a horse has hypertrichosis, a resting ACTH level test can be sufficient for diagnosis.

It is important to assess insulin function in PPID or potentially affected horses, as horses with both PPID and EMS/ID are at increased risk for laminitis.

Currently, Prascend (pergolide mesylate) is the only FDA-approved treatment for PPID. This medication is given orally and must be prescribed by your veterinarian.

It is recommended to start horses on the lowest dose of Prascend and monitor ACTH levels regularly when starting treatment. If ACTH levels remain uncontrolled on the lowest dose, your veterinarian may recommend increasing the dose your horse receives each day.

Remember, during treatment, changes in clinical signs associated with PPID (for example, hair coat) can take some time to see. Don't assume treatment isn't working because your horse's appearance doesn't change rapidly!

There are also non-pharmaceutical methods you can use to help manage your PPID horse, including:

  • Body clipping when they do not shed out;

  • Providing shelter from rain/wind/sun to assist with temperature regulation; and

  • Ensuring free access to water and a salt/mineral block.

Our goals are to:


  • Better understand how PPID may affect the immune responses of horses and their ability to resist to infection;

  • Learn why some horses develop PPID and others don't, along with why some horses with PPID are ID and others aren't; and

  • Find ways to manage horses with PPID nutritionally.

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