Cushing’s Disease in Horses

Equine Cushing’s Disease is the common name for Pituitary Pars Intermedia Dysfunction (PPID), is a common but complex endocrine disorder primarily affecting older horses. This condition results from an abnormal growth or dysfunction in the pituitary gland, leading to an overproduction of hormones, particularly adrenocorticotropic hormone (ACTH). This hormone imbalance affects various bodily functions and can cause a wide range of symptoms.

What causes Cushing’s?

PPID is caused by dysfunction in the pars intermedia, a part of the pituitary gland located near the brain. In healthy horses, this area helps regulate hormones like ACTH. In horses with Cushing’s, the gland enlarges (due to a benign tumour or hyperplasia), causing excessive ACTH production. This overstimulates the adrenal glands, resulting in excess cortisol in the bloodstream. High cortisol levels disrupt normal metabolic processes.

Cushing’s is most common in older horses but can affect younger horses in rare cases

What are the symptoms?

Cushing’s Disease can present with a variety of symptoms, especially in horses over the age of 12:

• Hirsutism: The most common sign of Cushing’s is the growth of an abnormally long, curly coat. Affected horses often fail to shed their winter coat properly.
• Laminitis Inflammation and damage to the tissues inside the hoof, is a serious condition, commonly linked to PPID.
• Other symptoms:  Horses with Cushing’s disease may experience increased thirst and urination, weight loss and muscle wasting, abnormal fat distribution (e.g., fat pads in unusual areas), lethargy and poor performance, and recurrent infections due to a weakened immune system.

How is it diagnosed?

Most cases of Cushing’s are identified through observation of the symptoms above and confirmed by blood tests. In younger horses or less obvious cases, there are three additional hormone tests:

• ACTH Measurement: this detects elevated levels of the hormone ACTH in the blood.
• Dexamethasone Suppression Test (DST): this measures cortisol response after administering a corticosteroid.
• TRH Stimulation Test: this newer test assesses the pituitary’s response to thyrotropin-releasing hormone.

How is it treated?

While there is no cure for Cushing’s Disease, treatment focuses on managing symptoms and slowing progression.

We usually treat with daily Pergolide, a medication which reduces excessive hormone production from the pituitary gland. To reduce the risk of Laminitis, a low-sugar, low-starch diet is essential. Your Stride vet can advise you on regular hoof care (again, for Laminitis), tooth care with regular check-ups and a suitable worming programme.

Regular blood tests every 6 months are advised while on Pergolide, to ensure the correct dose is being given.

Prognosis:

As an owner, you should be aware that this is a lifelong, progressive condition but, with appropriate management, many horses with Cushing’s can live comfortable, healthy lives. You should regularly monitor for changes in your horse’s general body condition and, if you have any doubts, speak to your vet immediately.