Other Names
- Cushing’s Disease
- PPID (Pituitary Pars Intermedia Dysfunction)
- Equine Cushing’s
What is Cushing’s Disease (PPID) in Horses?
Pituitary pars intermedia dysfunction (PPID), more widely known as Cushing’s disease, is the most common endocrine (hormonal) disorder in horses. It is a progressive, age-related condition caused by dysfunction of the pars intermedia — the middle lobe of the pituitary gland — resulting in the overproduction of several hormones, with wide-ranging effects on the horse’s metabolism, immune function, and coat.
PPID is primarily a disease of older horses. Studies suggest it affects around 20–30% of horses over 15 years of age, though it can occasionally occur in younger animals. There is no breed predisposition, though some evidence suggests native ponies, particularly those with a concurrent diagnosis of equine metabolic syndrome (EMS), may be more susceptible.
With early diagnosis and appropriate management, horses with PPID can have a good quality of life and remain in work for many years. The key is recognising the signs early and acting on them.
What Causes PPID in Horses?
The pars intermedia of the pituitary gland is normally regulated by dopaminergic neurons originating in the hypothalamus. These neurons produce dopamine, which acts as an inhibitory signal — keeping hormone production from the pars intermedia in check.
In PPID, these dopaminergic neurons progressively degenerate and die, an oxidative stress-related process associated with ageing. As dopamine levels fall, the pars intermedia is released from inhibitory control and begins to hyper-secrete a family of hormones collectively derived from a precursor protein called pro-opiomelanocortin (POMC). The most clinically significant of these is adrenocorticotrophic hormone (ACTH), though multiple POMC-derived peptides contribute to the clinical picture.
Elevated ACTH stimulates the adrenal glands to produce excessive cortisol, leading to many of the classical signs of Cushing’s disease.
Signs and Symptoms of PPID
PPID progresses gradually. Early signs are subtle and easily attributed to normal ageing. As the disease advances, signs become more pronounced and distinctive.
Classic Signs (Established Disease)
- Hypertrichosis (long, curly, failure-to-shed coat): The most recognisable sign of PPID. The coat grows abnormally long, thick, and curly, and fails to shed in spring. This is caused by altered cortisol regulation affecting the hair follicle growth cycle. It is seen in advanced disease and is rarely the first sign.
- Excessive sweating: Often disproportionate to exercise and temperature.
- Increased thirst and urination (polydipsia and polyuria): Significant increases in water consumption and urine output. Horses may drink noticeably more and urinate frequently.
- Weight loss and muscle wasting: Particularly over the topline and hindquarters, despite adequate feeding. Chronic elevated cortisol drives muscle protein catabolism.
- Pot-bellied appearance: Redistribution of fat and loss of core muscle tone leads to a pendulous abdomen.
- Lethargy and reduced performance: Loss of energy, dullness, decreased willingness to work.
- Recurrent laminitis: One of the most significant and dangerous consequences of PPID. Chronic elevated insulin (secondary to ACTH-driven cortisol) predisposes strongly to recurrent or seasonal laminitis.
Early Signs (Before Classic Coat Changes)
- Fat pads above the eye sockets (supraorbital fat deposition)
- Subtle coat changes — delayed shedding or coat that never looks quite right
- Mild weight or muscle changes in an otherwise well-fed horse
- Recurrent foot problems in a horse that has no prior history
- Increased susceptibility to infections — dental disease, skin infections, recurrent foot abscesses — due to immunosuppression caused by elevated cortisol
Diagnosis
Diagnosis of PPID combines clinical assessment with blood testing:
- Resting ACTH measurement: A blood sample measuring plasma ACTH is the most widely used diagnostic test. ACTH levels are normally elevated in autumn in all horses — reference ranges are therefore season-adjusted. Autumn samples require interpretation using autumn-specific reference ranges. A single elevated ACTH result in the appropriate clinical context is typically sufficient for diagnosis in horses with clear clinical signs.
- Thyrotropin-releasing hormone (TRH) stimulation test: Used in horses where PPID is suspected but resting ACTH is borderline. TRH is administered IV and ACTH is measured 10 minutes later. An exaggerated ACTH response indicates loss of normal pituitary regulation consistent with early PPID. This test is more sensitive for detecting early or mild disease.
- Dexamethasone suppression test: Previously the gold standard, now less commonly used in the UK due to the risk of precipitating laminitis in susceptible horses.
Treatment
Pergolide (Prascend)
Pergolide mesylate (licensed equine product: Prascend) is the primary and most effective treatment for PPID. It is a dopamine receptor agonist that replaces the function of the lost dopaminergic neurons, restoring inhibitory control over the pars intermedia and reducing ACTH overproduction.
Pergolide is given once daily, typically mixed into a small amount of feed. The standard starting dose is 2 micrograms/kg, which can be increased if the initial response is insufficient. Treatment is lifelong. Clinical improvement is usually seen within 1–3 months, with coat changes typically requiring a full season cycle to normalise.
The most common side effect is transient reduced appetite (“pergolide veil”) in the first 1–2 weeks after starting treatment or increasing the dose. Dividing the daily dose or reducing temporarily can help manage this.
Dietary Management
- Horses with PPID are at risk of insulin dysregulation and laminitis — their diet should be managed as for an EMS horse: low sugar, low starch
- Hay should be soaked for high-risk individuals to reduce non-structural carbohydrate content
- Avoid lush spring and autumn pasture or use a grazing muzzle
- Feed a balancer or appropriate supplementation to maintain condition and topline
Preventive Veterinary Care
Horses with PPID have compromised immune function and require more vigilant preventive care:
- Regular dental checks (at least annually, often 6-monthly) — dental disease is more prevalent and progresses faster
- Regular deworming with faecal egg counts — immune compromise increases parasite burden
- Vaccinations kept up to date
- Vigilant hoof care — frequent farrier visits, particularly in horses with a history of laminitis
- Body clipping in summer for horses with significant hypertrichosis to prevent heat stress
Monitoring Treatment
ACTH levels should be rechecked 4–6 weeks after starting treatment or changing dose to assess response. Annual (or biannual) ACTH monitoring is recommended for horses on long-term pergolide to ensure the dose remains adequate as the disease progresses.
Prognosis
PPID is a progressive condition with no cure. However, with appropriate pergolide therapy and management, most horses with PPID maintain a good quality of life and can remain in light to moderate work for years. The prognosis is significantly worsened by concurrent laminitis or advanced disease at the time of diagnosis.
Horses with PPID that are well-managed, have their coat condition normalised, and are protected from laminitis can live comfortably into old age. Regular monitoring and willingness to adjust treatment as the disease progresses are the keys to long-term successful management.
When to Call the Vet
Contact your vet if:
- A horse over 15 years old develops any of the classic signs — particularly hypertrichosis, increased thirst/urination, or recurrent laminitis
- An older horse shows unexpected weight or topline loss despite adequate feeding
- A previously well-managed PPID horse shows deterioration or signs of poorly controlled disease
- Autumn laminitis or unexplained foot issues recur each year in an older horse
Frequently Asked Questions About Cushing’s Disease in Horses
Is Cushing’s disease the same as equine metabolic syndrome (EMS)?
No — PPID and EMS are distinct conditions, though they share some features (particularly insulin dysregulation and laminitis risk) and can co-exist in the same horse. PPID is caused by pituitary dysfunction and primarily affects older horses; EMS is a metabolic disorder affecting insulin sensitivity and is more common in middle-aged, obese or easy-keeper horses. A horse can have both simultaneously — this is important to test for, as treatment requirements differ.
Can younger horses get Cushing’s disease?
PPID can occur in horses under 15 years, though it is much less common. Cases as young as 7–8 years have been reported. Young horses showing signs consistent with PPID — particularly unexplained hypertrichosis, polyuria, or recurrent laminitis without an obvious metabolic cause — should be tested.
How much does pergolide treatment cost?
Prascend is a prescription veterinary medicine. Costs vary but are generally considered manageable compared to the consequences of uncontrolled PPID. Your vet will prescribe the appropriate dose and can advise on the most cost-effective approach. Some horse insurance policies cover ongoing medication for conditions like PPID — check your policy.
Do horses with Cushing’s disease need a different vaccination or dental schedule?
Yes. Due to immunosuppression, horses with PPID benefit from more frequent dental checks (every 6 months is often recommended) and should be kept fully up to date on all vaccinations. Worm egg counts should be monitored regularly as parasite burdens tend to be higher. More frequent farrier visits are also advisable, particularly in horses with a history of laminitis.