Equine Cushing's Disease (PPID): Signs and Management

The Disease That Is Hiding in Your Senior Horse

If your horse is over 15 years old and you have not tested for Cushing's disease, you are flying blind. That is not alarmism — it is epidemiology. Studies estimate that Pituitary Pars Intermedia Dysfunction (PPID), the condition we commonly call Cushing's disease in horses, affects more than 20 percent of horses over 15 and possibly 30 percent or more of horses over 20. Those numbers mean that in any barn with a handful of senior horses, at least one of them likely has PPID, whether anyone knows it or not.

I have diagnosed PPID in horses whose owners were convinced they were "just getting old." The muscle wasting, the pot belly, the slow recovery from minor infections — all chalked up to age. And every time, when we start treatment and the owner sees their horse regain muscle, shed its coat normally, and move with energy again, the same reaction: "I had no idea. I thought this was just what getting old looked like." It does not have to be. PPID is treatable, and treatment changes lives.

Understanding the Disease: This Is Not Your Dog's Cushing's

First, let me clear up a common confusion. Cushing's disease in dogs and humans is caused by a pituitary tumor (adenoma) that produces excess adrenocorticotropic hormone (ACTH), which drives the adrenal glands to produce too much cortisol. It is a tumor problem.

PPID in horses is fundamentally different. It is a neurodegenerative disease — a slow, progressive loss of dopaminergic neurons (nerve cells that produce dopamine) in the hypothalamus. These dopaminergic neurons normally exert tonic inhibition on the pars intermedia of the pituitary gland. Think of dopamine as the brake pedal keeping the pars intermedia in check. As those neurons degenerate with age, the brake pedal is slowly released.

Without dopamine suppression, the pars intermedia enlarges (hypertrophy and hyperplasia — the cells grow bigger and multiply) and begins producing excessive amounts of pro-opiomelanocortin (POMC)-derived peptides, including ACTH, alpha-melanocyte stimulating hormone (alpha-MSH), and beta-endorphin. The excess ACTH drives cortisol production from the adrenal glands, but the relationship is more complex than simple hypercortisolism. The cocktail of POMC peptides creates a cascade of metabolic dysfunction that affects nearly every system in the body.

This distinction matters because it explains why PPID is age-related (neurodegenerative diseases are), why it is progressive (the neurons continue to die over time), and why the treatment targets dopamine replacement rather than tumor removal.

The Signs: Most Owners Catch It Late

The sign everyone knows is the curly, wavy coat that does not shed — hypertrichosis (commonly called hirsutism). The horse grows a long, often curly winter coat and fails to shed it in the spring. By summer, the horse is wearing a full winter blanket of fur while its pasturemates are sleek and shiny. This sign is unmistakable, and when I see it, the diagnosis is essentially clinical. No test needed — that horse has PPID.

But here is the problem: hirsutism is a late sign. By the time a horse has a persistently long coat, the disease has been progressing for months to years. The early signs are far more subtle, and catching them can make the difference between proactive management and crisis management.

Early Signs of PPID

• Muscle wasting along the topline: Loss of muscle over the back, croup, and haunches, even in horses that are still in work. The horse develops a "sway-backed" appearance. Many owners attribute this to aging, and while some muscle loss is normal in senior horses, the degree seen in PPID is disproportionate.
• Abnormal fat distribution: Instead of losing fat uniformly, PPID horses redistribute it. You will see fat deposits in characteristic locations — a thick, cresty neck, bulging supraorbital fat pads (the fatty pockets above the eyes that give the horse a "heavy-browed" look), and sometimes a pendulous belly contrasted with a wasted topline. The combination of topline wasting with a cresty neck and pot belly is highly suggestive.
• Recurrent infections: PPID suppresses immune function. The horse that gets one respiratory infection after another, develops skin infections (including recurrent rain rot and thrush) that respond slowly to treatment, or has slow wound healing should raise a red flag. Sole abscesses that recur frequently are another clue.
• Excessive drinking and urination (polyuria/polydipsia): Cortisol excess interferes with the kidney's ability to concentrate urine. The horse drinks more, urinates more, and the stall is wetter than normal. Water bucket consumption that is noticeably increased over baseline is worth monitoring.
• Laminitis: This is the big one, and I will address it in detail below. Any horse over 15 that develops laminitis — especially without an obvious dietary or mechanical trigger — should be tested for PPID. Period.
• Delayed shedding: Before the full hirsute coat develops, many PPID horses simply shed late. Their pasturemates are slick by May and they are still patchy in June. This is the earliest coat-related sign and it is easily dismissed.
• Lethargy and decreased performance: The horse seems dull, less willing to work, less interested in its surroundings. Again, often attributed to age, but the degree of change may be disease-related.

The Laminitis Connection: Why PPID Can Kill

PPID itself is not typically fatal. What kills PPID horses is laminitis — and the relationship between the two is direct and devastating.

Excess cortisol from PPID causes insulin dysregulation, a state in which the body's normal insulin signaling becomes impaired. Tissues become resistant to insulin, blood insulin levels rise (hyperinsulinemia), and the elevated insulin directly damages the lamellar tissue of the hoof. The laminae — the interlocking sheets of tissue that suspend the coffin bone inside the hoof capsule — weaken and fail. The coffin bone rotates, sinks, or both. In severe cases, the bone penetrates through the sole. It is agonizing, and in advanced cases, it is career-ending or fatal.

This is not theoretical. In my practice, the most common presentation for a new PPID diagnosis is a laminitis episode. The horse founders, I run blood work, ACTH is elevated, and suddenly the muscle wasting and cresty neck that the owner had noticed but dismissed make sense. The laminitis was not bad luck — it was the disease reaching a critical threshold.

My rule: any horse over 15 years old with unexplained laminitis should be tested for PPID. And by "unexplained," I mean laminitis that does not have an obvious dietary trigger (grain overload, lush pasture in an unacclimated horse) or mechanical cause. If the horse simply developed sore feet and there is no obvious reason, test for PPID. The blood draw takes five minutes and can save the horse's life.

Diagnosis: Timing Matters

PPID is diagnosed by measuring endogenous ACTH levels in the blood. But there is a critical nuance: ACTH has a natural seasonal rise in the fall (August through November in the Northern Hemisphere). Normal horses experience a surge in ACTH during this period, and if you use the same reference range year-round, you will get false positives in the fall and false negatives the rest of the year.

Resting ACTH

A simple blood draw measuring baseline ACTH is the first-line test. The sample must be handled carefully — ACTH degrades rapidly at room temperature. The blood should be centrifuged and the plasma separated within four hours, or the whole blood sample should be chilled immediately and shipped on ice to the lab. Using seasonally adjusted reference ranges (your lab should provide these) is essential for accurate interpretation.

TRH Stimulation Test

When resting ACTH is equivocal — borderline values that could go either way — I use the TRH (thyrotropin-releasing hormone) stimulation test. This involves drawing a baseline ACTH, injecting 1 mg of TRH intravenously, and drawing a second ACTH sample 10 minutes later. In PPID horses, the dysfunctional pars intermedia overreacts to TRH, producing an exaggerated ACTH spike that is diagnostically clear even when the resting level was borderline. This test is more sensitive than resting ACTH alone, particularly in early disease.

Practical note: TRH injection can cause transient side effects — muscle fasciculations, flehmen response, yawning, and brief anxiety. These resolve within 10 to 15 minutes and are harmless, but warn the owner so they are not alarmed.

Treatment: Pergolide Changes Everything

The mainstay of PPID treatment is pergolide mesylate, a dopamine agonist that replaces the inhibitory dopamine signal that the degenerating neurons can no longer provide. The FDA-approved equine formulation is Prascend (pergolide 1 mg tablets).

Starting and Titrating

I start most horses at 1 mg (one tablet) once daily, given orally. Some veterinarians start at 0.5 mg in smaller horses or horses they suspect have early disease, though the standard starting dose is 1 mg regardless of body weight (PPID is a pituitary disease, and dosing is based on the gland's dysfunction, not body mass).

The most common side effect during initiation is decreased appetite. Some horses become transiently inappetent for the first few days to a week. This is usually mild and self-limiting. If the horse stops eating entirely, I reduce the dose to 0.5 mg for a week and then increase back to 1 mg. Severe or prolonged inappetence is rare but does occur.

Recheck ACTH 30 days after starting treatment or after any dose change. The goal is to bring ACTH into the seasonally adjusted normal range. If ACTH remains elevated at 1 mg, increase to 1.5 mg or 2 mg and recheck in another 30 days. Some horses — particularly those with advanced disease — may require 3 mg or more. I have a handful of patients on 4 to 5 mg daily. Titrate to the lab values, not to clinical signs alone.

Long-Term Monitoring

Once ACTH is controlled and the dose is stable, I recommend rechecking ACTH every six months — once in the spring (non-seasonal baseline) and once in the fall (to assess control during the seasonal rise). Many horses need a dose increase during the fall months (September through November) and can return to their baseline dose in winter. This seasonal adjustment is part of responsible PPID management.

Management Beyond Pergolide

Medication alone is not enough. PPID horses need comprehensive management to stay healthy and comfortable.

Coat Management

Horses with hirsutism suffer in warm weather. A thick, curly coat prevents effective thermoregulation, and PPID horses can overheat, sweat excessively, and lose condition in summer. Body clipping in the spring — and sometimes a second clip in late summer — is not cosmetic; it is medical. Clip the horse as you would any horse in work: trace clip, hunter clip, or full clip depending on how heavy the coat is. It makes an immediate, visible difference in comfort and energy level.

Diet

Because of the insulin dysregulation associated with PPID, dietary management focuses on low non-structural carbohydrate (NSC) intake. NSC includes sugars and starches — the components of feed that spike blood glucose and insulin.

• Feed grass hay with an NSC content below 10 percent. If you do not know your hay's NSC, have it analyzed (most agricultural extension services offer this) or soak the hay for 30 to 60 minutes before feeding to leach out soluble sugars.
• Avoid grain-based concentrates. Use a low-NSC ration balancer or beet pulp (unmolassed) as a carrier for supplements.
• Manage pasture access carefully, especially in the spring and fall when grass NSC is highest. Grazing muzzles, limited turnout during high-sugar hours (late afternoon on sunny days), or dry lot turnout may be necessary.
• Provide adequate protein for muscle maintenance — senior feeds or ration balancers with 14 percent or higher protein help combat the muscle wasting associated with PPID.

Hoof Care

Given the laminitis risk, PPID horses need attentive hoof care. A regular farrier schedule (every five to six weeks), radiographic monitoring if there is any history of laminitis, and prompt attention to any sign of foot soreness are essential. I often recommend therapeutic shoeing or boots for PPID horses with compromised hoof structure.

Dental Care

PPID horses are often senior horses, and dental disease is common. Poor dentition leads to poor feed utilization, weight loss, and nutritional deficiency — all of which are worse in a horse already fighting PPID. Annual dental exams and floating are baseline; many PPID horses benefit from twice-yearly dental care.

Infection Prevention

Because PPID compromises immune function, vaccination and parasite management are particularly important. Keep vaccinations current, follow a fecal egg count-based deworming program, and address any infections (skin, respiratory, dental) promptly rather than waiting to see if they resolve on their own. They often will not, because the immune system is impaired.

Prognosis: Better Than You Think

Here is the good news, and I want to end on this because too many owners hear "Cushing's disease" and assume the worst. The prognosis for treated PPID is good to excellent. Most horses respond well to pergolide, regain muscle and energy, shed their coats, and live comfortably for years — often many years — on medication. I have patients that were diagnosed at 18 and are still going strong at 25, competing at lower levels, trail riding, and enjoying life.

The horses that do poorly are the ones diagnosed late — after severe laminitis has caused irreversible structural damage to the hooves, or after prolonged immunosuppression has allowed chronic infections to become entrenched. That is why early detection matters so much. A $50 blood test in a seemingly healthy 16-year-old horse can catch PPID before laminitis ever happens, and a $2-per-day medication can keep it under control for the rest of the horse's life.

If you have a horse over 15, ask your vet about PPID testing at your next routine visit. If your horse is showing any of the early signs I described — topline wasting, cresty neck, slow shedding, recurrent infections, unexplained laminitis — do not wait for the next routine visit. Call now. The earlier we catch this disease, the more years of comfortable, productive life your horse has ahead of it. That is a conversation worth having today.