Cushing's Disease in Horses (PPID): Signs, Testing, and Management

That old horse in the pasture with the long, curly coat that never quite sheds out? The one who's lost topline despite eating well, drinks water like he's emptying a pond, and maybe had a mysterious bout of laminitis last fall? There's a good chance he has PPID. Pituitary Pars Intermedia Dysfunction, still commonly called Cushing's disease, is the most prevalent endocrine disorder in horses over 15. It's also one of the most manageable once you understand what's happening and commit to the program. Horses with PPID live productive, comfortable lives for years after diagnosis. The ones that struggle are usually the ones diagnosed too late or managed inconsistently.

What PPID Actually Is

Cushing's disease in horses is fundamentally different from Cushing's syndrome in humans and dogs, which typically involves adrenal tumors or pituitary macroadenomas producing ACTH. In horses, the problem starts with neurodegeneration.

Deep in the brain, dopaminergic neurons in the hypothalamus normally send inhibitory signals to the pars intermedia, the middle lobe of the pituitary gland. Dopamine acts as a brake, keeping the pars intermedia's hormone production in check. In PPID, these dopaminergic neurons progressively degenerate and die. Nobody knows exactly why, though oxidative stress and aging appear central to the process.

Without dopamine's restraining influence, the pars intermedia cells proliferate unchecked. They produce excess pro-opiomelanocortin (POMC), which gets cleaved into multiple hormones including ACTH, alpha-melanocyte stimulating hormone (alpha-MSH), beta-endorphin, and CLIP. The resulting hormonal cascade produces the clinical signs we recognize.

Elevated ACTH drives the adrenal glands to produce excess cortisol. Chronically elevated cortisol suppresses immune function, disrupts glucose metabolism, alters fat distribution, and interferes with the normal hair growth cycle. Alpha-MSH likely contributes to the characteristic coat changes. Beta-endorphin may explain why some PPID horses seem unusually docile or "quiet" despite significant pathology.

This isn't a condition horses develop overnight. The neurodegeneration is gradual, spanning years. By the time clinical signs become obvious, significant pituitary changes have already occurred. That's why early detection matters.

Recognizing the Signs

The classic PPID horse has a long, curly coat (hypertrichosis) that fails to shed properly in spring. It's the most recognizable sign and the one that prompts most diagnoses. But it's also a relatively late sign. By the time coat changes are obvious, the disease has been progressing quietly for a while.

Earlier, subtler signs that deserve attention:

• Delayed or incomplete shedding: Not the full woolly coat yet, but patches that hang on longer than normal, especially along the jaw, behind the ears, and on the legs
• Muscle wasting: Loss of topline muscle along the back and hindquarters, sometimes described as a "sway-backed" appearance. The belly may appear pendulous as abdominal muscles weaken
• Fat redistribution: Abnormal fat deposits above the eyes (supraorbital fat pads), along the crest of the neck, and at the tail head, even as the horse loses muscle mass elsewhere
• Increased water intake and urination (PU/PD): Cortisol interferes with antidiuretic hormone, leading to dilute urine and compensatory thirst
• Lethargy and decreased performance: Often attributed to "just getting old" but may reflect hormonal disruption
• Recurrent infections: Skin infections, sole abscesses, sinusitis, or slow wound healing reflect cortisol-mediated immune suppression
• Laminitis: This is the big one. PPID is one of the leading causes of endocrinopathic laminitis in older horses. Laminitis in a horse over 15 with no other obvious cause should always trigger PPID testing

Regional fat deposits combined with muscle wasting create a distinctive body shape: thick cresty neck, fat pads over the eyes, pot belly, but prominent spine and hip bones. It looks contradictory, fat in some places and wasted in others, but the pattern makes sense when you understand that cortisol drives both fat deposition in specific locations and protein catabolism in muscle.

Testing: ACTH and the Seasonal Trap

The baseline ACTH blood test is the primary diagnostic tool for PPID. It's a simple blood draw, the sample gets spun and frozen promptly (ACTH degrades rapidly at room temperature), and results come back within a few days.

Here's where it gets interesting and where mistakes happen: ACTH levels in all horses, healthy and affected, rise significantly during late summer and fall. This seasonal rise, sometimes called the "fall rise" or "autumn increase," peaks between August and October in the Northern Hemisphere. Normal horses may have ACTH values in fall that would be considered abnormal at other times of year.

This means the reference ranges must be adjusted seasonally. Most labs now provide season-specific reference intervals:

• Non-fall period (November through June): Normal ACTH generally below 29-35 pg/mL (varies by lab)
• Fall period (July through October): Normal values can reach 50-100 pg/mL or higher depending on the exact timing and the lab's reference population

Testing during the fall has advantages and drawbacks. The seasonal ACTH rise amplifies the difference between normal and PPID horses, making early or mild cases easier to detect. But it also requires careful interpretation. A value of 80 pg/mL in September might be normal or might be elevated, depending on the lab's seasonal reference range.

Testing in the non-fall period provides cleaner results: a clearly elevated ACTH from January through June is highly diagnostic. However, early PPID cases may have normal baseline ACTH values outside of fall, potentially missing the diagnosis.

The Thyrotropin-Releasing Hormone (TRH) stimulation test improves sensitivity for early cases. A baseline ACTH is drawn, TRH is administered intravenously, and a second ACTH sample is taken 10 minutes later. PPID horses show an exaggerated ACTH response to TRH stimulation. This test is particularly useful when baseline ACTH is borderline or when you suspect early disease in a horse with subtle signs.

Practical advice: if you're suspicious of PPID, test. If the result is equivocal, retest in the fall or use the TRH stimulation test. Don't let a single borderline result end the conversation.

Treatment: Pergolide is the Cornerstone

Pergolide mesylate is a dopamine receptor agonist. It replaces the dopaminergic input that the degenerating hypothalamic neurons can no longer provide. By stimulating dopamine receptors on pars intermedia cells, it restores the inhibitory braking mechanism and reduces POMC-derived hormone production.

Prascend (manufactured by Boehringer Ingelheim) has been the FDA-approved pergolide product for horses since 2011. The standard starting dose is 2 mcg/kg once daily (one 1 mg tablet for a 500 kg horse). Tablets are scored for dose adjustment.

New in 2026: Zygolide has entered the market as a generic pergolide option, offering a more affordable alternative. The active ingredient and mechanism are identical. For owners managing PPID on a budget, this development is welcome. Discuss with your vet whether the generic is appropriate for your horse.

Most horses tolerate pergolide well. The most common side effect is transient appetite reduction when starting treatment or increasing the dose. This typically resolves within a few days. Starting at half the target dose and increasing over a week can minimize this. Rarely, horses may show lethargy or mild colic signs during the adjustment period.

Dose adjustments are guided by monitoring ACTH levels. Recheck ACTH 4-6 weeks after starting treatment (or after dose changes), ideally during the same season as the diagnostic test. The goal is an ACTH value within the normal reference range for the time of year. Many horses need dose increases over time as the disease progresses. Some require 3-4 mcg/kg or higher to achieve adequate control.

Pergolide is a lifelong commitment. Stopping the medication allows ACTH to rise again, and clinical signs return. This isn't a cure. It's management. Effective, reliable management, but management nonetheless.

Diet and Metabolic Management

PPID horses frequently have concurrent insulin dysregulation, especially those that develop laminitis. Managing the diet is not optional. It's as important as the pergolide.

Key dietary principles:

• Restrict nonstructural carbohydrates (NSC). Total diet NSC should stay below 10-12% on a dry matter basis. This means no sweet feed, no molasses-based supplements, and careful selection of hay. Have your hay tested. Cool-season grass hays can range from 8% to 25%+ NSC depending on species, growing conditions, and harvest timing
• Soaking hay for 30-60 minutes in cold water reduces NSC content by roughly 30%. It's not a perfect solution (it leaches some minerals too), but it's a practical tool when low-NSC hay isn't available
• Limit or eliminate pasture access during peak sugar periods: spring growth and fall regrowth, sunny afternoons after cool nights, and stressed grass during drought. Grazing muzzles reduce intake but don't eliminate risk
• Provide adequate protein. Muscle wasting is a hallmark of PPID, and protein restriction makes it worse. Ensure adequate quality protein through appropriate forage and supplementation if needed. Soybean meal or whey protein concentrates are cost-effective options
• Supplement minerals appropriately. Copper, zinc, selenium, and vitamin E often need supplementation, particularly in horses on restricted diets. A ration balancer formulated for metabolic horses covers these gaps without adding excess calories or sugar

The vaccination schedule deserves attention in PPID horses. Their compromised immune function means vaccines are both more important (reduced ability to fight infection naturally) and potentially less effective (blunted immune response). Work with your veterinarian to ensure appropriate vaccination timing, and consider checking antibody titers if vaccine efficacy is in question.

Laminitis Prevention: The Highest Stakes

Laminitis is the most serious complication of PPID and the leading cause of euthanasia in affected horses. The mechanism involves insulin dysregulation: chronically elevated insulin damages the lamellar tissues that suspend the coffin bone within the hoof capsule. Once laminitis develops, managing it becomes exponentially harder than preventing it.

Prevention centers on three pillars:

1. Adequate pergolide dosing to control ACTH and its downstream hormonal effects
2. Strict dietary management to minimize insulin spikes
3. Regular monitoring of insulin levels, ideally alongside ACTH rechecks, to catch insulin dysregulation before it produces clinical laminitis

Seasonal vigilance is critical. The fall ACTH rise often triggers laminitic episodes in PPID horses, even those that seemed controlled during other seasons. Some veterinarians preemptively increase pergolide doses by 20-30% during August through November to counteract the seasonal surge. Discuss this strategy with your vet.

Foot care matters enormously. Regular farrier visits on a 4-6 week cycle, appropriate trimming to maintain proper hoof balance, and prompt attention to any signs of foot soreness (shortened stride, reluctance on hard ground, increased digital pulses) can catch problems early.

For a deeper look at the anatomy involved in these conditions and how structures relate to each other, explore our interactive models and see the pituitary gland, hoof laminae, and endocrine connections in context.

Living with PPID: What to Expect Long Term

PPID is progressive. The dopaminergic neurons continue degenerating regardless of treatment. Pergolide manages the consequences but doesn't halt the underlying neurodegeneration. Over years, most horses need gradual dose increases. Clinical signs may wax and wane, particularly with seasonal ACTH fluctuations.

That said, many PPID horses do remarkably well for years. They're ridden, they enjoy turnout, they maintain reasonable body condition. The horses that thrive are typically those whose owners monitor consistently, adjust treatment proactively, and take the dietary restrictions seriously.

Quality of life assessment should be ongoing. Key indicators of adequate control include: normal shedding pattern, stable body weight and muscle mass, absence of laminitis, normal thirst and urination, healthy coat and hoof quality, and overall energy and demeanor consistent with the horse's age.

When control deteriorates despite escalating treatment, or when laminitis becomes recurrent and refractory, honest conversations about quality of life become necessary. That's never easy. But it's part of responsible ownership of a horse with a progressive condition.

To explore the PPID condition page for additional anatomical context and related structures, our interactive resources break down the endocrine pathways involved in this disease.

Frequently Asked Questions

Sources

• Equine Endocrinology Group - PPID Diagnosis and Treatment Recommendations
• American Association of Equine Practitioners - Cushing's Disease Overview
• Journal of Veterinary Internal Medicine - PPID and Insulin Dysregulation Research
• University of Minnesota College of Veterinary Medicine - Equine Endocrine Diagnostics