PPID, or pituitary pars intermedia dysfunction, is a progressive hormonal disorder in horses caused by overactivity in a specific region of the pituitary gland. You may know it by its older name, equine Cushing’s disease. It’s one of the most common endocrine conditions in older horses and ponies, and it can lead to serious complications like laminitis if left unmanaged.
What Happens Inside the Pituitary Gland
The pituitary gland sits at the base of the brain and has several distinct sections. In PPID, the problem is in the middle section, called the pars intermedia. Normally, dopamine produced in the brain acts as a brake on this region, keeping hormone production in check. In horses with PPID, the neurons that supply dopamine degenerate over time, and without that brake, the pars intermedia grows unchecked and eventually forms small benign tumors called adenomas.
These adenomas pump out excessive amounts of several hormones, most notably ACTH (the hormone that stimulates cortisol production) and a pigment-related hormone called alpha-melanocyte-stimulating hormone. The overproduction of ACTH drives many of the downstream effects of PPID, including the metabolic disruption and immune suppression that make the disease so damaging. Elevated levels of that pigment hormone are thought to be directly responsible for the abnormal coat growth that is the disease’s hallmark sign.
Signs to Watch For
The most recognizable symptom of PPID is a long, curly hair coat that doesn’t shed normally in the spring. This is often what first alerts owners that something is wrong, because a healthy horse’s coat should transition smoothly with the seasons. A horse with PPID may still have a thick, shaggy winter coat well into summer, sometimes with wavy or matted hair along the neck and barrel.
Beyond the coat, the signs are varied and can develop gradually:
- Muscle wasting along the topline, giving the horse a swayback appearance even if it was previously well-muscled
- A rounded, potbelly abdomen caused by loss of core muscle tone
- Abnormal fat deposits on the crest of the neck, above the eyes, and around the tailhead
- Increased thirst and urination, sometimes dramatically so
- Lethargy and exercise intolerance
- Abnormal sweating, either excessive or reduced
Horses with PPID also become immunocompromised. They’re prone to chronic skin infections, sinus infections, pneumonia, slow-healing wounds, and higher parasite burdens than you’d expect for their deworming schedule. If your older horse seems to pick up every infection going around the barn, PPID is worth investigating.
The Laminitis Connection
Laminitis is the most serious complication of PPID and often the reason the disease gets diagnosed in the first place. Nearly half of horses with PPID, about 49%, develop laminitis at some point. An Australian study found that horses with PPID are 4.7 times more likely to develop laminitis than healthy horses of the same age. The hormonal imbalances caused by PPID disrupt insulin regulation and blood flow to the hoof, creating the conditions for painful and potentially life-threatening damage to the laminar tissue that connects the hoof wall to the coffin bone.
If an older horse develops unexplained or recurrent laminitis, PPID testing should be high on the list. Controlling the disease often reduces laminitis risk significantly.
How PPID Is Diagnosed
Diagnosis relies on blood testing to measure ACTH levels, but there’s a critical timing issue. Healthy horses naturally experience a seasonal rise in ACTH during late summer and early fall. Concentrations peak between late August and early October, and during that window, normal ACTH levels can climb above 100 pg/mL, with the highest reference limits occurring in the second half of September (around 129 pg/mL on average). Outside that window, from November through July, normal ACTH levels are considerably lower.
This means a single ACTH measurement taken in September could look elevated even in a perfectly healthy horse. Veterinarians use different reference ranges depending on the time of year, and the seasonal peak timing varies slightly by latitude. In southern regions, the rise starts a bit earlier (early August) compared to northern regions (mid-August).
The TRH Stimulation Test
When a baseline ACTH level is borderline or when testing needs to happen during the tricky autumn period, veterinarians often use a TRH stimulation test. This involves drawing an initial blood sample, then injecting a small amount of thyrotropin-releasing hormone (TRH) intravenously. A second blood sample is drawn exactly 10 minutes later. In horses with PPID, the abnormal pituitary cells overreact to TRH, producing a spike in ACTH that is much higher than what a healthy horse would show. This test is more sensitive than a resting ACTH level alone, particularly for catching early-stage disease before obvious clinical signs appear.
Treatment With Pergolide
PPID is not curable, but it is manageable. The primary treatment is a daily oral medication called pergolide (brand name Prascend), which is the only FDA-approved drug for PPID in horses. Pergolide works by mimicking dopamine, essentially replacing the chemical brake that the horse’s brain can no longer supply on its own. This reduces ACTH output from the pituitary and brings hormone levels closer to normal.
Treatment typically starts at a low dose of 2 micrograms per kilogram of body weight, given once daily. For an average 500-kilogram horse, that’s a single 1 mg tablet. Most horses begin showing improvement within the first few weeks to months. The coat often improves first: shedding resumes, and the next season’s coat may grow in much more normally. Energy levels tend to pick up, and the cycle of chronic infections often breaks.
Some horses experience a temporary decrease in appetite when first starting pergolide. This usually resolves within a few days, and splitting the dose or giving it with food can help. The dose may need to be increased over time as the disease progresses, and periodic ACTH testing helps guide those adjustments.
Long-Term Management
Managing a horse with PPID goes well beyond medication. Because these horses are immunocompromised, they benefit from a proactive approach to health maintenance. Regular dental care, an aggressive but targeted deworming schedule, and prompt treatment of wounds or infections all help prevent small problems from becoming serious ones.
Nutritional management matters too, especially for horses that also have insulin dysregulation. A low-sugar, low-starch diet helps reduce laminitis risk. Soaking hay to leach out excess sugars, limiting pasture access during peak grass growth, and avoiding grain-based feeds are common strategies. Body condition should be monitored closely because PPID horses can simultaneously lose muscle and gain fat in abnormal locations, making weight assessment less straightforward than it looks.
Hoof care is particularly important given the high laminitis risk. Regular trimming on a consistent schedule, and close attention to any signs of foot soreness, can catch problems before they become emergencies. Many PPID horses do well for years with proper management, maintaining a good quality of life well into their twenties and beyond. The key is catching the disease early, starting treatment promptly, and staying consistent with monitoring.