PPID, or pituitary pars intermedia dysfunction, is the most common hormonal disorder in older horses. Often called equine Cushing’s disease, it affects an estimated 20 to 25 percent of horses over the age of 15. The condition results from a slow breakdown of nerve cells in the brain that normally keep part of the pituitary gland in check, leading to hormone overproduction that can cause a cascade of visible changes, from a long, curly coat to muscle wasting and a heightened risk of laminitis.
How PPID Develops in the Brain
A horse’s pituitary gland sits at the base of the brain and has several distinct regions. The one involved in PPID is the pars intermedia, a middle section made up of cells called melanotropes. Under normal circumstances, these cells are kept quiet by dopamine, a chemical messenger released by specialized nerve cells originating in the hypothalamus. Dopamine binds to receptors on the melanotropes and tells them to slow down: stop multiplying, stop producing hormones.
In PPID, those dopamine-producing nerve cells gradually degenerate due to oxidative damage. As fewer of them survive, less dopamine reaches the pars intermedia, and the brakes come off. The melanotropes begin to grow and multiply unchecked. They ramp up production of a precursor molecule called POMC, which gets broken down into several hormones, including ACTH (the hormone that drives cortisol production), along with other peptides that affect metabolism, immune function, and fat distribution. Over time, the pars intermedia physically enlarges, progressing from simple overgrowth to benign tumor-like changes.
Who Gets PPID
Age is the single biggest risk factor. The disease is rare in horses under 10 and becomes increasingly common with each passing year. One Australian study of horses aged 15 and older found that 21.2 percent had PPID, even though most owners didn’t realize their horse was affected. All breeds and types can develop it. Ponies and Morgan horses appear in the literature frequently, but no breed is immune.
Recognizing the Signs
PPID tends to creep in slowly, and the earliest changes are easy to dismiss as normal aging. A horse might seem a little more tired than usual, take longer to shed its winter coat in spring, or develop subtle changes in body shape. These early signs are worth paying attention to because treatment is most effective when started before the disease progresses.
The hallmark sign of advanced PPID is a long, wavy, or curling coat that fails to shed. This is technically called hypertrichosis: the hair follicles get stuck in their growth phase and never cycle into shedding. In severe cases, a horse can carry a thick, matted coat year-round, even in summer heat. This single sign is so strongly associated with PPID that many veterinarians consider it diagnostic on its own when it appears in an older horse.
Other common signs include:
- Epaxial muscle wasting: loss of the muscles along the topline, giving the back a sunken or swayed appearance
- Pendulous abdomen: a pot-bellied look that develops as core muscles weaken
- Bulging fat pads above the eyes: a puffy, rounded look to the supraorbital area
- Abnormal sweating: either excessive sweating or patchy, uneven sweating
- Increased thirst and urination
- Lethargy or unusual docility
- Recurrent infections: about one third of horses with PPID develop secondary infections such as sinusitis, dental abscesses, pneumonia, skin infections, or heavy parasite burdens
- Laminitis: painful inflammation of the tissues inside the hoof, which can be severe and recurrent
The immune suppression behind those recurring infections is likely driven by multiple factors at once: elevated cortisol, excess ACTH, and impaired white blood cell function all working together to weaken the horse’s defenses.
How PPID Is Diagnosed
Diagnosis usually starts with clinical observation. When a veterinarian sees three or more of the common signs (hypertrichosis, topline muscle loss, pendulous abdomen, bulging eye fat pads, and divergent hoof rings) in an older horse, the picture is already quite clear. But bloodwork is needed to confirm the diagnosis and to catch early cases before obvious signs appear.
Resting ACTH Test
The most widely used screening test measures resting ACTH levels in a blood sample. A value below roughly 35 pg/mL is generally considered normal. The important catch is that ACTH levels naturally spike in autumn, even in healthy horses. This seasonal surge means that fall testing requires adjusted reference ranges to avoid false positives. Your veterinarian will account for the time of year when interpreting results.
TRH Stimulation Test
For horses with suspected early-stage PPID whose resting ACTH comes back in a gray zone, a stimulation test can provide more clarity. A baseline blood sample is drawn, then the horse receives an intravenous injection of TRH (a hormone that stimulates the pituitary). A second blood sample is collected exactly 10 minutes later. In horses with PPID, the abnormal pituitary cells overreact to this stimulation and produce a disproportionate spike in ACTH. This test is particularly useful for catching the disease before the coat changes and muscle wasting become obvious.
How PPID Affects the Body Long Term
Left untreated, the hormonal excess from PPID touches nearly every system. The elevated cortisol-like activity breaks down protein in muscle tissue, which is why the topline wastes away while fat redistributes to unusual places like above the eyes and along the crest of the neck. The immune suppression makes horses vulnerable to infections that a healthy animal would fight off easily. Wounds heal slowly. Parasites gain a stronger foothold.
Laminitis is the most dangerous complication. The hormonal imbalance disrupts insulin regulation, and chronically elevated insulin damages the sensitive tissues (laminae) that attach the hoof wall to the bone inside the hoof. Repeated bouts of laminitis can cause permanent structural damage and chronic pain, and laminitis is the most common reason PPID horses are eventually euthanized. This is why early detection and treatment matter so much.
Treatment and Management
PPID cannot be cured because the nerve degeneration in the hypothalamus is irreversible. But it can be managed effectively, often for years, with medication and supportive care.
The standard treatment is pergolide, a drug that mimics dopamine and restores some of the inhibitory signal the pituitary has lost. It is given orally once daily, starting at a low dose that is then adjusted based on how the horse responds, both in terms of visible improvement and follow-up bloodwork. The goal is to find the lowest dose that controls symptoms and brings ACTH levels closer to normal. If a horse shows signs of intolerance (typically decreased appetite or mild lethargy), the dose is cut in half for a few days and then gradually increased again.
Most horses on pergolide show noticeable improvement within a few months. Coat quality often improves first: the horse begins shedding again and grows a more normal coat. Energy levels pick up. Laminitis episodes become less frequent. Some degree of muscle wasting and fat redistribution may persist, but further deterioration typically slows.
Beyond medication, management adjustments play a big role. Horses with PPID benefit from a diet that minimizes sugar and starch to help control insulin levels and reduce laminitis risk. Regular hoof care is critical, particularly for horses with any history of laminitis. Body clipping in warm weather helps horses with persistent coat growth stay comfortable and avoid overheating. Dental checks, deworming schedules, and prompt treatment of any infection take on extra importance given the weakened immune system.
Follow-up ACTH testing, typically every six months or so, helps your veterinarian fine-tune the pergolide dose over time. PPID is a progressive disease, meaning the underlying nerve degeneration continues, and many horses need gradual dose increases as they age. With consistent management, though, many horses with PPID maintain a good quality of life well into their late twenties and beyond.