1. What is PPID?
Previously known as Cushing’s disease, pituitary pars intermedia dysfunction (PPID) occurs when the pituitary gland becomes enlarged and overactive, producing excessive amounts of adrenocorticotropic hormone (ACTH). These elevated ACTH levels, in turn, lead to increased cortisol production. Although the exact cause of PPID cause remains unclear, the condition is especially common in senior horses. One study found that 85 percent of veterinarians are caring for at least one horse with the condition.
2. How does PPID affect horses?
Elevated hormone levels lead to a wide range of clinical signs in horses with PPID, such as a long hair coat that is slow to shed, muscle-wasting, lowered immune function and susceptibility to infection. Many affected horses also develop slow‑onset chronic laminitis, which may go unnoticed until hoof structures have been significant and irreversible damaged.
3. How is PPID diagnosed?
In addition to clinical signs, veterinarians utilize two laboratory tests to diagnose PPID. The first is a blood test to measure resting levels of ACTH. The second is the thyrotropin-releasing hormone stimulation test (TRH stimulation test), which involves taking a blood sample, giving an injection of TRH, and then collecting a blood sample again 10 minutes later. All horses show some increase in ACTH after TRH administration, but horses with PPID exhibit a significantly greater rise. Because of natural seasonal and geographic fluctuations in ACTH, veterinarians will use published reference ranges to interpret laboratory results.
4. What are the best PPID treatment options?
PPID can be controlled with the medication pergolide, sold under the brand name (Prascend) and as a generic product (Zygolide). Pergolide acts on dopamine receptors in the brain, reducing ACTH production. It is given daily and is widely considered the most effective treatment for controlling PPID.
5. What does the latest research tell us about PPID?
PPID is the subject of continued research, and recent studies offer encouraging results.
• Researchers from Michigan State University followed 30 horses from the original 2011 Food and Drug efficacy study for an additional 15 years evaluating the long‑term safety and effectiveness of pergolide. Five years into the study, all owners reported consistent improvement in their horses’ clinical sign, and nearly 60 percent of the horses had normal endocrine test results. Ten years into the study, 96 percent of horse owners agreed that pergolide had improved their horses’ quality of life. At the study’s conclusion there was no indication that horses had developed resistance to pergolide over time there were no adverse effects attributed to the drug among the study horses.
• A study from Purdue University found that, if managed with medication, PPID is not likely to shorten a horse’s lifespan. Researchers there examined the records of 132 horses diagnosed with the condition, along with the records of 274 healthy horses to serve as controls. Almost all the PPID horses—94.7 percent —were on a treatment regimen that included medication. The data showed that although horses with PPID were indeed more likely to develop dental issues, laminitis and impaired wound healing, they did not die any earlier than horses without the condition.