Owners have come to accept the fact that many horses will develop Cushing’s disease (pituitary pars intermedia dysfunction, or PPID) as they age. The FDA-approved drug pergolide has been commercially available since 2011 as a treatment for this complex endocrine dysfunction. But veterinarians and researchers have questioned whether the drug’s dose might need to increase as treatment progresses. They’ve also wondered whether pergolide improves or even prolongs a horse’s quality of life, along with how to monitor treated horses over time.

Harold Schott II, DVM, PhD, Dipl. ACVIM, professor of large animal clinical sciences at Michigan State University’s College of Veterinary Medicine, spoke to these concerns at the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah.

Since 2009 he and other researchers from Michigan State University have evaluated 30 horses confirmed positive for PPID before going on pergolide treatment. The team re-examined the horses six months, 2 ½ years, 3 ½ years, 4 ½ years, and 5 ½ years later to determine dose effectiveness.

Of the 30 horses, 14 were euthanized and four died by 2013 (5 ½ years into the study). At 4 ½ years, 18 study horses remained, 12 of which were doing well on their regular dose, and the owners were satisfied. At 5 ½ years, only 12 horses were left in the study. Endocrine blood testing revealed well-controlled PPID in most of them. Veterinarians increased the pergolide dose for four of the horses due to muscle wasting, slow shedding, and failed endocrine testing