Cushing's Horses' Long-Term Response to Pergolide

Cushing's Horses' Long-Term Response to Pergolide

There might be a need for a seasonal increase in dose for some horses, because the endocrine system is more sensitive in the autumn months.

Photo: iStock

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.

Schott said the most common medical problems they observed in these aged horses were colic and diarrhea, as well as intermittent flare-ups of laminitis. The causes of death or euthanasia were similar to those of aged horses without PPID. Although a decrease in appetite was the most common adverse effect reported at the start of pergolide treatment, it was only a minor problem during prolonged drug treatment. Schott explained that there was no significant difference between a horse’s clinical score (a measure of body condition and hormonal and blood chemistries) and development of laminitis: A cresty neck, decreased body weight, elevated blood glucose or triglyceride concentrations (indicative of Cushing's), or elevated ACTH levels (the pars intermedia secretes more of this hormone in affected horses) did not make a significant difference in whether a horse would survive.

“Approach to management and medical treatment should be performed on a case-by-case basis and should be monitored by physical examination and endocrine testing on a yearly basis (in the spring), unless the horse’s condition changes for the worse,” Schott stressed,

The researchers on this study showed that owners and veterinarians can manage PPID for a long time without a progressive increase in pergolide dose—50% of the study horses did not need an increase above the standard 2 μg/kg dose. That said, Schott suggested there might be a need for a seasonal increase in dose for some horses, because the endocrine system is more sensitive in the autumn months. Schott said there is also evidence that pergolide does improve a horse’s quality of life, but there is no evidence that it prolongs life.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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