Cover Story | Endocrine Disease

Long-Term Pergolide Beneficial, Even at Low Dose

By Adam Marcus

Long-term treatment with pergolide has a high rate of success in horses with pituitary pars intermedia dysfunction (PPID), a common endocrine disorder of older equids, according to a nearly 15-year look at the therapy.

The researchers tracked a subgroup of 30 animals enrolled in the original trial of clinical efficacy of pergolide submitted for FDA approval, which was granted in 2011. Of those, 13 received 2 mcg/kg per day orally and 17 received 4 mcg/kg per day, also orally in this extended use study.

Over the next 14.5 years, 29 of the 30 horses died, according to a survey of their owners. After 5.5 years of the extended use study, 11 surviving horses experienced sustained clinical improvement while taking pergolide, and endocrine test results had returned to normal ranges in three-fourths of these animals.

After nearly a decade of treatment, 6 horses were still alive, of which 2 had normal endocrine function, according to the researchers.

Although 7 of the horses surviving more than 5 years required a boost in the dose of pergolide, the rest did not, suggesting durable benefit even at the starting 2 mcg/kg-per-day dose, they added.

“The drug produces variable results, but nearly 70% of treated horses have substantial improvement, which is quite good for a medication,” said Harold Schott, DVM, PhD, a professor in the Department of Large Animal Clinical Sciences at Michigan State University College of Veterinary Medicine, in East Lansing. “Our findings are important for equine veterinarians to know that a progressive increase in pergolide dose—and cost to owners—may not be needed in many equids with PPID,” Dr. Schott said.

While roughly 80% of owners were highly satisfied with long-term treatment, and nearly 90% said they would give the drug to another horse as long as necessary, most said the expense could deter them from doing so: “26% and 57% of owners were willing to pay an annual cost of $500 or $1000, respectively, whereas only 17% of owners would be willing to pay ≥$1,500 annually,” the researchers reported.

In the initial 6-month clinical efficacy study, the most common adverse effect of pergolide was a transient decrease in appetite, observed in 30% of horses, Dr. Schott said. Horses enrolled in the extended use phase of the study were also reported to have transient appetite decreases by their owners, but it was unclear whether this problem was an adverse effect of the drug or simply due to aging.

Dr. Schott and his colleagues also were surprised that some owners reported their horses appeared to dislike the taste of the medication. “Owners of other horses not in the study occasionally cannot ever get their horses on the medication due to loss of appetite,” he said. “The reasons for loss of appetite and apparent aversion to the taste of pergolide are not known but appear to be an idiosyncratic response in some equids.”

Age of onset of PPID seems to be an important factor in the success of the drug. “In my clinical experience, the younger the horse is when clinical signs develop, the poorer the response to the drug and the poorer the long-term prognosis seems to be,” he said. “It helps to understand this when you compare PPID to Parkinson’s disease in people,” for whom an earlier onset of clinical signs might be associated with slower progression but also a shorter lifespan.

As a result, Dr. Schott said, it would be helpful to investigate how the timing of treatment—early in the course of PPID versus later—could affect prognosis. But such a study would require another long-term trial with a large number of both treated and untreated animals, at a high cost. “Unfortunately, that kind of study is unlikely to be completed,” he said. MeV

For more information:
Schott HC, Strachota JR, Marteniuk JV, et al. Long-term response of equids with pituitary pars intermedia dysfunction to treatment with pergolide. J Vet Intern Med. 2025;39(3):e70109. 
https://onlinelibrary.wiley.com/doi/10.1111/jvim.70109