A groundbreaking 15-year study from Michigan State University provides insights into how the drug pergolide aids in the long-term management of pituitary pars intermedia dysfunction (PPID), a common endocrine disorder affecting aging horses.

The Food and Drug Administration approved pergolide to treat horses with PPID in 2011. PPID, also known as Cushing’s disease, is caused by hormonal imbalances stemming from an overactive pituitary gland. It can produce a variety of signs, including a slow-to-shed hair coat, muscle-wasting and lowered immune function.

For the study, the researchers periodically examined the horses and regularly drew blood for analysis. In addition, owners were interviewed every three months to document any changes in their horses’ health status. Each horse’s medication was provided at no cost to the owners in exchange for their participation in the research.

The average age of the horses when the study began ranged from 17 to 29. The median survival duration after that was three years. “We lost them as a trickle at first,” says Schott, “then, by the five-year point, we had lost 20 of the 30 horses. But keep in mind that these horses were quite old when the study began.”

Consistent improvement

In 2016—five years into the study—all owners reported consistent improvement in their horses’ clinical signs. In addition, nearly 60 percent of the horses had normal endocrine test results. “I compare PPID to Parkinson’s in people,” says Schott. “It’s a slowly progressive disease with no cure. You can slow it down and stabilize the progression, but you cannot stop it.”

Some degree of abnormal shedding and muscle wasting was observed in but Schott says this is not unexpected. “The flip side of that is that 57 percent of the horses had improvement in shedding and muscle wasting,” he says. “No drug is 100 percent effective. And if this drug can offer clinical improvement to a majority of the horses—that’s great.”

Five of the horses were euthanized during the study period due to complications of chronic laminitis. “Pergolide isn’t a treatment for laminitis,” says Schott. “Chronic laminitis is associated with PPID, but it’s more related to the insulin dysregulation that these horses tend to have.” The remaining 24 horses died of other conditions associated with old age, like debilitating arthritis or strangulating lipomas. The only study horse still alive in early 2025 was a pony that lives on Schott’s property.

One significant finding of the study, Schott says, was that there was no indication that the horses developed resistance to the drug over time.

“When the study began, all the horses were on one or two milligrams of pergolide, which is a standard dose,” says Schott. “What was a bit surprising is that half the surviving horses were on the same dose after 10 years. They didn’t have to have their dose continually increased for pergolide to be effective, as some people assume is necessary.”

Perhaps most important, the researchers found no adverse effects attributed to pergolide among the study horses.

“Overall, I think this study confirms that pergolide is a great drug for keeping these horses comfortable as they age,” says Schott. Owners of the study horses agree: During interviews at the 10-year point of the study, 96 percent of owners agreed that pergolide had improved their horse’s quality of life, and 87 percent agreed they would provide similar treatment if they had another horse with PPID, even if they had to pay for the medication themselves.

Reference: “Long-term response of equids with pituitary pars intermedia dysfunction to treatment with pergolide,” Journal of Veterinary Internal Medicine, May 2025