Equine Cushing’s syndrome (ECS), or hyperadrenocorticism, is caused by an excess of cortisol. This syndrome produces signs including excessive hair growth (hirsutism), lethargy, fat redistribution under the skin, chronic/relapsing laminitis, and increased drinking and urination (polydypsia/polyuria). The drug pergolide is commonly recommended as treatment for ECS. However, data on its long-term effects is lacking, and it can cause anorexia and depression. Another drug, trilostane (Wanskerne Ltd., Billingshurst, UK), is used to treat canine Cushing’s disease. Trilostane controls not only clinical signs but also excess cortisol. For this reason, Catherine McGowan, BVSc, DipVetClinStud, MACVSc, PhD, working at the Royal Veterinary College in Hertfordshire, UK, set out to evaluate the effectiveness and safety of trilostane for the treatment of ECS.

Twenty ponies and horses enrolled in the study were referred to the Royal Veterinary College after previous diagnosis of ECS. The animals ranged in age from 15-30 years. Upon admission, baseline serum cortisol was measured, and endocrine function tests (dexamethasone suppression and thyrotropin releasing hormone–TRH–stimulation) were performed. Horses began receiving trilostane once daily. Horses weighing less than 250 kg received one 120-mg tablet, and horses heavier than 250 kg received two tablets. “We based this dosing strategy on pilot trials done by the drug company,” explains McGowan. “Basically, one tablet per small horse and two tablets per big horse.” Thirty days later, horses were re-examined, serum cortisol tests were repeated, and owners were asked about changes in their animals.

All 19 horses originally reported to be lethargic had improved, as had all 11 horses with polydypsia/polyuria. Thirteen of 16 horses with chronic/relapsing laminitis showed improvement as well. However, it appeared that horses responding in the first 30 days were most likely to continue to respond. Horses relapsing during the one- to two-year study were gradually increased to 1 mg/kg trilostane. According to McGowan, “This dosage stopped the relapsing. The 1 mg/kg dosage is the minimum I would use now.”

No adverse side effects were reported in any horse taking trilostane. In addition, trilostane therapy altered endocrine testing results to normal levels. Therefore, it appears that trilostane could be a useful, safe drug for treating ECS. (Note that trilostane is not available in the United States