Monitoring Horses with Cushing’s Disease

Cushing’s Disease, correctly called pituitary pars intermedia dysfunction or PPID, is common in older horses, but monitoring treatment of these cases is difficult. If you have a horse over 15 years of age, there is a chance your horse could have some degree of Cushing’s Disease. Horses over 25 years of age commonly have some degree of the syndrome although they may not develop clinical signs. The disease causes multiple symptoms, including laminitis, increased water consumption, decreased or increased sweating and long hair that is not shed in the spring. 

Diagnosis involves testing a hormone level called ACTH, either as a sole test or after injection of another hormone. The disease is treated with the drug pergolide (Prascend), and it can be difficult to determine in follow-up testing if the drug is helping or if the correct dose is being administered.

In some horses, clinical signs such as hair shedding can be used to determine if the disease is controlled, but more difficult if laminitis is involved.  In 20-75% of horses treated, there will be a decrease in ACTH when treating with Prascend, but it may not be reduced to a normal level. This is confusing because normal ACTH levels vary during the fall of the year and with stress.  Dr. David Rendle, BVSc, MVM, CertEM, DipECEIM, MRCVS, says that an improvement in clinical signs is the most important parameter. If the blood test value is decreased from pretreatment but not normal and clinical signs are improving, this tells us the drug is working. Blood test monitoring twice a year is recommended, as Cushing’s Disease is progressive, and your horse will likely need a higher dosage as the horse ages. Ask your equine veterinarian about checking your older horse for Cushing’s Disease.