Pergolide for Veterinary Use

Therapeutic Class: Dopaminergic agonist

Species: Horses

May Be Prescribed For: Pituitary pars intermediahyperplasia, "pituitary adenoma."

Pergolide is a human drug used to treat Parkinson's Disease. Pergolide is the drug of choice to treat pituitary pars intermedia hyperplasia or equine Cushing's Syndrome (ECS) in horses. Pergolide works by binding with drug receptors in the brain that control the production of dopamine.

Cyproheptadine is another drug that has been used to treat ECS although recent work indicates that it is not very effective and that pergolide is clearly the treatment of choice.

For many years, pituitary pars intermedia hyperplasia was called pituitary adenoma. In most cases, this commonly seen condition of older horses and ponies is caused by hyperplasia or hypertrophy of the pars intermedia rather than tumor. Although the hyperplasia eventually may progress to adenoma formation, that is considerably less common. The average age of horses diagnosed with ECS is 20 (range seven to 42). Ponies may be affected more commonly. Pituitary pars intermedia hyperplasia is a medical problem that is managed, not cured. Animals that are being treated usually are on medication for the rest of their lives.

Clinical signs associated with ECS include hirsutism, polyuria, polydipsia, abnormal fat deposition, abnormal sweating patterns, laminitis, and decreased immune competence. Horses usually are started on a low dose of pergolide (0.5 mg/day) and re-evaluated by six weeks. If there is no improvement, the dose should be increased gradually (by 0.25 mg) until improvement is seen. Recent work reported by J. Beech at the 2003 AAEP suggested that some horses may require higher doses than those that are used commonly.

A possible differential diagnosis for ECS is the recently hypothesized Peripheral Cushing's Syndrome (PCS). PCS is thought to be a different endocrine disturbance although there are some similar symptoms. Horses with PCS are overweight with cresty necks, abnormal fat distribution, and frequently suffer from laminitis. They are not hirsute and do not respond to pergolide.

• Decreased appetite during the first week of treatment is the most-commonly reported side effect of Pergolide. This may be managed by an initial reduced dose followed by a gradual increase.

• A number of side effects are listed in humans taking pergolide with other medication for Parkinson’s Disease. Side effects included low blood pressure, headache, GI upset, anemia, respiratory infections, dizziness, and hallucinations.

• It is important to identify and use the lowest-possible dose of pergolide. Based on information from human Parkinson’s Disease patients, there is some thought that this condition may, over time, become resistant to medication.

• High doses of pergolide have been tested in laboratory animals without causing detectable harm to the fetus but this work has not been done in horses. It is not known if pergolide is excreted in milk but this type of drug may interfere with lactation. No information was found on pergolide use in breeding stallions. Pergolide should be used only in pregnant or lactating animals if the benefits outweigh the risks.

Phenothiazine tranquilizers such as acepromazine may interfere with the action of pergolide.

Overdose in humans causes gastrointestinal upset and hallucinations.