Vitex agnus castus extract (Chaste Berry) has been reputed to have therapeutic efficacy in the treatment of Equine Cushing’s syndrome. However, results of a study at the University of Pennsylvania’s New Bolton Center repudiated these claims.  Jill Beech, VMD, professor of medicine in the School of Veterinary Medicine and chief of the section of large animal medicine at the New Bolton Center, was involved with the study and presented “Comparison of Vitex agnus castus Extract and Pergolide in Treatment of Equine Cushing’s Syndrome” at the AAEP convention’s Medicine II session on Saturday, Dec. 7.

Cushing’s syndrome in horses is due to pituitary pars intermedia hyperplasia. Affected horses frequently (but not always) have elevated secretion of ACTH from the abnormal pituitary.  Other peptides (pro-opiomelanocortins or POMCs) have also been increased in the small number of horses where they have been measured.  Another endocrine hallmark of the disease is failure of circulating cortisol levels to decrease in the blood following administration of dexamethasone; in normal horses this steroid suppresses ACTH production and thus cortisol production.  Pergolide, a dopaminergic agonist, has been shown to affect plasma POMC and plasma ACTH concentrations in horses.  The study compared pergolide with a commercially-available form of Vitex agnus castus extract (Hormonise)