AAEP 2002: Vitex Agnus Castus Extract for Treatment of Equine Cushing's Syndrome

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).

Fourteen horses were selected for the study based on clinical signs of pituitary hyperplasia and elevated ACTH concentrations or abnormal dexamethasone suppression test (DST) results.  Using the manufacturer’s recommended dose, Vitex agnus castus extract was administered to four horses for two months, to four horses for four months, and to six horses for six months.  After treatment was discontinued, nine of the horses received pergolide.  According to Beech, only one of the 14 horses was found to remain stable and not deteriorate clinically during treatment with the extract.  One other horse’s appetite improved.  In fact, many of the horses’ clinical signs worsened and plasma ACTH levels increased, sometimes doubling.  In addition, responses to DST continued to be abnormal.

In contrast, with the exception of one horse, pergolide had a beneficial but not curative effect, although individual horses often required dosages above what many practitioners customarily use, said Beech.  ACTH concentrations were abnormal in seven of the horses.  In two horses, ACTH concentrations were normal, but their DSTs were abnormal; one showed no improvement (but the dose was low for his body weight).  The other horse’s DST had become normal in 2000 after three months of 3 mg pergolide but in 2002 after two months of the same dose her DST remained abnormal; whether prolonged treatment or a higher dosage would have had an effect is unknown.  Two horses with elevated ACTH levels (that decreased with pergolide) also had abnormal DSTs, which became normal with pergolide treatment.

This study showed that Hormonise was ineffective in this population of horses with Cushing’s Syndrome, and that pergolide dosage requirement varies among individual horses.

About the Author

Sarah Evers Conrad

Sarah Evers Conrad has a bachelor’s of arts in journalism and equine science from Western Kentucky University. As a lifelong horse lover and equestrian, Conrad started her career at The Horse: Your Guide to Equine Health Care magazine. She has also worked for the United States Equestrian Federation as the managing editor of Equestrian magazine and director of e-communications and served as content manager/travel writer for a Caribbean travel agency. When she isn’t freelancing, Conrad spends her free time enjoying her family, reading, practicing photography, traveling, crocheting, and being around animals in her Lexington, Kentucky, home.

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