Suppressing reproductive activity in horses using GnRH vaccines, antagonists or agonists

There are a number of situations in which it is desirable to suppress part or all of the reproductive endocrine system in a horse, notably the competing animal whose tractability during training, or performance during competition, is compromised by the expression of sexual or aggressive behavior. The current therapeutic approaches to reproductive endocrine suppression include gonadectomy and progestagen administration, where the former carries surgical risks and entails irreversible loss of breeding potential, and effective progestagen therapy requires frequent administration for extended periods and is banned in some competing animals as potentially anabolic. In this context, preventing the action of gonadotrophin releasing hormone (GnRH) by blocking its pituitary receptors is an attractive alternative for reversibly rendering mares anestrus or depressing testosterone secretion or spermatogenesis in stallions. This paper reviews the data on effects, efficacy, reversibility, and side effects of GnRH vaccines, antagonists, and agonists for suppressing reproductive activity in horses, within the context of their potential place in the pharmacological armoury of the veterinary clinician.