Immunocastration of horses: A tool for behavioural modification?

Introduction Elsewhere in this issue, Meloen describes the techniques used by his group for the development of new vaccines. Over the last three years, the University of Utrecht's Faculty of Veterinary Medicine has been involved in collaborative experiments with Dr. Meloen to test the efficacy in horses of a vaccine against gonadotrophin releasing hormone (GnRH), the hypothalamic hormone that controls the reproductive endocrine system. In general, vaccination involves the injection of GnRH (or a modified form of the hormone), often conjugated to a non-native protein, to induce the formation of anti-GnRH antibodies. The antibodies bind to endogenous GnRH within the hypothalamic-pituitary portal vessels and prevent it from binding to receptors on pituitary gonadotrophes, thus removing the stimulus for gonadotrophin secretion (particularly luteinising hormone: LH). The follow-on effect is the loss of stimulation to the gonads and a subsequent reduction of gonadal steroid hormone secretion to baseline levels; in effect, an immune-mediated endocrine castration. In farm animals, anti-GnRH vaccination is seen as a potentially cost-effective and welfare-friendly way of reducing testosterone production in male animals to prevent androgen-induced odours that can affect the meat (e.g. 'boar taint' in pigs [6]). In addition, it is hoped that immunocastration will improve the feed conversion efficiency of male animals and reduce aggression directed towards other animals or towards handlers. In man, anti-GnRH vaccination is being developed as a treatment for hormone-induced neoplasia (e.g. prostate cancer) and in cats, dogs and wild animals it has been proposed as a means of contraception. In horses, the main desire is to develop a way of controlling inappropriate sexual or aggressive behaviour without resorting to surgical castration or long-term hormone administration.