Multi-year fertility reduction in free-roaming feral horses with single-injection immunocontraceptive formulations

Context. Contraception is increasingly used as a management technique to reduce fertility in wildlife populations; however, the feasibility of contraceptive formulations has been limited until recently because they have required multiple treatments to achieve prolonged infertility. Aims. We tested the efficacy and evaluated potential side effects of two contraceptive formulations, a porcine zona pellucida (PZP) formulation, SpayVac@ and a gonadotrophin-releasing hormone (GnRH) formulation GonaCon-B, in a population of free-roaming feral horses (Equus caballus). Both formulations were developed to provide several years of infertility with one injection. Methods. Females were treated in June 2005 with either GonaCon-B (n = 24), SpayVac (n = 20), adjuvant only (n = 22), or received no injection (n = 18). Females were monitored for fertility status year round for 3 years after treatment. Key results. Both contraceptive treatments significantly reduced fertility for 3 years. Fertility rates for GonaCon-B mares were 39%, 42% and 31%, respectively, and 37%, 50% and 44% for SpayVac mares. During the same seasons, 61%, 67% and 76% of control females were fertile. We found no significant effects from contraceptive treatment on the sex ratio of foals, birthing season or foal survival. Conclusions. These results demonstrated that both vaccines are capable of significantly reducing fertility for several years without boosters. Implications. Contraceptive vaccines examined in the present study represent a useful tool for the management of feral horses, because of their being efficacious for 3 years in the absence of booster immunisations.