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

Abstract

CONTEXT: Contraception is increasingly used as a management technique to reduce fertility in wildlife populations, but the feasibility of contraceptive formulations has been limited until recently as 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 three years post treatment. KEY RESULTS: Both contraceptive treatments significantly reduced fertility for three 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 demonstrate that both vaccines are capable of significantly reducing fertility for several years without boosters. IMPLICATIONS: Contraceptive vaccines examined in this study represent a useful tool for the management of feral horses, due to their being efficacious for three years in the absence of booster immunisations