Four-year contraception rates of mares treated with single- injection porcine zona pellucida and GnRH vaccines and intrauterine devices

We evaluated the multiyear contraceptive efficacy of the gonadotrophin-releasing hormone (GnRH) vaccine GonaCon, the porcine zona pellucida (PZP) vaccine SpayVac and the human intrauterine device (IUD) 380 Copper ‘T’ in mustang mares provided by the State of Nevada. Eight untreated control mares were compared with 12 mares treated with SpayVac, 16 mares treated with GonaCon and 15 mares treated with the copper-containing IUD. Rates of contraception for Years 1, 2, 3 and 4 respectively for SpayVacwere 100% (12 of 12), 83% (10 of 12), 83% (10 of 12) and 83% (10 of 12), rates forGonaConwere 94% (15 of 16), 60% (9 of 15), 60% (9 of 15) and 40% (6 of 15) and rates for IUD-treatedmareswere 80% (12 of 15), 29% (4 of 14),14% (2 of 14) and 0% (0 of 14). Antibody titres against PZP andGnRH declined over the four-year study. For mares given SpayVac, uterine oedema was commonly observed. IUDs were visible by ultrasonography in nonpregnantmustangmares, suggesting that pregnantmares did not retain their IUD. IUD retentionmay be a function of uterine size: pony mares with IUDs had high retention and contraception rates for 4–5 years. We conclude that long-term contraception of mustang mares with a single shot of either the SpayVac or GonaCon vaccine is possible.