Immunocontraception Limits Foal Production in Free-Roaming Feral Horses in Nevada

Abstract

We determined contraceptive effectiveness and duration of effect after porcine zona pellucida (PZP) vaccination in free-roaming feral horses in Nevada. We captured, freeze-branded, treated and subsequently released 132 adult feral mares. Treatment consisted of 1 or 2 injections (1 month apart) of vaccine emulsion consisting of aqueous PZP and Freund's complete adjuvant (Sigma, St. Louis, Mo.; FCA; inj. 1) or Freund's incomplete adjuvant (FIA; inj. 2). We gave controls saline and Freund's adjuvant as above. Among 1-injection mares, we gave one group a single dose of raw antigen, and we gave the other group raw antigen plus a second dose of antigen sequestered in controlled-release microspheres. We also studied 63 untreated mares. We administered inoculations in December 1992, and monitored the mares via fecal analysis for pregnancy and via helicopter and ground survey for foal production through September 1994. We determined pregnancy via measurement of estrone sulfate, progesterone metabolites and total estrogens, in fresh feces collected from the ground. Among antigen-treated mares, reproductive success across 1 year was 4.5% (2-inj.), 20.0% (1-inj. + microspheres) and 28.6% (1-inj., no microspheres). Reproductive success in placebo (55.0%) and untreated (53.9%) mares was significantly greater than in treated groups. The following year, without further treatment, reproductive success was 44.0% (2-inj.), and 54.5% (untreated) and these values were not different. Data from other groups were insufficient for comparison. The results of this study confirm the feasibility of capturing large numbers of mares in the'field for treatment and demonstrate: (1) 1-year contraceptive efficacy of several PZP vaccine protocols, (2) greater effectiveness in 2-injection protocols than in 1-injection protocols, and (3) a return to fertility in the second year after vaccination.