Immunocontraception in Feral Horses: One Inoculation Provides One Year of Infertility

Abstract

We determined contraceptive effectiveness of a 1-inoculation, 1-year porcine zona pellucida (PZP) vaccine in free-roaming feral horses (Equus caballus) in Nevada. We captured, freeze-branded, treated, and subsequently released 267 adult feral mares given (1) 2 inoculations (13-17 days apart) of vaccine emulsion consisting of aqueous PZP and Freund's Complete Adjuvant (FCA; injection 1) or Freund's Incomplete Adjuvant (FIA; injection 2), (2) same 2 inoculations as in (1) except also containing a carbomer adjuvant in inoculation 1, or (3) a single inoculation of an emulsion of PZP and FCA containing a second dose of PZP and carbomer adjuvant in controlled-release polymer microspheres. We administered inoculations in January 1996, and monitored the mares via fecal analysis for pregnancy and via ground survey for foal production through October 1997. We determined pregnancy via measurement of estrone sulfate and progesterone metabolites in fresh feces collected from the ground. Among 2-inoculation mares, reproductive success across 1 year was 12.8% (carbomer adjuvant absent) and 10.6% (carbomer adjuvant present). In mares given 1 injection containing microspheres, reproductive success was 11.3%. The concurrent rate in 72 untreated mares was 62.5%. This study revealed marked and equivalent 1-year contraceptive efficacy in 1-and 2-inoculation PZP vaccine, indicating that controlled-release technology can replace a second inoculation and thereby increase PZP vaccine cost effectiveness and potential for use in feral horse management.