We determined contraceptive effectiveness of a 1-inoculation, 1-year porcin
e zona pellucida (PZP) vaccine in free-roaming feral horses (Equus caballus
) in Nevada. We captured, freeze-branded, treated, and subsequently release
d 267 adult feral mares given (1) 2 inoculations (13-17 days apart) of vacc
ine emulsion consisting of aqueous PZP and Freund's Complete Adjuvant (FCA:
injection 1) or Freund's Incomplete Adjuvant (FM; injection 2), (2) same 2
inoculations as in (1) except also containing a carbomer adjuvant in inocu
lation 1, or (3) a single inoculation of an emulsion of PZP and FCA contain
ing a second dose of PZP and carbomer adjuvant in controlled-release polyme
r microspheres. We administered inoculations in January 1996, and monitored
the mares via fecal analysis for pregnancy and via ground survey fur 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 I injection containing microspheres, reproductive success w
as 11.3%. The concurrent rate in 72 untreated mares was 62.5%, This study r
evealed marked and equivalent 1-year contraceptive efficacy in 1- and 2-ino
culation PZP vaccine. indicating that controlled-release technology can rep
lace a second inoculation and thereby increase PZP vaccine cost effectivene
ss and potential for use in feral horse management.