THE FOLLOWING SET OF DOCUMENTS COMPRISES;
1. A LIST OF RELEVENT REFERENCES FOR PZP
IMMUNOCONTRACEPTION IN WILD HORSES
2. THE ASSOCIATED COMPLETE PAPERS (PDF FILES)
FOR THIS LIST NUMBERED ACCORDINGLY.
Annotated References List for PZP Immunocontraception
in Wild Horses
(Prepared by J.W. Turner Jr., PhD and J.F. Kirkpatrick, PhD)
(1) Kirkpatrick, JF, Liu, IKM and Turner, Jr., JW 1990. Remotely delivered
immunocontraception in feral horses. Wildlife Society Bulletin 18: 326-330.
This is the first recorded study of PZP contraception in free-roaming wild horses (Assateague
Island). The findings indicated that PZP could be delivered remotely, by darts, contraception
was effective, that two inoculations worked as well as three and that administration to pregnant
animals had no deleterious effects on either the pregnancy or the foal.
No Abstract
(2) Kirkpatrick, JF, Liu IKM, Turner, Jr., JW and Bernoco, M. 1991. Antigen recognition in
mares previously immunized with porcine zonae pellucidae. J. Reprod. Fertil. (Suppl. 44): 321325.
This study reported that contraception could be maintained by a single annual booster
inoculation and that the contraceptive effects were reversible.
Abstract: Twenty-six free-roaming feral mares were immunized against porcine zonae
pellucidae (PZP) between February and May, 1988. Eight sexually mature mares received 2
inoculations 2 weeks apart, and 18 mares received 3 inoculations at intervals of 2 and 4 weeks.
Analysis of urinary oestrone conjugates (E1C) and non-specific progesterone metabolites
(iPdG) in samples collected in October, 1988, revealed that none of the 18 mares that received
3 and only 1 of the 6 mares that received two inoculations were pregnant, whereas 3 of 6 shaminjected control mares and 5 of 11 untreated mares were pregnant. In February and March,
1989, 14 of the immunized mares that were not pregnant were given a single booster
inoculation of PZP. Urinary E1C and iPdG analysis, from samples collected in August and
October, 1989, revealed only a single pregnancy among the 14 boosted mares, whereas 33% of
mares treated in 1988, but not given a booster inoculation in 1989, and 7 of 16 untreated
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controls, were pregnant. Foaling was reflected accurately by the urinary steroid metabolite
analyses. These results suggest that once recognition of PZP antigen occurs in mares,
subsequent annual inoculations will raise antibody titres sufficiently to inhibit fertility.
(3) Kirkpatrick, JF, Liu, IKM, Turner, Jr., JW, Naugle, R, and Keiper, R. 1992. Long-term
effects of porcine zonae pellucidae immunocontraception on ovarian function of feral horses
(Equus caballus). J. Reprod. Fert. 94:437-444.
This paper reported on the effects of four years of contraception in the Assateague horses.
Results showed a 90% efficacy, no effects on pregnancies and normally functioning ovaries, as
determined with urinary estrogen conjugates. The significance of this paper is that long-term
use does not compromise ovarian function.
Abstract: Ten feral mares free-roaming in Maryland, USA, were inoculated with porcine zonae
pellucidae (PZP) protein before the breeding season for three consecutive years (1988–90).
Ovarian function was monitored for 51 days during the peak of the breeding season after the
third annual PZP inoculation, in seven of these mares and in four untreated control mares, by
means of urinary oestrone conjugates and nonspecific progesterone metabolites. None of the
ten inoculated mares became pregnant in 1990, compared with 55% of 20 control mares, which
included two of the four monitored for ovarian function. Three of the untreated mares
demonstrated apparent normal ovarian activity, characterized by preovulatory oestrogen peaks,
concurrent progesterone nadirs at ovulation, breeding activity, and luteal-phase progesterone
increases after ovulation. Two of the seven monitored PZP-treated mares demonstrated
ovulatory cycles that did not result in conception. One was pregnant as a result of conception in
1989 and demonstrated a normal, late-gestation, endocrine profile. The remaining four PZPtreated mares revealed no evidence of ovulation, and urinary oestrogen concentrations were
significantly depressed. The experiments indicated that (i) a third consecutive annual PZP
booster inoculation is > 90% effective in preventing pregnancies in mares and (ii) three
consecutive years of PZP treatment may interfere with normal ovarian function as shown by
markedly depressed oestrogen secretion.
(4) Kirkpatrick JF, Liu IKM, Turner JW, Lasley BL. 1993a. The control of fertility and
monitoring ovarian function in free-roaming and captive wildlife: New approaches to old
problems. Proc. Soc. Theriogenol., Aug. 12-14, Jacksonville, FL, pp. 9-17.
This paper reports on the effectiveness of PZP contraceptive vaccine in 4 free-roaming
ungulate species and 8 captive exotic ungulate species, with the use of urinary and fecal sex
steroids monitoring in 5 of the 12 species to assess estrus cycling. The results of these studies
demonstrate marked PZP vaccine inhibition of fertility in all tested species and advantage of
remote monitoring of estrus cycles via urine/fecal steroid analysis.\
No Abstract
(5) Kirkpatrick, JF, Naugle, R, Liu, IKM, Bernoco, M and Turner, Jr., JW. 1995. Effects of
seven consecutive years of porcine zona pellucida contraception on ovarian function in feral
mares. Bio. Reprod. Monograph Series 1: Equine Reproduction VI: 411-418.
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This paper reports on seven years of continuous contraception. The significance of this study is
that no long-term deleterious affects were noted, and the ovaries in animals treated up to five
years functioned normally.
Abstract: A seven-year study was conducted to determine the effects of long-term treatment of
wild mares with a porcine zona pellucida (PZP) immunocontraceptive vaccine on fertility,
urinary estrogen and progesterone metabolite concentrations, ovulation rates, and reversibility
of contraceptive effects. Fifty free-roaming mares were inoculated remotely with PZP in
March over 7 consecutive years (1988-1994). During the initial year of treatment, each mare
received either one (n = 14), two (n = 18), or three (n = 18) inoculations (2-3 wk apart) of 65 µg
of native PZP; each received a single annual booster inoculation thereafter. Alternate-day urine
samples were collected directly from the sand after witnessed eliminations. Collections covered
60 days during the peak of the breeding season. Ovarian endocrine function was monitored by
means of urinary estrone conjugates (E1C) and nonspecific progesterone metabolites (iPdG).
PZP treatment over 6 yr, equaling 105 mare-years, resulted in four foals (3.8%) compared to a
46.2% foaling rate for the untreated population. Twenty-seven mares received their initial
inoculations during the later stages of pregnancies (280-340 days), and all 27 delivered foals.
Twenty-one of these foals lived to 1 yr, and this survival rate (77.7%) did not differ
significantly from that of 25 foals (84% survival rate) born to untreated mares. As evidenced by
pregnancies or luteal phase iPdG patterns and concentrations, ovulation rates declined over 6
consecutive years of treatment. The ovulation rates after a single year of treatment, 3
consecutive years, and 7 consecutive years were 73.3% (11 of 15), 55.5% (5 of 9), and 10% (1
of 10), respectively. One mare was withdrawn from treatment after 5 consecutive years of
treatment and ovulated during the sixth and seventh years of the study.
After a single year of treatment, 12 of 15 mares (80%) had urinary E1C concentrations
that fell within the normal range of 100-385 ng/mg creatinine (Cr). After 2, 3, 6 and 7 yr of
consecutive treatment, the percentage of mares with normal urinary estrogen concentrations
declined to 46.1% (6 of 13), 33% (2 of 6), 40% (2 of 5), and 0% (0 of 5), respectively. Some
mares with decreased urinary E1C concentrations continued to demonstrate cyclic peaks and
nadirs, suggesting continued but diminished follicular activity. Reproductive behaviors were
correlated with elevated urinary E1C concentrations. Reversibility of contraceptive effects after
1, 2, 3, and 4 consecutive years of treatment was 70% (7 of 10), 66% (2 of 3), 10% (1 of 10),
and 50% (1 of 2), respectively. The results of this study indicate that long-term PZP treatment
leads to decline in both ovarian estrogen production and ovulation rates, and that reversibility
of effects occurs in some animals.
(6) Kirkpatrick, JF, Zimmermannn, W, Kolter, L, Liu, IKM and Turner, Jr., JW. 1995.
Immunocontraception of captive exotic species. I. Przewalski;s horse (Equus przewalskii) and
banteng (Bos javanacus). Zoo Bio. 14: 403-413.
This paper reports on the first use of PZP in Przewalski’s horses (at the Koln Zoo in Germany.
Contraception was effective.
Abstract: A contraceptive vaccine made of porcine zonae pellucidae (PZP) was tested in three
Przewalski's mares and five banteng cows. The vaccine antigen consisted of the complete
family of glycoproteins of the porcine zona pellucida, including the sperm receptor ZP3. All
mares and three of five banteng were inoculated with 2 or 3 i.m. injections of approximately 65
μg of antigen given over a 6 week period. Two other banteng received inoculations of only 35
μg of antigen on the same schedule. Two of the three mares and three of five banteng cows
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were pregnant at the time of inoculation. No new pregnancies, as a result of postinoculation
breedings, occurred among either the mares 36 months after 65 μg antigen inoculations or
among the banteng for 24 months after 65 μg inoculations. One postinoculation pregnancy
resulted among the two banteng receiving only 35 μg of antigen. Differences in fertility
between treated and control mares and between preinoculation and postinoculation
reproductive performance of the banteng were significant (P < 0.05). Urinary ovarian steroid
metabolites and behavioral observations indicated follicular development and ovulations were
occurring among treated mares during the year following PZP inoculations. PZP immunization
produced progressively elevated anti-PZP antibodies in both species, which provided
contraceptive protection. PZP immunization appears to be an effective form of contraception in
both species.
(7) Turner, JW, Jr., Liu, IKM and Kirkpatrick, JF. 1996. Remotely-delivered
immunocontraception in free-roaming feral burros. J. Reprod. Fert. 107:31-35.
This is the first reported study of PZP application in wild burros (Virgin Islands National Park).
The significance of this project was that the PZP will work in non-seasonal breeders (burros) as
well as seasonal breeders (horses).
Abstract: Regulation of local overpopulations of free-roaming feral equids is in demand
worldwide for ecological balance and habitat preservation. Contraceptive vaccines have proven
effective in feral horses, which breed seasonally, but no data are available for equids such as
the burro, which is reproductively active all year round. In the present study, 27 individually
identified female feral burros (Equus asinus) roaming free in Virgin Islands National Park (St
John, US Virgin Islands; Lesser Antilles) were remotely treated with pig zonae pellucidae
(PZP) vaccine. Between January and May, 16 burros were darted with a 1 ml emulsion of PZP
plus Freund's adjuvant. Ten to twelve months later each treated burro was given a single
booster injection of PZP plus adjuvant to maintain contraception through a second year. Eleven
adult untreated jennies served as controls. Beginning one year after initial vaccination, these
burros were monitored for pregnancy and foal production. Collection of data to determine
treatment effect was not begun until 12 months after initial treatment to ensure that pregnancies
existing before vaccination were not included. Pregnancy was assessed using previously
validated methods for steroid metabolite measurement in fresh faecal samples. None of the
PZP-treated burros produced foals between 0 and 12 months after the last inoculation. One
PZP-treated burro tested positive for pregnancy at 10 months after the final inoculation. During
this same period, six of 11 untreated burros tested pregnancy-positive, and four were observed
with foals. There was no difference in pregnancy rates among treated, control and randomly
sampled jennies between 12 and 24 months after the last inoculation. The results demonstrate
that, in free-roaming feral burros that are reproductively active all year round: (1) burros can be
accessed for remotely delivered PZP vaccination; (2) PZP contraception is effective; (3) PZP
contraception is reversible; and (4) pregnancy can be reliably detected by faecal steroid
analysis.
(8) Kirkpatrick, JF, Liu, IKM and Turner, Jr., JW. 1997. Contraception of wild and feral
equids. In Kreeger, T. J. , tech. coord. Contraception in Wildlife Management. tech. Bull. 1853.
Wash., D.C.:U.S. Dept. Agriculture, Animal and Plant Health Inspection Service: 161-169.
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This paper was originally given at the Third International Symposium on Wildlife
Contraception, in Denver, in 1993. It reviews all PZP progress in wild horses to date.
Abstract: Fertility control in wild horses has been attempted with bost stallions and mares.
Nonreversible surgical sterilization by means of vasectomy has been successful in inhibiting
reproduction in wild horses in Montana and Nevada. Administration of a microencapsulated
form of testosterone to wild stallions reduced sperm counts and motility and foal counts. In a
third approach, intraperitoneal Silastic ™ implants containing ethinylesdtradiol and
progesterone blocked ovulation in wild mares for up to 3 years.
The first immunological fertility control of free-ranging wildlife was accomplished with
wild horses, Initial experiments demonstrated that immunization with porcine zonae pellucidae
was capable of causing contraception in domestic mares. Later, contraception was achieved
with the vaccine in free-ranging horses. That study demonstrated that the vaccine (1) could be
delivered remotely via darts, (2) was safe to administer to pregnant animals, and (3) did not
alter social behavior. A followup study revealed that a single annual booster inoculation would
extend the contraceptive effects for a second year, and the vaccine’s effects are reversible after
short-term use.
After 6 years of treating 52 different mares with porcine zonae pellucidae, contraceptive
efficacy exceeded 95 percent. In more recent studies, investigators are studying the effects of
long-term treatment (4 to 7 consecutive years) upon ovarian function. The porcine zona
pellucida (PZP) vaccine has also proved to be effective in contracepting free-roaming feral
donkeys in Virgin Islands National Park, captive Przewalski’s horses and onagers. Tests are
currently under way on 150 feral horses in Nevada for the purpose of developing a oneinoculation form of the PZP vaccine that will deliver from 1 to 3 years of contraceptive
protection. An initial field test of this vaccine indicated a high degree of success with a single
inoculation over a single year, and a field test of a second-generation of microcapsules also
indicated a high degree of contraceptive efficacy over a single year.
(9) Turner, JW, Jr., Liu, IKM, Rutberg, AT and Kirkpatrick, JF. 1997. Immunocontraception
limits foal production in free-roaming feral horses in Nevada. J. Wildl. Manage. 61: 873-880.
This is the first reported application of PZP to large numbers of western wild horses (Nevada),
in 1992. The significance of this paper includes the efficacy (90%), lack of injection site
reactions, and inoculations at the time of gathers.
Abstract: We determined contraceptive effectiveness and duration of effect after porcine zona
pellucida (PZP) vaccination in free-roaming feral horses in Nevada. We captured, freezebranded, 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 antigen5
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.
(10) Powell DM 1999. Preliminary evaluation of porcine zona pellucida (PZP)
immunocontraception for behavioral effects in feral horses (Equus caballus). J. Appl. Anim.
Welfare Sci. 2:321-335.
This paper originated from the National Zoological Garden and reports on a lack of behavioral
effects after treatment with PZP in wild horses (Assateague).
Abstract: Successful management of captive populations of wild animals requires effective
control of reproduction. Contraception is one tool for controlling reproduction of animals in
zoos; however, the options available to the animal manager are limited. Contraceptives vary in
efficacy, reversibility, and side effects, and thus may not be suitable for widespread use. One
consideration when selecting a contraceptive is its potential for side effects on behavior,
especially given the fact that reproduction plays such a prominent role in the biology of any
species. To date, there have been few evaluations of contraceptives for behavioral effects, and
those that have been conducted have focused on hormone-based contraceptives. This study
sought to evaluate a novel method of population control, imrnunocontraception, for behavioral
effects in a population of feral horses. Porcine zona pellucida (PZP) immunocontraception
prevents fertilization of ova and does not alter normal hormone secretion patterns. It therefore
should leave the animal behaviorally intact in terms of reproductive behavior. The study
examined the behavior of 43 sexually mature mares on Assateague Island during the 1997
breeding season and, with help from Earthwatch volunteers, collected observations over a 3month period. The study found no significant differences between treated and untreated mares
in general activity budget, aggression given or received, and spatial relationships relative to the
stallion. These preliminary findings indicate that PZP contraception seems to have no acute
behavioral effects on the behavior of individuals. The study findings also suggest that PZP
could be a desirable and effective management tool for captive species in which social behavior
plays an integral role in group dynamics. Analyses of group level effects and population level
effects are continuing.
(11) Barber, MR, Fayrer-Hosken, RA 2000. Evaluation of somatic and reproductive
immunotoxic effects of the porcine zona pellucida vaccination. J. Exptl. Zool. 286:641-646.
This paper originates from the University of Georgia and reports on a lack of cross-reactivity
between anti-PZP antibodies and somatic (non-reproductive) tissues and reproductive
hormones. This is a significant paper because it shows that the PZP immunocontraception is
not plagued by the same problems associated with sperm vaccines.
Abstract: Immunological, immunocytochemical and fertility analyses were performed to
determine the potential toxic side effects of porcine zona pellucida (pZP) vaccinations on target
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animals, including horses and dogs. The study was designed to determine the effect of
antibodies, raised against highly purified pZP, on somatic tissues. Immunocytochemical studies
performed with fixed tissues showed that rabbit anti-pZP antiserum did not crossreact with
brain, heart, lung, kidney, liver, bladder, stomach, small intestine, large intestine, muscle, skin,
spleen, pancreas, or lymph node of either the dog or horse. To determine the effect or oral
intake on nontarget animals, female rabbits were fed a contraceptive vaccine containing pZP
glycoproteins and the synthetic trehalose dicorynomycolate in drakeol (S-TDCM) adjuvant.
Enzyme-linked immunoadsorbent assay (LISA) analyses showed that rabbits fed with the
adjuvanted pZP proteins did not develop circulating anti-pZP IgG antibodies that crossreacted
with pZP. Furthermore, fertility studies performed on rabbits fed with adjuvanted pZP revealed
no significant differences in the number of embryos or stage of the embryos produced between
the treated and control animals. Results of these studies suggest that the pZP vaccine delivered
to dogs or horses in field studies have no recognizable somatic tissue effects. Moreover, there
were no side effects on nontarget animals should they eat the vaccine. This substantiates field
trials results about the safety of the pZP immunocontraceptive vaccine.
(12) Turner, JW, Jr., Liu, IKM, Flanagan, DR, Rutberg, AT and Kirkpatrick, JF. (2001)
Immunocontraception in feral horses: One inoculation provides one year of infertility (J. Wildl.
Manage. 65:235-241.
This paper is the first to describe the efficacy of a one-inoculation form of the PZP vaccine in
wild horses in Nevada. The study demonstrated that a one-inoculation application of PZP in
lactide-glycolide microspheres was as effective as the standard two inoculation protocol.
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 2inoculation 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.
(13) Powell DM, Monfort SL 2001. Assessment: effects of porcine zona pellucida
immunoconraception on estrous cyclicity in feral horses. J. Appl. Anim. Welfare Sci. 4:271-284.
This paper, originating from the National Zoological Garden, demonstrated that ovarian
cyclicity was normal in PZP-treated mares and that the mares treated for seven consecutive
years had which remained infertile, were in fact cycling.
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Abstract: The study used noninvasive fecal hormone monitoring to assess whether
contraception with porcine zona pellucida (PZP) had an impact on estrous cycle characteristics
in a herd of feral horses living on Assateague Island, Maryland. The study assayed longitudinal
fecal samples, collected over a 2-year interval, for total estrogens (estradiol and estrone) and
progestagens. Follicular (6.4 ± 2.4 days) and luteal (11.2 ± 3.2 days) phase durations were
within the range reported for domestic horses, with an average estrous cycle duration of 18.6 ±
2.5 days. There were no differences (p > .05) in phase durations between currently treated and
currently untreated mares in either year of the study. Endocrine data suggested that ovulatory
failure occurred frequently in Assateague mares; however, we could not link this phenomenon
directly to contraception. Overall, no estrous cycle characteristics differed between currently
treated and currently untreated mares. However, all mares in this study had been treated with
PZP at some point in the previous 10 years. Because PZP has been shown to induce decreased
ovulation rates in several mammalian taxa, including Assateague horses, we speculate that in
addition to blocking sperm binding PZP may adversely impact ovarian function in mares.
(14) Turner, A, and Kirkpatrick, JF. 2002. Effects of immunocontraception on population,
longevity and body condition in wild mares (Equus caballus). Reproduction (Suppl. 60): 187-195.
This paper is a summary of six years of PZP management on Assateague. The significance of
this study was that (1) the population could be reduced given enough time, that body condition
scores increased and that longevity of the herd was significantly increased (by up to 35%). In
general, it supported the idea that PZP can be an effective management tool for wild horse
populations.
Abstract: Contraception is becoming a common approach for the management of captive and
wild ungulates yet there are few data for contraceptive effects on entire populations.
Management-level treatment of mares with porcine zona pellucida (PZP) vaccine resulted in
zero population growth of the Assateague Island wild horse population within 1 year of
initiation of treatment. Contraceptive efficacy was 90% for mares treated twice in the first year
and annually thereafter. For mares given a single initial inoculation, contraceptive efficacy was
78%. The effort required to achieve zero population growth decreased, as 95, 83 and 84% of all
adult mares were treated in each of the first 3 years, compared with 59 and 52% during the last
2 years. Mortality rates for mares and foals after the initiation of management-level treatments
decreased below historic and pretreatment mortality rates of approximately 5%. Two new age
classes have appeared among treated animals (21-25 years and > 25 years), indicating an
increase in longevity among treated animals. Body condition scores for all horses, all adult
mares and non-lactating mares increased significantly between summer 1989 and autumn 1999
but did not change significantly in lactating mares. These results provide reliable data for the
construction of realistic models for contraceptive management of free-roaming or captive
ungulate populations.
(15) Kirkpatrick, JF, and Turner, A. 2002. Reversibility of action and safety during pregnancy of
immunizing against porcine zona pellucida in wild mares (Equus caballus). Reproduction
(Suppl. 60): 197-202.
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This paper reports on 12 years of reversibility data after PZP administration and gives percents
and times to reversal for a return to infertility. The significance is that only horses treated for
seven consecutive years have a risk of permanent infertility, but that there are no long-term
deleterious health effects.
Abstract: Contraceptive management of publicly valued wildlife species requires safeguards to
ensure that these populations are preserved in a healthy state. In addition, reversibility of
contraceptive effects and safety in pregnant animals are major concerns. A population of wild
horses has been immunized against porcine zona pellucida (PZP) over a 12 year period on
Assateague Island National Seashore, MD (ASIS). Mares initially received one or two 65
microg inoculations and once a year 65 microg booster inoculations, all delivered by dart. All
young mares aged > 2 years were treated with PZP for 3 consecutive years regardless of
whether they have bred successfully and they were then removed from treatment until they had
foaled. All mares vaccinated for 1 or 2 consecutive years became fertile again and 69% of
mares treated for 3 consecutive years returned to fertility. All five mares treated for 4 or 5
consecutive years have also returned to fertility, but over longer periods of time. Mares treated
for 7 consecutive years have not returned to fertility, but several, while still infertile, have
started ovulating again. There was no difference in survival rates between foals born to treated
and untreated mares, and PZP treatment of pregnant mares did not affect subsequent fertility of
their female offspring.
(16) Turner, JW, Liu, IKM, Flanagan, DR, Bynum, KS, Rutberg, AT 2002. Porcine zona
pellucida (PZP) immunocontraception of wild horses (Equus caballus) in Nevada: a 10 year
study. Reproduction (Suppl. 60):177-186.
This study reports the efficacy of a number of one-inoculation PZP formulations and the
problems and advantages associated with each.
Abstract: Porcine zona pellucida (PZP) immunocontraception was investigated for possible use
in free-roaming wild horses in the western USA. A protocol of two injections (3-4 weeks apart)
of vaccine lasting 1 year was first used and a single-injection controlled-release vaccine of 1
year duration was developed and tested in the field. Studies of a presumptive vaccine of 2 year
duration were initiated. The parameters of anti-PZP antibody titre response, pregnancy testing
and offspring production were used, and PZP vaccine was found to provide up to 94%
infertility in free-roaming wild mares. In addition, a single-injection PZP vaccine of 1 year
duration and containing a controlled-release component of PZP in a polymer matrix can
provide infertility equivalent to the two-injection PZP vaccine. All the PZP vaccine
preparations tested were associated with a return to normal fertility within 1 year. During the
course of these studies, attention was given to practical aspects of management application of
PZP contraception. Preparation of the controlled-release portion of the vaccine in pellets, which
fit into the needle of a dart or syringe, has simplified vaccine handling and permitted long-term
storage of the controlled-release component. Vaccine delivery is now performed using a
jabstick on captured mares restrained in a field stock chute during routine horse gathers.
Provision of a vaccine-training programme has maximized personnel safety during vaccine
preparation and use.
(17) Kirkpatrick, JF. and Turner, A. 2003. Immunocontraception and effects on seasonal birth
patterns and foal survival among barrier island horses. J. Appl. Anim. Welfare Sci.6:301-308.
9
This study reports on the lack of effects of PZP immunocontraception on seasonality of foaling,
and on the health and survival of foals born to treated mares, over 14 years. In general, there
were no differences between those parameters in non-treated and treated horses.
Abstract: Despite a large body of safety data, concern exists that porcine zonae pellucidae
(PZP) immunocontraception-used to manage wild horse populations-may cause out-of-season
births with resulting foal mortality. Our study at Assateague, Maryland indicated the effects of
immunocontraception on season of birth and foal survival between 1990 and 2002 on wild
horses from Assateague Island. Among 91 mares never treated, 69 (75.8%) of foals were born
in April, May, and June (in season). Among 77 treated mares, 50 (64.9%) were born in season.
Of 29 mares foaling within 1 year after treatment (contraceptive failures), 20 (68.9%) were
born in season. Of 48 mares treated for greater than 2 years then withdrawn from treatment, 30
(62.5%) of 48 foals were born in season. There were no significant differences (p < .05)
between either treatment group or untreated mares. Survival did not differ significantly among
foals born in or out of season or among foals born to treated or untreated mares. Data indicate a
lack of effect of PZP contraception on season of birth or foal survival on barrier island habitats.
(18) Kirkpatrick JF, Turner A. (2007). Immunocontraception and increased longevity in
equids. Zoo Biology 26:237-244.
This study describes the link between PZP treatment and increased longevity at the individual
animal level.
Abstract: Intensive population management by means of fertility control has been shown to
change the age profile of a wild horse herd. The primary change has been an increase in the
number and percent of older animals, as expected, but also the appearance of new and older age
classes. An examination of direct effects of fertility control on two groups of treated animals
shows a significant increase in longevity over non-treated animals that is associated with
contraceptive treatment. The mean age at death (MAD) was calculated for 128 wild horses for
which precise birth and death dates were known, including 56 stallions, 42 untreated mares, 11
mares treated with a porcine zona pellucida contraceptive vaccine for 1–2 years, and 19 mares
treated with the same vaccine for ≥3 years. The MAD for stallions (10.3±0.84 [SEM] years),
and mares treated for 1–2 years (10.2±0.56), was significantly greater (P<0.05) than for
untreated mares (6.4±0.85), and significantly <19.9±1.66 for mares treated ≥3 years
(19.9±1.66).
(19) Lyda R., Hall R and Kirkpatrick JF 2005. A comparison of Freund’s Complete and
Freund’s Modified adjuvants used with a contraceptive vaccine in wild horses. J. Zoo Wildl.
Med.36:610-616.
This paper shows that the use of Freund’s Modified adjuvant is equally effective as Freunds
complete adjuvant when used in wild horses, and without significant injection site reactions.
Abstract: Fifteen captive wild mares (Equus caballus) were treated with porcine zona pellucida
contraceptive vaccine and either Freund's Complete Adjuvant (n = 7) or Freund's Modified
Adjuvant (n = 8). All mares received a booster inoculation of porcine zona pellucida plus
Freund's Incomplete Adjuvant a month later. Anti–porcine zona pellucida antibodies were
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measured over 10 mo following the initial inoculation. There were no significant differences in
antibody titers at any point during the 10 mo, and seven of the eight mares in the Freund's
Modified Adjuvant group were above the 60% level at the end of the study, which is
considered to be the contraceptive threshold for horses. There were no significant differences in
titers between pregnant and nonpregnant horses, nor was there a significant correlation between
age and titers. One local injection site reaction occurred after booster treatment with Freund's
Incomplete Adjuvant, and 11 healthy foals were born during the course of the study. These data
suggest that Freund's Modified Adjuvant is an acceptable substitute for Freund's Complete
Adjuvant in certain free-ranging and captive wildlife species.
(20) Kirkpatrick JF, Turner A. (2008) Achieving population goals in long-lived species (Equus
caballus) with fertility control. Wildl. Res. 35:513-519.
This paper describes, for the first time, how entire populations of wild horses can be altered or
modified through the exclusiveuse of contraception.
Abstract: The ultimate goal of any wildlife contraceptive effort is some alteration of the target
population, either through a slowing of growth, or stabilisation or reduction of the population.
Early population models suggested that short-term contraceptive agents applied to long-lived
species would not achieve significant population changes. Native porcine zona pellucida
antigen (PZP), a short-term contraceptive vaccine, was applied to a herd of wild horses
inhabiting Assateague Island National Seashore, MD, USA, over a 13-year period, with an
immediate goal of achieving zero population growth, a secondary goal of reducing the
population from 175 to 150 and a tertiary goal of reaching 120 individuals, all without the
physical removal of animals. Contraceptive efficacy ranged from 92 to 100% on an annual
basis (96.28 ± 2.49%), and the percentage of adult females that was treated on any given year
ranged from 42 to 76% (67.78 ± 18.21%). The goal of zero population growth was achieved in
2 years, an initial decline in the population became apparent in 8 years and by Year 11, the
population declined to 135, a decrease of 22.8%. The lengthy period required for achieving a
population decline was caused by increasing body condition scores, reduced mortality and
significantly increased longevity among treated females.
(21) Turner JW, Liu IKM, Flanagan DR, Rutgberg, AT, Kirkpatrick JF, 2007.
Immunocontraception in wild horses: One inoculation provides two years of infertility. J. Wildl.
Manage. 71: 662-667.
This paper describes a two-year contraceptive effect from a single application of PZP as a
primer emulsion with lactide-glycolide pellets as boosters. It also addresses the use of fecal
steroids for pregnancy detection in free-roaming mares.
Abstract: Previous studies reported one year of contraception associated with a 1-injection
porcine zona pellucida (PZP) vaccine. We have subsequently determined contraceptive
effectiveness of a presumptive 1-injection, 2-year-duration PZP vaccine in free-roaming wild
horses (Equus caballus) in Nevada, USA. In January 2000, we captured, freeze-branded,
treated, and subsequently released 96 adult females that received 1) a primary dose of vaccine
emulsion consisting of aqueous PZP and Freund's Complete Adjuvant, and 2) booster doses of
PZP and adjuvant in controlled-release polymer pellets. We determined PZP release
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characteristics of pellets in vitro, prior to field use. We determined reproductive success in
treated and untreated females through October 2004 via measurement of estrone sulfate and
progesterone metabolites in fresh feces collected from the ground and by twice-annual foal
counts. Among treated females, annual reproductive success from 2001 though 2004
sequentially was 5.9%, 14.0%, 32.0%, and 47.5%. Untreated females showed average
reproductive success of 53.8 ± 1.3% across this period. This study revealed that: 1) PZP acted
as an effective contraceptive for 2 years posttreatment; 2) some residual contraceptive effect
remained in year 3; and 3) fertility returned to control levels by year 4 posttreatment. It appears
that controlled-release technology can replace both the second (1-month) and annual booster
injection of PZP vaccine, thereby decreasing cost and increasing efficiency of use of this
vaccine in wild horse management.
(22) Liu IKM, Turner JW, Van Leeuwen EM, Flanagan DR, Hedrick JL, Muruta K, Lane
VM, Morales-Levy MP. 2005. Persistence of anti-zonae pellucida antibodies following a
single injection of porcine zoane pellucidae in the domestic mare. Reproduction 129:181190.
This paper describes controlled trials with domestic horses, of tests with a one-shot long acting
form of the PZP vaccine.
Abstract: In this study of equids, we investigated the antibody response and the effect on the
estrous cycle following a single inoculation of porcine zonae pellucidae (pZP) employing
controlled-release methodology. We also investigated the use of two different water-soluble
adjuvants as an alternative to oil-based adjuvants. Twenty-seven domestic mares were
inoculated with various formulations of pZP and adjuvant. We showed that the anti-pZP
antibodies generated as a result of the inoculations persisted for at least 43 weeks (length of the
study). Of the various formulations used in the study, pZP and QS-21 water-soluble adjuvant,
administered in combination with an emulsified preparation of pZP and Freund’s Complete
Adjuvant generated a significantly (P < 0.05) higher titer of anti-pZP antibodies when
compared with other formulations employing the water-soluble adjuvant, Carbopol. Hormone
analyses for cyclicity indicated a high incidence and extended duration of persistent corpora
lutea among the treated mares. The positive control group of mares receiving two standard
inoculations of pZP and Freund’s Complete and Incomplete Adjuvants, as well as the placebo
group of mares injected with QS-21 only, also exhibited high incidences of persistent corpora
lutea. However, all mares eventually returned to normal cyclicity. The basis for the high
incidence and extended duration of persistent corpora lutea was unexplained. The results
demonstrate for the first time the persistent generation of anti-pZP antibodies following a single
inoculation of pZP incorporated into a controlled-released preparation in the horse. This study
further suggests that a single inoculation of pZP sequestered in a controlled-release lactideglycolide polymer may serve as an alternative to traditional two-inoculation protocols for
contraception investigations in the equine.
(23) Turner JW, Rutberg AT, Naugle RE, Manpreet KA, Flanagan DR, Bertschinger HJ, Liu
IKM. 2008. Controlled-release components of PZP contraceptive vaccine extend duration of
infertility. Wild. Res. 35:555-562.
This paper describes field trials of a one-shot long acting form of the PZP vaccine in Nevada
wild horses. Results indicated that this form of the vaccine is quite effective over three years.
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Abstract: Successful immunocontraception of wildlife relying on repeated access to individuals
for boosters has highlighted the need to incorporate primer and booster immunizations into one
injection. We have investigated use of controlled-release polymers (lactide–glycolide) in small
pellets to provide delayed in vivo delivery of booster porcine zona pellucida (PZP) antigen and
adjuvant. This report reviews pellet-making methodology, in vitro testing of controlled-release
pellets and in vivo effects of controlled-release PZP vaccine. We assessed 3 different
manufacturing approaches for producing reliable, cost-effective pellets: (1) polymer melting
and extrusion; (2) solvent evaporation from polymer solution; and (3) punch and die polymer
moulding. In vitro testing of release patterns of controlled-release formulations, towards
development of a 3-year duration vaccine, provided estimates for in vivo use of pellet
preparations. These in vitro studies demonstrated protein release delay up to 22 months using
100% l-lactide or polycaprolactone polymers. For in vivo tests, pellets (1-, 3-, and 12-month
release delay) serving as boosters were administered intramuscularly with PZP/adjuvant liquid
primer to wild horses (Equus caballus), white-tailed deer (Odocoileus virginanus) and African
elephants (Loxodonta africana). Horse field studies assessed fertility via offspring counts
and/or faecal-hormone pregnancy testing. Treatment decreased fertility 5.3–9.3-fold in Year 1
and 3.6-fold in Year 2. In preliminary testing in deer, offspring counts revealed treatmentassociated fertility reduction of 7.1-fold Year 1 and 3.3-fold Year 2. In elephants, treatment
elevated anti-PZP titres 4.5–6.9-fold from pretreatment (no fertility data).
(24) Killian G, Thain D, Diehl NK, Rhyan J, Miller L. 2008. Four-year contraception rates of
mares treated with single-injection porcine zona pellucida and GnRH vaccines and intrauterine
devices. Wildl. Res. 35:531-539.
This paper describes the comparative results of three approaches to horse contraception,
including a GnRH vaccine, the SpayVac form of PZP, and IUDs. The SpayVac was effective
but caused poorly described behavioral problems and significant uterine problems.
Abstract: 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 SpayVac were 100% (12 of 12), 83% (10
of 12), 83% (10 of 12) and 83% (10 of 12), rates for GonaCon were 94% (15 of 16), 60% (9 of
15), 60% (9 of 15) and 40% (6 of 15) and rates for IUD-treated mares were 80% (12 of 15),
29% (4 of 14),14% (2 of 14) and 0% (0 of 14). Antibody titres against PZP and GnRH declined
over the four-year study. For mares given SpayVac, uterine oedema was commonly observed.
IUDs were visible by ultrasonography in non-pregnant mustang mares, suggesting that
pregnant mares did not retain their IUD. IUD retention may 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.
(25) Ballou JD, Traylor-Holzer, K, Turner, A, Malo, AF, Powell, D, Maldonado, J, Eggert,
L. 2008. Simulation model for contraceptive management of the Assateague Island feral
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horse population using individual-based data. Wildl. Res. 35:502-512.
This paper provides the most sophisticated, accurate and current approach to modeling the
effects of contraception in a wild horse herd. What makes this paper so unique is that the data is
derived from individual life histories of horses and actual contraceptive results over 20 years.
Abstract: The National Park Service (NPS) manages a culturally significant population of feral
horses (Equus caballus) inhabiting the Maryland portion of Assateague Island, a barrier island
in the eastern United States. Rapid growth of this population over the past few decades from 28
to 166 horses negatively impacts native species and ecological processes on the island. Since
1994, contraception via porcine zona pellucida vaccine has been used to control horse numbers,
although herd reduction has been slower than initially expected, leading NPS to consider other
management options. An individual-based stochastic simulation model was developed using
the Vortex software program to examine the effects of different management strategies on the
population. Data from the managed population were used to populate the model parameters.
Model projections over the next 50 years using current management practices show an average
rate of population decline of 13% per year, suggesting that the population will reach the
management target of 80–100 horses in 5–8 years. The effectiveness of contraception to reduce
the herd and maintain it at various target sizes of 20–100 horses was also assessed. The
accumulation of inbreeding at each target population size was also modelled.
(26) Roelle, JE, Ransom, J. 2009. Injection-site reactions in wild horses (Equus caballus)
receiving an immunocontraceptive vaccine. Scientific Investogations Report 2009-5038,
U.S. G. S., Department of Interior, Reston, VA. 15 Pp.
This report summarizes the frequency and type of injection-site reactions from delivery of the
PZP vaccine to wild horses in Montana, Wyoming and Colorado.
Abstract: The U.S. Geological Survey and the Bureau of Land Management are conducting
research on the efficacy of the immunocontraceptive agent porcine zona pellucida (PZP) in
reducing fertility of wild horses (Equus caballus). As an antigen, PZP stimulates antibody
production when injected into many mammalian species. These antibodies bind to the external
surface of the ovum, preventing fertilization. By itself, PZP is only weakly immunogenic and is
therefore delivered with an adjuvant, most commonly one of the Freund adjuvants, designed to
further stimulate antibody production. Freund’s complete adjuvant (FCA) in particular is
known to be very effective, but may also be associated with undesirable side effects such as
formation of abscesses at injection sites. Such reactions may be exacerbated when accompanied
by the additional trauma of a remotely delivered dart. Because horses in our three study herds
were individually identifiable by color markings and harem association, we were able to
monitor mares for injection-site reactions (abscesses, nodules, swelling, and stiffness)
following inoculation with PZP.
In 100 injections delivered by hand we observed a single nodule, two instances of
swelling, and no other reactions. In two herds that received remotely delivered (dart) injections,
the frequency of reactions was about 1 and 6 percent for abscesses, 25 percent for nodules
(both herds), 11 and 33 percent for swelling, and 1 and 12 percent for stiffness. Abscesses were
too infrequent to allow meaningful analysis of the relation to covariates, but for the other types
of reactions we used logistic regression to examine the relation of occurrence to the delivery
method (rifle or CO2-powered blowgun), adjuvant (FCA, Freund’s modified adjuvant, and
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Freund’s incomplete adjuvant), dart trauma (normal or abnormal), and age of mare. Abnormal
dart trauma included cases where the dart hit bone or the needle broke off. We found strong
evidence (odds ratio = 5.023, P = 0.001) for a higher probability of occurrence of swelling
when darts were delivered by blowgun. We found some evidence (odds ratio = 8.729, P = 0.07)
that abnormal dart trauma led to a higher frequency of nodule formation. Nodules were the
most common reactions observed and often persisted for a year or more, but in our observations
they did not appear to change any animal’s range of movement or locomotor patterns and in
most cases did not appear to differ in magnitude from naturally occurring injuries or scars. We
were unable to perform histological examinations of these nodules, but they may be similar to
granulomas reported by other investigators following administration of Freund’s adjuvant.
(27) Kirkpatrick JF, Rowan A, Lamberski N, Wallace R., Frank K, Lyda RO. 2009. The
Practical Side of immunocontraception:zona proteins and wildlife. J. Reprod. Immuniol.
83:151-157.
This is a review of the performance of PZP in wildlife with regard to population changes and
safety. It is most useful for safety issues.
Abstract: With shrinking habitat, the humane control of certain wildlife populations is relevant.
The contraceptive vaccine based on native porcine zona pellucida (PZP) has been applied to
various wildlife populations for 20 years. Prominent efforts include wild horses, urban deer,
zoo animals and African elephants, among others. This approach has been successful in
managing entire populations and to date, no significant debilitating short- or long-term health
effects have been documented.
(28) Ransom JI 2009. Understanding the influences of immunocontraception of equids
through competing models of behavior. MS Thesis, Colorado State University, Fort
Collins, CA. 61 Pp.
This thesis reports on more than a thousand hours of observation of PZP treated wild horses
and concludes that there were few behavioral changes of consequence. (Thesis not included in
pdf list.)
Abstract: The rapid increase in human populations over the last several decades has lead to
shrinking habitats and limited resources for many large vertebrates around the world. In
response, managers seek effective tools for limiting population growth in some species, yet
little is known about impacts of fertility control on the behavioral ecology of wild, freeroaming animals. Feral horses (Equus caballus) in the western United States are ideal
candidates for contraceptive management due to broad scale federal protection, high fecundity,
and finite public land allocated for them. The complex mating system of feral horses relies on
behavioral manipulation of females by a polygynous male and a balance between reproductive
and maintenance behaviors to promote fitness gains. I investigated influences of the
immunocontraceptive porcine zona pellucida (PZP) on individual and social behavior within
bands of feral horses in three discrete populations.
(29) Ransom JI, Cade BS, Hobbs NT. 2010. Influences of immunocontraception on time
budgets, social behavior, and body condition in feral horses. Appl. Anim. Behavior Sci.
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124:51-60.
This paper reports on changes in time budgets of PZP-treated wild horses during the breeding
season, and concludes that such treatments “contribute few short-term bahavioral modifications
in feral horses.”
Abstract: Managers concerned with shrinking habitats and limited resources for wildlife seek
effective tools for limiting population growth in some species. Fertility control is one such tool,
yet little is known about its impacts on the behavioral ecology of wild, free-roaming animals.
We investigated influences of the immunocontraceptive porcine zona pellucida (PZP) on
individual and social behavior in bands of feral horses (Equus caballus) in three discrete
populations and used 14 hierarchical mixed effect models to gain insight into the influences of
PZP treatment on feral horse behavior. A model of body condition was the strongest predictor
of feeding, resting, maintenance, and social behaviors, with treated females allocating their
time similarly to control females. Time spent feeding declined 11.4% from low condition to
high condition females (F1,154 = 26.427, P < 0.001) and was partially reciprocated by a 6.0%
increase in resting (F1,154 = 7.629, P = 0.006), 0.9% increase in maintenance (F1,154 = 7.028,
P = 0.009), and 1.8% increase in social behavior (F1,154 = 15.064, P < 0.001). There was no
difference detected in body condition of treated versus control females (F1,154 = 0.033,
P = 0.856), but females with a dependent foal had lower body condition than those without a
foal (F1,154 = 4.512, P = 0.038). Herding behavior was best explained by a model of treatment
and the interaction of band fidelity and foal presence (AICc weight = 0.660) which estimated
no difference in rate of herding behavior directed toward control versus treated females
(F1,102 = 0.196, P = 0.659), but resident females without a dependent foal were herded 50.9%
more than resident females with a foal (F3,102 = 8.269, P < 0.001). Treated females received
54.5% more reproductive behaviors from stallions than control mares (F1,105 = 5.155,
P = 0.025), with the model containing only treatment being the most-supported (AICc
weight = 0.530). Treated and control females received harem-tending behaviors from stallions
equally (F1,105 = 0.001, P = 0.969) and agonistic behaviors from stallions equally
(F1,105 < 0.001, P = 0.986). Direct effects of PZP treatment on the behavior of feral horses
appear to be limited primarily to reproductive behaviors and most other differences detected
were attributed to the effects of body condition, band fidelity, or foal presence. PZP is a
promising alternative to traditional hormone-based contraceptives and appears to contribute
few short-term behavioral modifications in feral horses.
(30) Eggert, LS, Powell DM, Ballou JD, Malo, AF, Turner, A, Kumer, J, Zimmerman, C,
Fleischer RC, Maldonado JE. 2010. Pedigree and the study of the wild horse population of
Assateague Island National Seashore. J. Wildl. Manage. 74:963-973.
This is probably the definitive paper on the genetics of a wild horse population treated
chronically with PZP.
Abstract: Recently, a number of papers have addressed the use of pedigrees in the study of wild
populations, highlighting the value of pedigrees in conservation management. We used
pedigrees to study the horses (Equus caballus) of Assateague Island National Seashore,
Maryland, USA, one of a small number of free-ranging animal populations that have been the
subject of long-term studies. This population grew from 28 in 1968 to 175 in 2001, causing
negative impacts on the island ecosystem. To minimize these effects, an immunocontraception
16
program was instituted, and horse numbers are slowly decreasing. However, there is concern
that this program may negatively affect the genetic health of the herd. We found that although
mitochondrial DNA diversity is low, nuclear diversity is comparable to that of established
breeds. Using genetic data, we verified and amended maternal pedigrees that had been
primarily based on behavioral data and inferred paternity using genetic data along with
National Park Service records of the historic ranges of males. The resulting pedigrees enabled
us to examine demography, founder contributions, rates of inbreeding and loss of diversity over
recent generations, as well as the level of kinship among horses. We then evaluated the strategy
of removing individuals (using nonlethal means) with the highest mean kinship values.
Although the removal strategy increased the retained diversity of founders and decreased
average kinship between individuals, it disproportionately impacted sizes of the youngest age
classes. Our results suggest that a combined strategy of controlled breeding and
immunocontraception would be more effective than removing individuals with high mean
kinships in preserving the long-term health and viability of the herd.
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