Immunocontraception and Increased Longevity in Equids
American Association of Zoo Veterinarians Conference 2006
Jay F. Kirkpatrick1, PhD; Allison Turner2, BS
1The Science and Conservation Center, Billings, MT, USA; 2Assateague Island National Seashore, Berlin, MD, USA


Intensive population management of wild equids by means of fertility control has been shown to change the age profile of the treated population.6 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. A similar increase in body condition scores and a decrease in mortality was also revealed within a population under intensive contraceptive management.2,6 An examination of direct effects of fertility control on individual animals in a herd of wild horses on Assateague Island National Seashore (ASIS), MD, over a 30-year period, reveals a significant increase in longevity that is associated with contraceptive treatment and decreased pregnancies. Undetected neonatal losses were accounted for by means of remote pregnancy testing by means of urinary or fecal estrone conjugate and immunoreactive progesterone metabolites.3-5

The standard treatment protocol in the ASIS management plan applies initial contraception at 2 years, and booster inoculations at age 3 and 4 years. Previous studies demonstrated that ASIS mares treated for 3 years consecutively would take anywhere from 1–4+ years to become fertile again, based on current reversibility data,2 which would cause a significant decrease in foaling from age 5 to beyond 9 years, which are the years of peak reproductive success. On ASIS, a study of 14 mares, all 3 years or older, over an 8-year period and prior to any contraceptive treatment, revealed that they produced a mean of 5.0 foals during that 8-year period.1 Foals among a different set of 14 ASIS mares, all 3 years of age or older and treated with PZP for varying periods of time, produced a mean of 0.5 foals during their lives.

The mean age at death (MAD) was calculated for 128 wild horses over a period of 19 year for which precise birth and death dates were known, including 56 stallions, 42 untreated mares, 11 mares treated with a porcine zona pellucida (PZP) contraceptive vaccine for 1–2 years, and 19 mares treated with the same vaccine for ≥3 years. Results are given in Table 1.

Table 1. Mean ages at death for Assateague horses as a function of length of PZP treatment



Untreated males

Mares treated <3 years

Mares treated ≥3 years






Mean age at death (years)










The mean age at death (MAD) of stallions was not significantly different from the MAD of mares treated for <3 years (p=0.973; t=0.032) but was significantly greater than that of untreated mares (p=0.0006; t=3.53), and significantly less than the MAD of mares treated for 3 years or > (p=0.005; t=5.51).

The MAD of untreated mares was significantly less than that of mares treated for <3 years (p=0.064; t=2.84), and significantly less than the MAD of mares treated for 3 years or > (p=0.0001; t=9.67).

The MAD of mares treated for <3 years was significantly less than the MAD of mares treated for 3 years or > (p=0.005; t=3.92).


The management of long-lived ungulate populations by means of contraception is possible, but there will be an increase in the longevity of the treated animals, and a significant change in the age profile of the herd. These changes must be accompanied by a concomitant adjustment of breeding strategies and/or de-accession plans if the herd numbers are to be kept constant.


This research was supported by the National Park Service Grant CA-1600-30005, National Institute of Health Grant 1 R15 HDZ6898-01A1, Assateague Island National Seashore, and the Science and Conservation Center. Special thanks to J. Kumar, B. Rodgers, G. Olson, and C. Zimmerman, of the National Park Service.

This study is pending publication in Zoo Biology.

Literature Cited

1.  Keiper RR, Houpt K. Reproduction in feral horses: an eight-year study. Am J Vet Res. 1984;45:991–995.

2.  Kirkpatrick JF, Turner A, Reversibility of action and safety during pregnancy of immunization against porcine zona pellucida in wild horses (Equus caballus). Reproduction (Suppl.). 2002;60:197–202.

3.  Kirkpatrick JF, Kasman L, Lasley BL, Turner JW. Pregnancy determination in uncaptured feral horses. J Wildl Manage. 1988;52:305–308.

4.  Kirkpatrick JF, Lasley BL, Shidleler SE. Urinary steroid evaluations to monitor ovarian function in exotic ungulates. VII. Urinary progesterone metabolites in the Equidae assessed by immunoassay. Zoo Biology. 1990;9:341–348.

5.  Kirkpatrick JF, Shideler SE, Lasley BL, Turner JW. Pregnancy determination in uncaptured horses by means of fecal steroid conjugates. Theriogenol. 1991;35:753–759.

6.  Turner A, Kirkpatrick JF. Effects of immunocontraception on population, longevity and body condition in wild mares (Equus caballus). Reproduction (Suppl.). 2002;60:187–195.


Speaker Information
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Jay F. Kirkpatrick, PhD
Science and Conservation Center
Billings, MT, USA

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