USDA National Wildlife Research Center Symposia


Date of this Version

October 1993


Contraception in wildlife management. APHIS Technical Bulletin No. 1853. USDA, Animal and Plant Health Inspection Service, Washington, D.C., USA.


Fertility control in wild horses has been attempted with both stallions and mares. Nonreversible surgical sterilazaiton by means of vasectomy has been successful in inhibiting reporcution 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 SilasticTM 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 follow-up 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 males 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) free-roaming feral donkeys in Virgin Islands National Park, captive Przewalski’s horses, and onagers. Tests are currently underway on 150 feral horses in Nevada for the purpose of developing a one-inoculation 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.