Remotely Administered Immunocontraception as an Effective and Humane Management Tool for Feral Horses (Equus caballus) in Overpopulated Rangelands

Since the advent of world travel and exploration, humans have been introducing animals to new countries and environments to which they were not native. Wild horses in North America are protected by the Wild Free-Roaming Horses & Burros Act, and their growing populations can damage native species and ecosystems. These feral horses organize themselves into harems consisting of several mares, a dominant stallion, and occasionally subordinate stallions. In the breeding season, a peak in stallion libido and mare ovulation elicit distinct reproductive behaviors. Population numbers of feral horses (Equus caballus) need to be humanely controlled without the disruption of these key behaviors. The Adopt-A-Horse program, a current program consisting of roundup and public adoption for a fee, is not effective on its own. The proposed study examines two minimally-invasive immunocontraceptive methods that may be effective in reducing population growth: Gonadotropin releasing hormone (GnRH) agonists, antibodies that prevent mares from entering estrous, and porcine zona pellucida (PZP), an antibody that changes the conformation of ova sperm receptors to prevent fertilization. This potential study proposes that 8 independent populations of feral horses will be tracked for 4 years to assess normal behavior. After 4 years, populations will undergo one of four treatments: control (n=1), roundup and adoption (n=1), mares treated with 2-year remotely administered PZP vaccine (n=3), and mares treated with 2-year remotely administered GnRH agonist (n=3). Urinalysis will be used to test for pregnancy, and behavior of mares will be monitored over the course of 4 years after administration. Combined observations of behavioral effects and growth rates will be used to determine the most efficient and humane method of population control. Both chemical methods of fertility control should greatly reduce the overall number of foals produced. Where PZP will potentially be the better choice for immunocontraception due to its minimal interference with the endocrine system of feral horses, GnRH agonists are likely to affect normal behavior and may not be suitable for implementation in wild rangelands. Efforts to control populations will be most effective when the current Adopt-A-Horse program is combined with administration of PZP every second year. Success of an immunocontraception program for feral horses in overpopulated rangelands may pave the way for more invasive populations to be controlled in this manner