The National Park Service (NPS) is currently studying the use of fertility control vaccines to control feral horse populations on NPS lands.

Jenny Powers, DVM, PhD, an NPS wildlife veterinarian, shared preliminary results during a presentation at the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah.

Managers at Theodore Roosevelt National Park in North Dakota are seeking ways to control feral horse populations that include alternatives to helicopter roundups and sales. The horses, which some consider culturally significant and suggest are the origin of the Nokota breed, share the same range as elk, pronghorn, mule and white-tail deer, and bison in the 46,000-acre park, necessitating careful resource allocation and equine population management, Powers said.

In October 2009 NPS started a study using a gonadotropin-releasing hormone (GnRH) immunocontraceptive vaccine on mares in the park. First, park managers rounded up and removed half the park’s horses. They then blocked the remaining mature mares by age, body condition, and reproductive status. They separated the horses into a treatment group of 29 and a nontreatment group of 28 individual mares.

The treatment group received 2 mL of GnRH vaccine. The control group received 2 mL of saline. Most mares were pregnant at the time of treatment. “The GnRH vaccine we used showed promise as a multiyear vaccine after giving a single dose," Powers said. "This is highly unusual for a vaccine to work for more than a year and would greatly improve the efficiency over currently available fertility control vaccines.”