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.”
Over the next four years, NPS researchers observed the mares for:
- Foaling rates;
- Foal survival;
- Mare body condition;
- Mare reproductive behaviors; and
- Time budgets, such as time spent eating, moving, resting, or doing other behaviors.
In the second foaling season (the first expected to be affected by vaccination), the proportion of treated mares that foaled was 35% less than untreated mares. The third foaling season saw the proportion of treated mares that foaled 30% less than untreated mares. But the fourth foaling season, researchers observed no statistical difference in foaling rates between treated and untreated mares.
NPS managers observed minimal changes to reproductive behaviors or time budgets, Powers said. Of the treated mares, 80% had injection site swelling one to four years after treatment.
Overall, NPS researchers found a single vaccination with the GnRH vaccine provided a modest decrease foaling rates for two years post-vaccination and had little effect on social behaviors, she said, adding that treated mares had no more or less reproductive interaction with stallions than untreated mares.
She also said treated mares did have apparent inflammatory reactions at injection site, but this did not appear to affect their well-being, as they did not become lame or lose body condition.
“The NPS is currently studying the efficacy of revaccination four years after the first vaccination, and researchers are interested in how long a booster vaccine might last and how effective it can be at preventing pregnancy,” Powers concluded.
Source: The Horse