Controlling Wild Horse Populations Via Immunocontraception

Immunocontraception is a technique that uses the body's own immune system to prevent pregnancy.

"For almost four decades we have known that antibodies against the membrane of a mammal's unfertilized egg can be used to inhibit fertility," explained Jay F. Kirkpatrick, PhD, director of the science and conservation center at ZooMontana, located in Billings, Mont.

Kirkpatrick said these antibodies--produced in response to a group of proteins derived from the membrane surrounding pig eggs--will block fertilization. The proteins can be administered to a variety of mammals like a vaccination to control reproductive rates.

Because some mammals pose significant population problems, researchers evaluated the use of a porcine zona pellucida (PZP) vaccine to control wildlife populations.

"To date, PZP has been tested in dozens of populations including equids, deer, and even African elephants," Kirkpatrick said. "PZP is a safe, effective, and practical method for controlling wildlife fertility."

The researchers came to this conclusion after studying the effect of PZP in a population of wild horses on Assateague Island National Seashore located off the coast of Maryland. Mares were administered PZP via a small dart, followed by a booster dose one year later. After three consecutive years of treatment, the mares were left untreated until they produced a single, healthy foal. Mares were then subsequently placed back on treatment.

"After 14 years of immunocontraception, the population was reduced from 175 to 122, which approached the goal of 120 horses," Kirkpatrick relayed.

After analyzing 35 years of data on this and other animal groups, Kirkpatrick and colleagues concluded that the PZP vaccine:

  • Can be delivered remotely;
  • Is safe to administer to pregnant mares;
  • Does not change seasonal birth patterns, foal survival, or subsequent fertility of foals exposed to the vaccine during gestation;
  • Is reversible;
  • Void of debilitating long-term effects, and
  • Only rarely results in injection site reactions, such as abscess formation.

"The PZP is close to meeting the characteristics of the ideal wildlife contraceptive and has an important role in the management of wildlife populations," said Kirkpatrick.

Additional research focusing on long-acting PZP vaccines might alleviate the need for annual boosters and creating newer forms of the vaccine that will expand the number of animals that could be treated. PZP is presently labor-intensive to produce.

The study, "The practical side of immunocontraception: zona proteins and wildlife," was published in the December 2009 edition of the Journal of Reproductive Immunology. The abstract is available on PubMed.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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