Reducing Population Growth Rates: Fertility Control in Wild Horse Mares
Fort Collins Science Center, U.S. Geological Survey, Accessed 26 December 2014
Largely unchecked by natural predators, many wild horse populations grow at rates of 18–25 percent per year. . . . The Wild Free-Roaming Horses and Burros Act of 1971 requires the U.S. Bureau of Land Management (BLM) and USDA Forest Service to manage these free-roaming herds for a “thriving natural ecological balance.” . . .
. . . a National Park Service research team on Assateague Island National Seashore turned to an immunocontraceptive agent, porcine zona pellucida (PZP), which had been reported to block fertilization in dogs, rabbits, and primates. Experimental PZP application on the wild horses of Assateague Island commenced in 1988, resulting in promising reductions in the pregnancy rates of mares: by 1994, population growth began to stabilize solely through the use of PZP immunocontraception. . . .
. . . In addition, in 2011 the USGS, BLM, and the Animal and Plant Health Inspection Service (APHIS; Department of Agriculture) began studies on the safety and efficacy of SpayVac®, a form of PZP made using a proprietary technology developed by Immunovaccine Inc. (Halifax, Nova Scotia, Canada).
How PZP works
In order for sperm to attach to the ovum and fertilize the egg, there must be complementary proteins on both the surface of the sperm and the zona pellucida (ZP) of the ovum. PZP acts as a foreign protein against which the treated mare produces antibodies (thus, the PZP fertility control agent is actually a vaccine). These antibodies attach to the mare’s zonae sperm receptors on the ovum and block fertilization. Domestic pig ovaries (obtained from slaughterhouses) are minced and the PZP is obtained from screening filtration. An adjuvant is mixed with the PZP to enhance its effectiveness when it is injected into mares intramuscularly. Once injected, it causes an immune response, making the mare infertile. Over time, the antibody titers fall and fertility returns. With the liquid vaccine, a booster injection can be given at 10 months to raise the titers back to the infertile range. . . .
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