PZP Fertility Control: FAQs and Insights
Porcine Zona Pellucida (PZP) is a fertility control vaccine administered to female horses on the range through remote darting, effective for one year. Another formulation, PZP-22, is injected by hand and can last for 22 months. PZP is scientifically proven, with over three decades of use, and is recommended by the National Academy of Sciences for federally protected wild horse herds. It offers a humane alternative to roundups and removals, providing a promising strategy for managing wild horses in their natural habitat.
Why the Need for Fertility Control?
Wild horses in the United States are confined to Herd Management Areas (HMAs) and Wild Horse Territories (WHTs) on public lands managed by the Bureau of Land Management (BLM) and U.S. Forest Service (USFS). These lands are used for various commercial and recreational activities, often prioritizing livestock grazing over wild horse preservation. As a result, wild horses are frequently rounded up, leading to compensatory reproduction and an endless cycle of removals.
Natural predators like mountain lions and wolves are insufficient to regulate wild horse populations due to hunting and government control programs. Thus, wild horse populations, if unmanaged, stabilize near their food-limited ecological carrying capacity, posing threats to their welfare and the environment.
The American Wild Horse Conservation (formerly American Wild Horse Campaign) aims to secure fair resources for wild horses and protect predators to restore ecological balance. However, managing wild horse population growth is necessary, and PZP fertility control vaccines offer a humane, minimally intrusive solution that preserves natural behaviors.
What Is PZP?
The PZP vaccine, produced by the Science and Conservation Center and registered with the Environmental Protection Agency (EPA) as ZonaStat-H, prevents fertilization by inducing an immune response that stops sperm from binding to the egg. It does not affect hormone production, minimizing behavioral impacts, and is reversible.
Is PZP Effective?
Yes, the National Academy of Sciences found PZP to have a 91.5% efficacy rate. Research shows that mares treated for consecutive years take longer to return to fertility, requiring fewer treatments. PZP is administered to a percentage of breeding-age female horses to reduce population growth rates effectively.
How Does it Work?
PZP is administered via a dart injected into the muscle at the horse's hip, preventing fertilization through an immune response. It is safe for pregnant and nursing mares. Initially, mares receive a primer and booster dose, followed by annual boosters. After consecutive treatments, the vaccine's reversibility decreases, reducing treatment frequency while maintaining infertility. Each PZP program is customized based on herd dynamics and geography.
Is PZP Expensive?
No, a PZP dose costs $30, and total treatment costs about $220 per horse. In contrast, the BLM spends $48,000 per horse in off-range holding facilities. Expanding PZP programs could significantly reduce long-term costs.
Fertility control is “a more affordable option than continuing to remove horses to long-term holding facilities.” — National Academy of Sciences, 2013
Is it Difficult to Locate the Horses to Administer PZP Vaccines?
No, darting teams track horse movements and use game cameras and drones to locate herds. Horses are often darted at natural water sources or through baiting. Each PZP program is tailored to the specific herd and geography.
The American Wild Horse Conservation's Virginia Range Program in Nevada is the world's largest fertility control program, managing over 3,000 wild horses. The program uses a scientific database updated by field photographers, with facial recognition software in development. No helicopter roundups are necessary for PZP's success.
Learn more about the Virginia Range Fertility Program.
What’s the Main Barrier to Using PZP?
The BLM has not voluntarily adopted PZP, relying instead on mass roundups and removals, which exacerbate population growth issues. The National Academy of Sciences noted that removals are counterproductive. The BLM should engage with organizations, purchase equipment, and support training to expand PZP programs.
Where is PZP Being Used?
- Assateague Island, Maryland: PZP has been used by the National Park Service since 1988, achieving zero population growth in 2 years and negative growth in 8 years.
- Spring Creek Basin, Colorado: PZP has stabilized the wild horse population since 2012, with no removals since 2011.
- Fish Springs, Nevada: PZP has achieved zero population growth since 2018.
- Virginia Range, Nevada: The largest wild horse fertility control program, achieving near-zero growth since 2019.
- Salt River, Arizona: PZP has reduced foal births significantly since 2019, achieving below zero growth by 2021.
- PZP is also used in several other HMAs, including Sand Wash Basin, Little Book Cliffs, and Pryor Mountain Wild Horse Range.
How Can BLM Begin to Implement an Effective Fertility Control Vaccine Program?
The BLM could expand existing PZP programs and implement new ones in suitable HMAs without further roundups. Effective implementation requires treating sufficient mares and administering boosters, with specific plans for each HMA.
Does “Zero Population Growth Rate” Mean Extinction?
No, zero population growth means births equal deaths annually, maintaining a stable population size and genetic diversity.
Is PZP a Pesticide?
No, PZP is an immunocontraceptive vaccine used for wildlife management, registered with the EPA due to its environmental application. It provides a humane way to manage wild horses, preserving them for future generations.