For more than half a century, the Humane Society of the United States and the Humane Society Legislative Fund have campaigned for the safety and preservation of wild horses and burros on our western rangelands. There is room for debate on how best to manage wild horses and burros on public lands—even among those who care deeply about the fate and welfare of these animals—but it is essential that wild horse and burro advocates use the best available science to support the implementation of non-lethal management approaches.

It is understandable that some advocates are concerned about how contraception may affect the health, welfare and behavior of wild horses and burros. The HSUS is attentive to these concerns, but we have neither seen nor found any evidence to suggest that porcine zona pellucida (PZP) vaccines (i.e. native PZP, ZonaStat-H and PZP-22) cause any potentially negative effects in treated herds. To the contrary, there is ample evidence that hundreds of wild horses and burros treated with PZP are healthy and thriving  on the range.

The HSUS has worked with key scientists to conduct wildlife contraception research for nearly 30 years. We are confident in PZP as a valuable tool for the humane and cost-effective management of wild horse and burro populations.  As with most emergent technology, our understanding of fertility control vaccines evolves over time on the basis of new research and practical experience. We are committed to using the best available science when developing our recommendations for PZP’s use in managing wild horse and burro populations.


How can PZP (branded as ZonaStat-H) be applied in a way that ensures the long-term genetic and population health of wild horse and burro herds? 

Reversible contraception using safe, proven vaccines like PZP safeguards the population and genetic health of wild herds much more effectively than current removal practices on our Western ranges. Unlike removals, reversible contraception keeps treated mares in the gene pool and may actually extend their lives, increasing generation time and slowing the loss of genetic diversity that can occur through chance (“genetic drift”).

The maintenance of genetic diversity in wild equine populations is subject to other influences besides the use of PZP. Genetic and population health depend on the size of the herds and whether new animals join them. Even one newcomer per generation can offset the loss of genetic diversity. Relatively few wild horse and burro herds are completely isolated, and many nearby herds regularly exchange members. 

Whatever decisions or actions they may take to manage a wild horse or burro population, managers must watch for signs of inbreeding (such as low foal survival rates) when overseeing small, isolated herds.  

How does the application of PZP affect the behavior of treated mares?

After more than 30 years of research and study all over the country, there is no evidence that PZP alters the social organization of wild horse herds. Mares continue to live in stable groups with immature offspring, and stallions live in loose “bachelor” groups and compete to control groups of mares. While mares without foals may behave differently than mares with foals, any documented increases in movements of mares between groups in treated populations are similar to those observed in untreated herds.   

How does the application of PZP influence the survival of foals born as the effects of the vaccine wear off in mares?

Research shows that PZP does not negatively affect the survival of foals. Data from treated herds in Utah and Colorado show 94%-97% foal survival, similar to untreated herds.[1] While treated herds generally have wider and later birth peaks, observational data collected at Utah, Colorado and Maryland PZP management sites has shown that a break from continuous foaling improves a mare’s body condition, leaving her better prepared to nurse a foal whenever she gives birth again.[2] [3]

What is the role of helicopters and bait trapping in gathering mares to treat them with PZP?

PZP (and PZP-22, a version of the vaccine that incorporates timed-release pellets) can be injected by hand or by dart. To effectively stabilize and/or reduce populations, a high proportion (>80%) of mares and jennies (female burros) within a given herd must be treated. In some herds, including island herds and herds whose members are somewhat acclimated to people, the vaccine can be delivered to most mares by dart. In other contexts, where wild horse and burro herds occupy challenging terrain or are scattered across enormous ranges, and thus may be more wary of people, attempts to directly approach them and dart is not feasible. The majority of BLM Herd Management Areas fall into this category.

Where darting is not practicable, the options for gathering and treating mares with fertility control vaccines are bait trapping and helicopter. The HSUS has collaborated in studies to explore how, where and when baited traps can be used for hands-off administration of fertility control vaccines by dart. However, darting and bait trapping are not realistic  options for most wild horses and burros on public lands. For this reason, managers will continue to rely on helicopter to support gathers, even as we continue our work to develop new techniques and delivery systems that will eliminate any stressful management practices.  

What is PZP-22?

PZP-22 is a PZP vaccine preparation (not yet federally registered) that adds a set of three timed-release PZP pellets to facilitate successful delivery. PZP-22 is designed to provide multiple years of contraception with a single treatment, essential for herds who cannot be easily accessed for treatment every year.

All PZP-22 trials in wild horses have demonstrated at least one year of effectiveness with a single initial shot. Field studies have shown that PZP boosters given two or three years after initial treatment add at least three more years of contraceptive effectiveness.[4] Research is also underway to replicate the two to three years of contraception observed in early PZP-22 trials. 

What determines the cost of the PZP vaccine?

PZP is produced, distributed and sold at or below cost by the Science and Conservation Center in Billings, Montana. The HSUS and HSLF do not profit from sale and distribution of PZP vaccines: None of the funds from the sale of PZP are distributed to the HSUS or HSLF. The Environmental Protection Agency registers these types of fertility control vaccines for use as management tools on specific wildlife species. The HSUS obtained the registration for use of PZP on wild horses and burros as a management tool.

What is the Next Generation PZP project?

In many areas, geography makes repeated immunizations of free-ranging wildlife populations challenging and impractical. For example, a mare lives about 20 years and may need to be treated as many as five times over the course of her natural life. For that reason, we need to develop longer acting vaccine formulations as well as practical application improvements to expand their use.

In 2016, Purdue University, in partnership with the HSUS, launched the Next Generation PZP research project with the goal of producing a vaccine to provide multi-year contraception to help manage populations of wild horses, burros, urban deer, African elephants and other species. A longer-lasting formulation would mean fewer vaccinations over the course of an animal’s life, offering important time and cost benefits to local, state and federal agencies, NGOs and other entities seeking to incorporate immunocontraceptive vaccines into their wildlife management programs.

What is the Platero Project?

The Platero Project is a collaboration between the HSUS and the Bureau of Land Management to assess the feasibility of using PZP to manage the wild burro population in the Black Mountain Herd Management Area in northwestern Arizona. The four-year pilot project started in August 2017 with support from an HSUS donor and a supplementary grant from the BLM. The project is named for the Spanish Nobel Laureate Juan Ramon Jimenez's book about a faithful and friendly burro.

What is GonaCon? Is it as effective as PZP vaccines?

GonaCon is a gonadotropin-releasing hormone (GnRH) immunocontraceptive vaccine developed by scientists at the U.S. Department of Agriculture’s Wildlife Services’ National Wildlife Research Center. The single‐shot, multi-year vaccine stimulates the production of antibodies that bind to GnRH, a hormone in an animal’s body that signals the production of sex hormones (e.g., estrogen, progesterone and testosterone). By binding to GnRH, the antibodies reduce its ability to stimulate hormone release. Sexual activity is decreased, and animals remain in a nonreproductive state. GonaCon is registered with the EPA for use in wild horses, wild burros and white-tailed deer.

GonaCon has been shown to be effective for two or more years with a single administration in wild horses, but thus far only in a fraction of treated individuals.[5] [6] [7] In 2009, scientists launched a field study to determine the efficacy and duration of infertility from both a single immunization and a subsequent reimmunization of GonaCon on a herd of free-roaming wild horses in Theodore Roosevelt National Park in southwestern North Dakota.[8] As expected, in the first two years of the study, a single immunization with GonaCon was minimally effective in reducing fertility in treated mares. However, as with PZP-22, reimmunization of previously treated mares resulted in a significant reduction in fertility for three or more consecutive years. In short, GonaCon, like PZP-22, can produce five to seven years of infertility in free-roaming wild horses with one initial treatment and a single booster two to three years later.

[1] Rutberg, A., Grams, K., Turner, J.W., Hopkins, H. 2017. Contraceptive efficacy of priming and boosting doses of controlled-release PZP in wild horses. Wildlife Research 44(2), 174-181, (27 April 2017).

[2] Kirkpatrick, J.F., and A. Turner. 2003. Absence of effects from immunocontraception on seasonal birth patterns and foal survival among barrier island horses. Journal of Applied Animal Welfare Science 6: 301-308.

3] Kirkpatrick, J.F., and J.W. Turner, Jr. 1983. Seasonal patterns of LH, progestins and estrogens in feral mares. Journal of Equine Veterinary Science 3:113-118.

[4] Rutberg, A., Grams, K., Turner, J.W., Hopkins, H. 2017. Contraceptive efficacy of priming and boosting does of controlled-release PZP in wild horses. Wildlife Research 44(2), 174-181, (27 April 2017).

[5] Gray M, Thain D, Cameron E, Miller L. Multi-year fertility reduction in free-roaming feral horses with single-injection immunocontraceptive formulations. Wildl Res 2010; 37: 475–481

[6] Killian G, Thain D, Diehl N, Rhyan J, Miller L. Four-year contraception rates of mares treated with single-injection porcine zona pellucida and GnRH vaccines and intrauterine devices. Wildl Res 2008; 35: 531-539.

[7] Baker D, Powers J, McCann B, Oehler M, Bruemmer J, Galloway N et al. Gonadotropin releasing hormone vaccine (GonaCon-Equine) suppresses fertility in free-ranging horses (Equus caballus): limitations and side effects of treatment. 8th International Conference on Wildlife Fertility Control. Washington, DC; 2017. p. 111.

[8] Baker, D.L., Powers, J.G., Ransom J.I., McCann, B.E., Oehler, M.W., Bruemmer, J.E., et al. (2018) Reimmunization increases contraceptive effectiveness of gonadotropin-releasing hormone vaccine (GonaCon-Equine) in free-ranging horses (Equus caballus): Limitations and side effects. PLoS ONE 13(7): e0201570.