For years, the Humane Society of the United States (HSUS) has advocated for the protection of wild horses and burros. We work collaboratively with many wild horse and burro advocacy groups to preserve and properly manage these animals in their habitats in the West. Free-roaming horses and burros deserve first to be given every chance to live out their lives wild and free, if and when intervention is required, we owe them our best efforts to ensure that any human actions that affect their lives—such as gathers, fertility control, transportation, confinement and adoption—are conducted in a way to assure their humane treatment.


The HSUS supports contraception as a humane means of managing wild and domestic animal populations where active management is needed or beneficial. We have forged partnerships with public agencies, communities, parks, zoos and other entities to test the use of immunocontraceptive vaccines to manage wildlife populations on the landscape level. For instance, the HSUS collaborates with biologists and officials to use immunocontraception to control the size of wild horse and burro herds and have conducted field studies in places such as Arizona, Colorado, Maryland, Montana, New Mexico, South Carolina and Utah.

In recent years, criticism has been directed towards the HSUS regarding our efforts to work with the BLM to expand the use of the immunocontraceptive vaccine Porcine Zona Pellucida (PZP) for managing wild horse and burro herds in the U.S. Some advocates oppose the use of any form of fertility control, including PZP, to manage wild horses and burros because they believe that herds should be managed using a “hands-off” approach—meaning the animals should live their lives free from any form of human intervention whatsoever.

Today, the law requires the BLM to manage our public lands using the multiple-use concept, and to balance the needs of the land, wildlife and wild horses and burros against those of livestock growers and a wide variety of other public interests. In short, the HSUS recognizes that, for now, political pressures from competing resource users on lands occupied by free-roaming equids will force active management of wild horse and burro populations.

The multiple-use mandate does not, in any way, absolve the BLM from managing wild horse and burro herds using the most humane, sustainable and fiscally responsible approaches. The HSUS has specific ideas about how this management should occur. Gathers and removal of horses and burros from public lands should never be conducted without simultaneous large-scale implementation of fertility control to break the agency away from the current cycle of large roundups and removals, and should focus on resolving major conflicts with endangered species, protecting the health or well-being of the horses and burros themselves, or preventing dramatic degradation of the landscape and must minimize stress to animals during every phase of wild horse and burro management activities.

Frequently asked questions

The HSUS has been conducting wildlife contraception research for over 20 years and is confident that PZP is a valuable tool for the humane and efficient management of wild horses and burros. We understand that, as with any new technology, those advocating for animals may be skeptical or concerned that PZP may have unintended consequences and the HSUS welcomes questions, observations and information regarding PZP. The following questions and concerns have recently been brought to our attention:

1. Does the HSUS profit from the use of PZP?

The HSUS and the Science and Conservation Center (SCC, the nonprofit organization that produces the drug) provides PZP at or below cost to our collaborators. Neither the HSUS nor the SCC profit from the use and sale of PZP.

2. Some claim that 75% of wild horse herds are not large enough to meet the minimum genetic viability requirements (i.e. 150–200 adult animals). How can the HSUS justify using PZP to manage these herds?

The estimate of 150–200 adult animals for genetic viability is derived from theoretical models that do not incorporate data from actual wild horse herds. These models assume, moreover, that there is absolutely no movement of horses into the herds. This is simply not true for many wild horse herds, which habitually exchange members with adjacent herds.

3. Some claim that mares treated with PZP will cycle repeatedly, which causes undue stress to the treated mares and/or out-of-season births (i.e. August-February). Is this true?

No. Based on 20 years of rigorous research, this claim is false and unsubstantiated (Kirkpatrick and Turner 2003). For example, mares treated with PZP in the Pryor Mountain herd did not cycle repeatedly if they failed to conceive following treatment with PZP. In fact, the mares did not extend their season of ovulation even when placed on high planes of nutrition (Kirkpatrick and Turner 1983).

Also, out-of-season births are rare but do occur in all wild horse populations—whether the mares have been treated with PZP or not. For example, of the 467 foals born in the Pryor Mountains herd since 1996, only 5% were born out-of-season—50% of these births were from untreated mares and 50% were from treated mares. In short, there is no known correlation between mares treated with PZP and an alleged “spike in out-of-season births” in the Pryors.

What we do know is that mares treated with PZP actually show comparable or better body condition than females who continue to have offspring. In wild horses, this condition actually leads to longer life spans. For instance, on Assateague Island, there are PZP-treated mares approaching 30 years old, which is about two times the expected lifespan of untreated wild horse mares.

4. Don’t mares have to be gathered in order to hand-inject the vaccine? If so, how can you safely gather mares in the late winter and early spring–the optimum time to treat mares?

Not always. While roundups may be necessary on some of the more remote herd management areas, mares do not always have to be gathered to treat them with PZP. PZP can and has been delivered remotely to wild horses on the range. The NPS has remotely-darted mares with PZP on Assateague Island, Cape Lookout National Seashore, and elsewhere for more than 20 years with no ill effects and without the need for stressful gathers.

Some herds are more difficult to access due to remote terrains, and for this reason, the HSUS is working in the field now with the BLM and private consultants to develop techniques for more passive gathering of wild horses and burros and for delivering longer-acting versions of the PZP vaccine by dart.

However, if the choice is between a winter gather to vaccinate with PZP so the herd can continue to live on the range or a removal at another time of year, the HSUS believes that the PZP treatment is the more humane alternative.


Although there is plenty of room for debate on how best to manage wild horses on public lands–even among those who care deeply about the fate of these animals—it is essential that wild horse advocates base policies and positions on the best available science. It is completely understandable for those who care about these animals to question how contraception may affect the health, welfare and behavior of horses (i.e. whether PZP causes mares to cycle repeatedly resulting in harassment of mares, out-of-season births and the needless suffering of foals, for example). The HSUS shares these concerns, but to date, we have found no evidence to suggest that PZP is causing any of these potentially negative impacts in treated wild horse and burro herds. But we have ample evidence that hundreds of horses and burros treated with PZP are now healthy and running wild and free with their herds on the range.


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.

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.

Dillon, M.personal comm.