Sterilization for Wild Horses

Wild Horses & Burros home

Sterilization for Wild Horses

Wild horses have roamed in North America for centuries, and in fact, evolved as a species on this North American continent 50 million years ago. Wild horses belong on the public lands in the United States, yet greedy special interest groups that want to reap personal profit from public lands hungrily desire their removal. The Bureau of Land Management (BLM), under the United States Department of the Interior, has succumbed to pressure from these special interest groups to put into practice management plans, including cruel and permanent surgical sterilization, that would eliminate these historic and iconic animals from their native rangeland homes.

The BLM uses artificial and scientifically unsubstantiated numbers, called Appropriate Management Levels (AML), to arbitrarily assign the number of wild horses and/or burros allowed to exist on specific areas of public lands where they are allowed, known as Herd Management Areas (HMAs). These AML numbers for wild horses and burros are often too low to sustain genetic viability in many HMAs, and too low in general, to reflect the accurate density that should be allowed.



Ranchers want to profit by grazing their own exploited animals at public expense on public lands. They want more forage and resources that belong to the American people and other animals for their own personal use. However, the American public would rather see the wild horses using their public lands.

For decades, to attain these unsupported AML numbers for wild horses and burros, the BLM has continued to follow its own failed policies of costly roundups and removals of horses and burros. When removed from their homes, wild horses and burros are cruelly ripped apart from their families and warehoused in cramped, filthy, and unsheltered pens at taxpayer expense. Often, these animals end up being sold to slaughter buyers.

Because of a scientific phenomenon called “compensatory reproduction,” wild herds of horses and burros compensate for the removals by increased breeding to fill the void. The roundups and removals are not only brutally cruel, using helicopters to chase the panicked animals into traps, but they are a costly failure as a protocol to bring HMAs to their dubious population goals. Therefore, the BLM has come up with another draconian plan to limit population growth: permanent surgical sterilization.


Surgical Sterilization is Cruel and Unethical

The preferred method of sterilization the BLM proposes to use on wild mares is a gruesome procedure so inhumane that many veterinarians refuse to perform it. The surgery, called ovariectomy via colpotomy, is controversial even for domestic mares who are used to human handling and given normal surgical protections such as sterile conditions, anesthesia, and complete long-term aftercare. The BLM plan is not conducive to any of those conditions which domestic mares can tolerate better.


Proponents of ovariectomy via colpotomy for wild horses and burros have tried to convince others that the barbaric sterilization procedure, which literally rips the ovaries out of wild mares using a metal rod and chain (called an ecraseur), is comparable to spaying a domestic dog or cat. It certainly is not! It is a highly dangerous procedure which is nothing like a spay procedure for animal companions. Scientists have weighed in, and in 2013 the National Academy of Sciences advised that the risk of prolonged bleeding and infection makes this procedure inadvisable for wild equines.

Each female is immobilized in a chute before suffering intense pain and distress as the practitioner violates the mare by blindly reaching in with the ecraseur to find and pull out the ovaries. Complications regularly occur, and many wild horses and burros can die as a result from hemorrhage and their insides falling out. The very nature of wild horses does not allow for the post-operative care necessary, and which is afforded to the few domestic horses unfortunate enough to undergo this surgery. Wild mares cannot stand calmly in cross-ties for days of recovery. Horses roll on the ground when they are in pain, and this alone would cause the rupture of their internal wounds.

Surgical sterilization is not the solution to population growth in wild herds for many reasons. Obviously, the inhumanity and mortality rate should be sufficient reason enough to stop this plan, but even more scientific reasons abound as well. If herds with only sterilized mares are returned to their HMAs, the herd will eventually go extinct as the herd members die and no new offspring are produced. If a few mares are left intact and returned along with the majority of sterilized mares, the gene pool will be so reduced that natural genetic viability will be lost and inbreeding will occur. Natural behaviors will be lost as the family bonds are altered. Dangerous stallion behavior will also ensue with too many stallions fighting more aggressively for the few fertile mares.


PZP Is a Humane, Reversible, Cost-Effective Solution

There is a humane, scientific, and viable alternative to permanent surgical sterilization. That alternative is humane, reversible fertility control. Beloved scientist, Dr. Jay Kirkpatrick, who passed away in 2015, was a pioneer of fertility control who specialized in the porcine zona pellucida (PZP) contraceptive. Dr. Kirkpatrick detested the wild horse roundups, stating, “They’re flat-out inhumane.” He was determined to find a way to manage herd populations so roundups would no longer be necessary. In 1998, he founded the Science and Conservation Center (SCC), in Billings, Montana, as an independent, non-profit organization. The SCC is the world’s only dedicated facility for the development of wildlife contraceptives, and now produces PZP on a not-for-profit basis and provides training for its use.

PZP is a reversible vaccine that has been scientifically studied on wild horses for decades. The PZP vaccine has been used successfully on the popular wild horses on Assateague Island National Seashore in Maryland since 1988! The Assateague herd is treated as wildlife and are not rounded up or removed from the island. Since then, other wild horse herds have also proven the safety and efficacy of PZP, yet government agencies do not allocate enough funds to administer it, and have even inexplicably halted some successful PZP programs in wild horse herds!

There are some common misconceptions about PZP that are simply untrue. One of these is that PZP is a “pesticide.” PZP is not a “pesticide,” despite being dumped into that category because there was no applicable category for its classification. PZP is simply a natural, isolated animal protein that does not contain chemical or synthetic components of any kind.

PZP can be administered by remote dart without capture or handling. Its effects are reversible and do not interfere with hormonal cycles (behavior). PZP has been proven safe, effective, and humane for over 30 years of usage in wild horse herds. Because PZP is a natural protein, it is readily destroyed in digestion, reduced to amino acids, and therefore cannot pass through the food chain intact and with biological activity to any animal that might ingest it.

PZP works by blocking fertilization in the treated mares. When the vaccine is injected into the hip muscle of the target female animal, it stimulates her immune system to produce antibodies against the vaccine. These antibodies also attach to the sperm receptors on her own eggs and distort their shape, thereby blocking fertilization. The specificity of the antibodies for the sperm receptor is absolute, and there is no cross-reactivity with any other organs, tissues or molecules in the mammalian body.

In the training that the Science and Conservation Center provides for people who administer PZP in wild herds, attendees learn the long history of studies proving both the safety and efficacy of the PZP vaccine. Included in the training is considerable “hands on” work, learning about the different dart guns to choose from to deliver the darts at varied distances, and how to use them.

In addition to learning how to administer the vaccine, identification and documentation are also critical to the success of a wild horse fertility control program. Horses must be definitively identified so the mares designated to receive the vaccination are darted. Records must be meticulously kept to preserve the genetic viability of the targeted herd, including photographs, notes, and regularly updated databases.

The darting of wild horses is humane because it is basically like a bee sting. The sound of the dart gun is more startling to the band of horses than the sting of the dart itself. A mare will jump and run from the sound of the gun and sting of the dart, but she and the rest of her band that run with her will usually come to a stop after just a few strides. The dart falls out immediately and is retrieved by the shooter.


Facts about PZP safety and efficacy:

1. Pregnancy is prevented approximately 90% of the time in treated animals;

2. The vaccine can be delivered remotely by small darts;

3. Contraceptive effects are reversible (up to five years in wild horses);

4. PZP is effective across many species;

5. No debilitating health side effects have been observed, even after long-term use;

6. No effects on social behaviors have been observed;

7. The vaccine cannot pass through the food chain;

8. It is safe to administer the vaccine to pregnant animals and has no effect on unborn foals.


Summary of PZP Fertility Control vs. Inhumane Surgical Sterilization

The BLM and U.S. Forest Service are charged with managing wild horses and burros on our public lands. Both agencies acknowledge that their current policies of roundups and removals are not working. Yet, they obtusely continue with them, largely ignoring the better option of reversible fertility control. The majority of the BLM’s wild horse and burro budget is spent on capture and holding, costing taxpayers many millions of dollars every year. Currently, less than 1% of its budget is spent on proven PZP fertility control.

The failed roundup and removal policies do not work, and after their capture, the holding of these animals in government run facilities is costly and unsustainable, which directly leads to “sale authority” animals, which puts them at risk of being sold for slaughter). Now, the BLM is proposing an equally bad plan: permanent sterilization.

Humane fertility control is the answer to saving these wild equines and preserving their freedom on our public lands.


What You Can Do To Help Wild Horses

You can support our work by donating