February 15, 2010 – PZP’s Reversibility

As we’ve gone over in previous posts, PZP is a vaccine. It influences immune responses that lead to the disruption of fertilization. PZP must be administered periodically as its ability to influence immune responses is reduced over time. Depending on the method of delivery, treatments must be given every one to two years at a minimum. But let’s think about the other side of all this: How many PZP treatments can a mare receive before her immune system has been so bolstered that she is unlikely to get pregnant again? That is, are PZP’s effects reversible? This will be the topic of today’s discussion.

When we talk about reversibility, we are really thinking about how the use of PZP in the short-term can lead to undesired effects in the long-term. Reversibility is something that is often desired in a wildlife contraceptive. As we’ll eventually see, PZP is used in many other wildlife species; and many of these species are endangered and in captive breeding programs. In this setting, it is important to prevent widespread pregnancy; but it is also important that non-reproductive animals will be capable of ensuring survival of the species. Similarly, it is important that wild horses, and other wildlife, can return to fertility after being treated with PZP as well. Further, it is important that the female offspring of treated animals also be healthy and be able to successfully reproduce. The goal of contraception is to stop growth and stabilize populations; it is not to harm individuals or to cause populations to go extinct.

A relevant study was (Kirkpatrick and Turner 2002) was done over a 12 year period, 1988 through 1999, on Assateague Island. (Though Assateague’s fertility control program started in 1994, research on PZP started in 1988.) The study sought to understand relevant issues, such as the reversibility of PZP was and the fertility of females born to treated mares. Treatments on Assateague Island consisted of one year doses of PZP delivered with field darts.

Reversibility was determined by making observations on the time it took for females to get pregnant after they were treated for a certain consecutive number of years and then no longer treated. Pregnancy was detected with remote pregnancy testing. Following is data on reversibility for treated females:

  • 16 females were treated for one year. 88% of the females were pregnant one year after treatment, while the remaining females were pregnant three years after treatment.
  • 5 females were treated for two consecutive years. 100% of the females were pregnant one year after treatment.
  • 32 females were treated for three consecutive years. 15% of the females were pregnant one year after treatment. By the last year of observation, year 4, 69% of the females had been pregnant. The time frame of the study didn’t allow for further observations to be made on the time it took for the remaining females to be fertile again.
  • 3 females were treated for four consecutive years. One of these females was pregnant three years after treatment, another was pregnant four years after treatment, and the last was pregnant eight years after treatment.
  • 2 females were treated for five consecutive years. One of the females was pregnant one year after treatment while the other was pregnant six years after treatment.
  • 5 females were treated for seven consecutive years. None of these females had gotten pregnant after 8 years of observation.

These results seem to show that mares will become fertile again if they have had a maximum of five consecutive years of treatment. Also, mares can get pregnant anywhere from one to eight years after their last treatment; an increased amount of time to return to fertility seems to correlate with higher numbers of consecutive treatments. During this study, the reasons for these reversibility patterns were not known. However, it was found that there were mares treated for at least seven years that were still ovulating; they were just not getting pregnant. This likely means that there were no significant effects on the health of mares treated for at least seven consecutive years.

While it is important to know that it is possible for mares to foal again after being treated with PZP, it is again important to also know that females born to treated mares are also capable of reproducing. During this study, approximately 84% of foals born to mares treated with PZP survived their first year. Within this group of surviving foals, 14 females were at least 6 years old during the study. Of these 14 females, 8 were not treated with PZP. All 8 of these untreated mares have produced at least one surviving foal.

Researchers have long worked to develop contraceptives that are reversible. The results of this study show that on Assateague Island, PZP’s effects can be reversed if a female has been treated for no more than five consecutive years. Because this is an obvious area of importance, it is likely that similar studies from other areas will also be coming out in the future. The data from such studies will further aide managers in developing fertility control programs that allow for the responsible management of wild horses.


Kirkpatrick, J.F. and Turner, A., 2002, Reversibility of action and safety during pregnancy of immunization against porcine zona pellucida in wild mares (Equus caballus): Reproduction (Suppl. 60), p. 197-202

Published in: on February 14, 2010 at 3:04 pm  Comments (12)  

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  1. Matt, this is very good to know, thank you again for all this information on PZP; I hope that it’s use will help roundups become a thing of the past. It would be great to know that CLoud, his family and the rest of the beautiful Pryor horses would never have to experience the terror of a helicopter again.
    take care,

    • Jan,
      I hope this too. Dr. Allen Rutberg once said something that I think is very true about all of this: “It works. It’s more of a political and regulatory fight than a technical issue.” That is to say, the use of PZP on wild horses isn’t about safety, efficacy, and the like. Genuine research has demonstrated what PZP can and cannot do. What prevents PZP from being used instead of, or along with, gathers seems to be sentiment from the public and sometimes from agencies. We’ll just have to wait and see what happens next here in the Pryors and in other wild horse herds of the West.

      • “Sentiment” seems to include issues to do with removal of vast acreage once available to the horses, it would seem, in my opinion. Case in point: fences. And other removals over many decades.

      • “What prevents PZP from being used instead of, or along with, gathers seems to be ♘ sentiment from the public. . . .♘ ”

        Matt, I think that is a statement that is blatantly obscuring the realities of the public lack of support for PZP.

        Reasons for public lack of support for PZP surely have included the issues of the removal of millions of acres of public lands in the West once available to the horses, for instance, or a lack of accurate inventories possibly going back to 1971. Or perhaps a lack of application in wild horse range planning and design.

        I understand that on Sable Island there is a herd which has regulated its own growth adequately since 1960. How that has occurred I do not know, but it again Sable Island sounds like a finite area. Hopefully there has been more at work there than huge die-offs due to starvation or other factors.

      • http://www.greenhorsesociety.com/horses/horses.htm

        (website for The Sable Island Horses and more info)

      • Hello,
        As I understand it, Sable Island’s wild horses experience starvation and die-offs; and this is how that population has been “managed.”

        I won’t disagree that other factors relating to past wild horse management actions have made some people hesitant to support the use of PZP. I think another major factor is also the amount of misinformation that has been spread on PZP; this has definitely had an effect on the way people perceive it. In my experience, I most often find that not fully understanding PZP leads to a lack of support for it. I was also originally skeptical about it until I took the time to learn about it.



      Capture Stress Affects the Free-Roaming Horse:

      It was reported on February 4th, 2010 that approximately twenty-five formerly free-roaming horses had abortions in the hours and days after strenuous captures in the Calico Complex Mountain Range of Nevada. Bloggers blogged, journalists scribbled, and wild horse advocates blamed the abortions and other horse deaths on a devious Bureau of Land Management (BLM) staff beholden to the cattle and energy lobbies. The advocates often cite the tortures found in the feature film, The Misfits, and quote Thelma Johnson while public observers recited their pledge to restore the protections of the now-gutted Wild Free-roaming Horses and Burros Act of 1971.

      Nonetheless, some physical, chemical, or mechanical processes caused more than twenty-four equine abortions in the hours and days just after capture at the Calico Complex. Did these processes contribute to what we veterinarians call an apparent “abortion storm?”

      In 2001, Holcomb and Ashley, at the University of Nevada-Reno, reported that “successful management of many species often relies on actions that involve intensive handling of individuals. Knowledge of how such handling may affect reproduction of a particular species is important and may be applicable to managing other species.” They used pregnancy testing, field observation, Bureau of Land Management (BLM) records, and adopter surveys to determine effects of stress induced by gather and removal management practices on the reproductive success of feral horses in the Garfield Flat (GF) Herd Management Area (HMA) in Nevada. They analyzed pre-release confinement effect data from gathers conducted in August 1993 and January 1997 and data on the additional effect of removal in 1997. The data for an un-gathered population in the Granite Range (GR) HMA in Nevada were used as controls for both years. Data from un-gathered GR horses also were used as controls for habitat effects on reproduction. “Granite Range and un-gathered GF mares in 1997 had similar reproductive success rates. Pregnant removed mares in 1997 had less reproductive success than un-gathered mares at GR (P=0.003) and GF (P=0.005). Gathered and released GF mares had less reproductive success than un-gathered GF mares (P=0.05). The results suggest that minimizing time that mares are held prior to release will reduce fetal loss.”
      During the fast and furious hours of any stimulating aerial herding the equine body goes through a series of stress induced biochemical, physiological, neurological, and mechanical effects. The heart and lungs accelerate, normal gut movement slows leading to increased gas production, blood vessels constrict in many parts of the body and dilate in the muscles, salivation increases, the colon evacuates, hearing is diminished, tunnel vision results in loss of peripheral vision, instantaneous reflexes accelerate, sweating begins, muscles fatigue, shaking results, and in the mare – occasionally abortion storms appear. All of these conditions could be considered signs of stress, but did the stress of the actual chase cause the abortions in an animal that evolved able to run while pregnant? Or was it some other capture related phenomenon?
      “Abortion in horses may result from a variety of causes. Infectious agents, such as bacteria, viruses or fungi, may attack the fetus or its membranes, resulting in fetal death and expulsion. Other factors attributable to the mare, fetus or external forces may also cause loss of the embryo or fetus. These factors include twinning, hormonal deficiencies, congenital anomalies, ergot alkaloid toxicity, and ingestion of tent caterpillar setae. ” According to some experts, nutritional deficiencies have not been associated with abortion in mares and in spite of common beliefs, experimental rough manual manipulation of the pregnant uterus has not caused abortion or embryonic death .
      I think we can all agree that major equine physiological occurrences result during the acute stress response of being chased by aircraft, land vehicles, and humans on foot. Those powerful effects are often mediated by way of chemical and electrical signals originating in the brain and affecting the adrenal gland and numerous other fear responsive organs. As a result, during the chase and for hours after, high-levels of many stress related hormones are produced or released by the adrenal glands, including adrenaline and cortisone. Though not definitively proven, increased cortisone levels have been implicated in contributing to abortion in horses. Some reports indicate that abortion in mares [occurring within hours to days of capture] is most likely caused by an increase in cortisone levels that initiate the parturition cascade in the mare. In one study, mares experiencing artificially induced colic who subsequently aborted had higher cortisone levels than untreated mares carrying to full term.
      If 50 horses perished as a result of gathering and handling 2,500 horses, the capture fatality rate is about two-percent. Though no deaths or some fatality rate lower than two-percent is preferred, two-percent is not ridiculously high for the capture and disposition of 2,500 formerly free-roaming horses. The exception that I know of for lower abortion and other fatality rates during capture and removal of more than 1,800 free-roaming horses was at White Sands Missile Range (WSMR) in 1995. After slow, multiple helicopter captures of 10 to 27 miles duration I observed 1 death due to capture technique, less than 10 deaths due to the combined veterinary processes, freeze-branding, other handling, and transporting to holding facilities, and I attended 2 abortions on WSMR. Neither of the abortions appeared related to the capture process. Of the 1,800 plus horses of all ages removed from WSMR during the birthing months of January to August, the adoption herds produced more than 300, 1995 foals. What might have caused the abortion storm at Calico?

      In my experience, and not judging a capture that I neither attended nor participated in, I make the following generalizations. The abortions at Calico were not likely the result of some faulty technical strategy during the actual aerial herding. Helicopter facilitated captures are the safest, least stressful, current strategy for capturing free-roaming horses on vast tracts of land. I doubt that the abortions can be attributed directly to the weather, the time of year, or the length of incarceration. Assuming water was available on the Calico Complex, poor range conditions did not cause the abortions unless poisonous plants were recently consumed in pathological quantities. If the horses had consumed adequate plant toxins they would likely have aborted in the harsh winter months prior to capture and did not have adequate photoperiod for recycling and breeding by early January 2010. Diagnostic testing and the age of each fetus at the time of abortion might help answer the toxin or infectious disease questions. Increased cortisone levels from acute stress during and just after capture may well have induced potential for the seemingly large numbers of abortions. So what really caused the pathological levels of stress at Calico?

      In my assessment, the most stress a free-roaming horse undergoes results from the close proximity of human handlers in the hours and days just after capture. Any presence of movement by a human on foot causes stress to the free-roaming horse. As a result, at WSMR, the technical specifications for capture demanded that the freshly captured horse be left alone with abundant average quality forage and free-choice clean water for not less than three contiguous days. I propose that the near presence of humans on foot and human related processing of freshly captured horses immediately after the Calico captures and not the actual aerial pursuit by itself was probably the causative-agent for untenable stress resulting in higher than normal cortisone levels in the mares. It is possible that the combined stress conditions led to an acute parturition cascade resulting in abortion in some mares at the Calico capture facility. However, technical capture strategy changes limiting humans on foot in or around the BLM corrals for three-days post-capture might reduce such abortion tragedies in future captures. That strategy is not without logistics, time, and cost considerations.

      Few of the claims of BLM mismanagement or advocacy legal maneuvers have slowed the western states’ public horse captures or resolved the free-roaming horse breeding-herd dilemmas we face. On the other hand, I have searched the various arguments about mismanaged public horse policy for fair-minded references to the good federal, state, and private free-roaming horse management found along the coasts of Virginia and the Carolinas. Those programs have produced effective public policy and precedent setting management of public horses. Perhaps we should study the Outer Banks horse management solutions and determine if those policies and practices apply to the management of public horses in the ten western states. Nevertheless, in order to understand the federal free-roaming horse policy and to offer palatable solutions for management of the western states’ free-roaming horses, we might remind ourselves of how America got to this crossroads in the mustangs’ long road to freedom . After all, when we capture a free-roaming horse, we destroy an aspect of the very freedom we treasure as Americans.

      Historical Perspectives on the Free-roaming Horse:
      At the beginning of the nineteenth century nearly two million wild horses, or mustangs, graced the western ranges, the rearing stallions – tails and manes flying – were almost mythical symbols of freedom, independence, and endurance. By 1970 free-roaming horse numbers on western states’ public lands were pared down to nearly 20,000 animals. The Wild Free-roaming Horses and Burros Act of 1971 (Public Law 92-195) was enacted to protect the remaining wild horses where then found on forty-seven million acres which became divided into three-hundred and three herd management areas.
      Sadly, within the last thirty years, some members of congress have worked tirelessly to zero-out over a quarter of the originally preserved acreage and have eliminated one hundred and eleven of the wild horse herd areas. The traumatic capture and adoption of free-roaming horses have been so successful, that six of the original sixteen western states that were home to wild horses in 1971 no longer have herds. Due to recent federal legislation by politicians beholden to private enterprises, free-roaming horses could vanish from the American west. What will the future hold for these noble creatures who are direct descendants of horses that once hauled us across the frontiers of America and helped us build a nation?
      Today, the attitude of “Live and let live,” when applied to the environment, just doesn’t work. Human population growth, urban sprawl, and natural resource development have made it necessary to manage everything – even wild things. So, to preserve the wildness and the world we love, we must intervene to save these free-roaming horses. The first intervention might be to detoxify the vocabulary.
      If you ask anyone vaguely familiar with horses about free-roaming American equine issues, be prepared to spend considerable time in debate. There are plenty of valid viewpoints, several of which are quite emotional. The multitude of positions has created a deafening whirl in the halls of Congress as well as out on the western rangelands. Concerns for and about wild horses pit urbanites against ranchers, public servants, and horsemen. For instance, just referring to free-roaming horses as wildlife is a great way to get bogged down in arguments, even law suits, about definitions. What distinguishes wild from exotic, mustang, feral (ranch horses gone loose), or indigenous? The truth is that, when cornered, free-roaming horses have plenty of equine wildness in their behavior, and it is that very wildness that captures the human imagination. Calling them ‘free-roaming’, as found in federal language, allows the discussion to concentrate on the horses themselves as a unique environmental and national treasure. The key is to find palatable, long-term solutions to preserve them, while at the same time protecting the environment and the interests of private citizens.
      As an equine veterinarian and professional horse welfare advocate with 25 years experience in dealing with and training thousands of un-gentled horses captured in western and eastern states, I completed seven annual contracts as the attending veterinarian with the Bureau of Land Management and New Mexico prison inmate horse training programs. I also provided the onsite technical support, veterinary care, and adoptions program for the rescue of the White Sands Missile Range horses from which Nobody’s Horses arose.
      From my perspective, in order to comprehend this often emotional debate, we must understand that there are two major horse populations in the United States; the domestic, private property horses and the publicly-owned, free-roaming horses found in the wild in many states or held in federal captivity awaiting fate. Only some of the free-roaming horses in the western states fall under the federal protections of ‘wild’ horse law. The other free-roaming horses, east or west of the Mississippi River, are on sovereign lands, military reservations, park service lands, federal lands not covered by ‘wild’ horse law, or on private property. In these various places the horses are either not protected by any law, as in the feral designation, or they are only protected by state and/or local laws. I consider only the federally protected, free-roaming horses in this discussion.

      At first glance and because Nobody’s Horses is a book about free-roaming horse rescue, the domestic group of equine might easily lope out of this discussion because they fall under the purview of domestic property law. Not so fast. Though the domestic population of 9 million horses is privately owned, current congressional potential for prohibiting horse slaughter for human consumption is locked between forces supporting and opposing such federal legislation. Because debate revolves around the acceptability of the slaughter of any horses, the fate of free-roaming horses enters the fracas. The publicly owned, free-roaming horse populations – hoofing out a living mostly on public lands – counts at about 64,000 head total. About one-half of these highly regarded horses – in effect, John Q public horses and taxpayer assets – range freely in ten western states. The discussion will return to their destiny later. The other 34,000 head, to the disquiet of some vocal sympathizers, are imprisoned in protected holdings called sanctuaries or in federal stockyards awaiting adoption to loving homes or death by law.

      As of this publication and regardless of logical or emotional perspective, three statements can be made. Free-roaming horses are still being captured even though more than 34,000 are already detained and munching out a life in taxpayer funded holding areas. Some captured, free-roaming horses are not federally protected from commercial processing. Domestic horses including some of the refuges from the wild life are not protected from human consumption. How did this trifurcation and resultant anxiety come about, this time? I say Senator Burns and his colleagues in Congress forgot about the determination of wild horse advocates. Or did they?

      In 2004, due to the Burns Amendment change in wild horse law, 8,000 of the imprisoned equine captives that are deemed not-adoptable are likely headed for a cold meat-hook. As the Burns Amendment would have it, regulatory protections for wild horses managed by the staff at the Bureau of Land Management and its counterpart in the Department of Agriculture were changed. Senators had written into the 3,300-page federal budget a mandate that the BLM sell to the highest bidder wild horses deemed not-adoptable. In April of 2007 the House of Representatives passed H.R. 249 which would have restored the prohibition of the commercial sale and processing of captured, free-roaming horses and burros. That bill would have overturned the Burns Amendment. However, sessions of Congress last two years and at the end of each session all proposed bills and resolutions that haven’t passed both houses of congress are cleared from the books. H.R. 249 later evolved into H.R. 1018 that passed the House in July, 2009 and hibernates peacefully in the Senate – Committee on Energy and Natural Resources – as S.R. 1579, a companion bill to the House version. Like it or not, as of February, 2010, the fate of the free-roaming horse is regulated under the protections of legislation formed and passed on or before fiscal year 2005.

      My current concern is for the destiny of the few remaining free-roaming horses that fall under the often amended and notably weakened protections of the Wild free-roaming Horses and Burros Act of 1971. The land management policies that apply to public horses seem to revolve around the concepts of health of the public lands, capture for adoption, maintenance in sanctuaries or holding facilities, or in-the-wild management.

      Dr. Kathleen Fagerstone of the USDA Wildlife Services tells us that “for most of the last century, federal and state wildlife conservation agencies in the United States have focused on conserving or increasing populations of many species of wildlife. The changing cultural values and increasing urbanization of the United States are curtailing traditional wildlife management tools used to effectively manage conflicts between human and wildlife populations. A growing interest in non-lethal methods for [animal] population control [in-the-wild] of nuisance or damaging wildlife species has fostered research in wildlife contraception.” Due to the fact that contraception for wild or other free-roaming animals is now possible, thanks to advances in scientific research, this method of population control is gaining favor in the hearts and minds of horse advocates and wild horse critics alike.

      Dr. Gary Killian, a distinguished professor and fertility control expert, notes, “There are lots of factors that need to be considered, not to mention the fact that mares that are contracepted tend to live longer. Decreasing the death rate will add to the total population. I guess if fertility control of free-roaming horses were easy, it would have been done already. At least we have some tools now like immunocontraceptives that have potential to be useful if we could overcome the politics and figure out how best to apply them.”

      So what are equine immunocontraceptives ? How do they act to reduce birth rates? Who is against their use, and why?

      USDA Wildlife Services at the National Wildlife Research Center in Fort Collins, Colorado began developing wildlife contraceptives in 1991. Scientists both here at affiliated universities and private companies have steadily worked toward developing and registering contraceptive products that are practical and safe for animals and humans. In 2005, the regulatory authority for wildlife contraceptives was changed from the Food and Drug Administration (FDA) to the Environmental Protection Agency (EPA).

      Currently, two immmunocontraceptive vaccines, the Gonadotropin-Releasing Hormone (GnRH) and the Porcine Zona Pellucida (PZP) vaccines have been tested to prevent pregnancy in free-roaming horses. Both come in a liquid form for injection by needle or bio-bullet. (Inter-uterine devices (IUDs) have also been tested, but with only limited success.) These two vaccines use the animal’s immune system to produce antibodies against reproductive hormones, gamete (egg) proteins, and other proteins essential for successful reproduction. In other words, the antibodies interfere with the natural activity of the reproductive agents. The vaccines can now be made so as to be effective contraceptives from 1 to 4 years or longer. Neither is a hormone, nor do they have hormone-like activity.

      GnRH Vaccines are not Hormone Injections:

      The GnRH vaccine creates antibodies that interfere with the reproductive hormone in both sexes by causing blocking of biologically active GnRH. The result causes a reduction in the release of other reproductive hormones and silencing the activity of the gonads.

      Fagerstone et al report that blockade of mammal GnRH is effective in reducing fertility in most mammals, including rodents. The contraceptive effects of a single-shot GnRH vaccine lasts for at least four years. As of October, 2006 in the fourth year of Dr. Killian’s well-refined study of the GnRH vaccine GonaCon ™ , half of the mares are still open (not pregnant). In preliminary studies with white-tailed deer using a modified GnRH vaccine, GonaCon-Blue™, 80% of the females are infertile four years following a single vaccination.

      A recent USDA “Target Safety Study” completed for the FDA in collaboration with Dr. Killian’s group at Penn State University, “did not find any contraindications associated with GonCon use.” GnRH vaccines can cause sterilization, but only if administered repeatedly. Hence, GnRH vaccines need to be used judiciously by knowledgeable, experienced handlers. The GnRH vaccine has been submitted to the EPA for registration.

      PZP Vaccines:

      The Zona Pellucida (ZP) is a naturally produced glycoprotein layer located on the outer surface of the egg which is produced in the ovary of the mare. Use of the PZP vaccine results in infertility either by blocking sperm from penetrating the zona pellucida layer or by interfering with egg maturation. According to Fagerstone, “The advantages of PZP are that, because PZP is a protein broken down in the gastrointestinal tract when consumed, it does not enter the food chain [if a vaccinated horse were eaten]. Also, its effects are normally reversible. It is not species specific and is effective in reducing fertility in most mammals tested. The disadvantages are that PZP vaccines must be applied by injection.” Other significant disadvantages with PZP vaccines exist. The vaccinated mare continues cycling and thus remains a target for stallions, where as pregnancy would limit that harassment. Also, it is unknown if ovary destruction occurs as a result of the antibodies. In some instances, when the antibodies level drops below critical thresholds (as the vaccine effects wear off), late-term pregnancies may occur complicating the winter life of the mare and the new born foal.

      Long-term studies involving GnRH and PZP vaccines on white-tailed deer (Miller et al. 1999 and 2000) showed no adverse effects on the animal’s health. For the GnRH vaccine, altered heat-cycles in the deer were seen and expected. The same can be anticipated for horses.

      There are of course concerns that must be addressed when using contraceptives to control overpopulation in free-roaming horses. Any program will need to be funded, easy to administer, cost-effective, effective for multiple years, and have few or no known contraindications (adverse effects). Currently, since the vaccines are injectable, horses must be captured and processed or shot from a remote location with a dart or bio-bullet. Even though capture for processing is traumatic and inherently violent, study of vaccine efficacy and horse health necessitates a close relationship to the target animal. Aerial facilitated darting or use of bio-bullets can be inefficient, and these types of vaccination methods make research study of the health and biomedical effects of the contraceptive difficult. The darting process has the potential to leave a mechanical dart in the environment. Further, annual capture of free-roaming horses for booster vaccination usually becomes more difficult with each subsequent attempt. This is not economical and from a behavioral perspective it is not logical. A vaccine that is effective for multiple years is better and less costly than a one – or two-year vaccination.

      There are other considerations to take into account when attempting to control herd size. Is the population “open” or “closed” to migration from other herds? What are the sex ratios, age structure, and the natural estimated increase or mortality of the targeted herd? Researchers have produced various models, some of which suggest that animals which give birth at 3 years of age, as horses generally do, will need more interventions than simply contraception in order to maintain a herd at a desired population. These additional interventions include capture and removal, redistribution of family groups to lands allotted for herd management, or in the case of game or nuisance wildlife, lethal control. Of course, lethal control was not an option for free-roaming horses, until recent legislation.

      Contraceptive treatment to manage free-roaming horse populations in the wild must take into consideration the average age of the animals involved. Dr. Killian astutely observes that ‘if younger mares have had one or two foals, they have contributed to the gene pool, and additional foals with their genes are not needed” to keep the genetic pool intact. In addition, due to the difficulties of gathering an entire free-roaming herd for vaccination, there will always be a number of untreated mares in any given population.

      Bottom Line:

      I agree with Dr. Killian’s suggestion that “The bottom line is there are some reasonable one-treatment options that will be available for contraception of mustangs in the near future. These far surpass anything currently being used for mustang contraception. What has to happen is that the public needs to drive the course of action. In my experience, the people with the dollars and currently in charge of dealing with the [horse] problem are set in their ways and have different ideas about how to proceed.” Given the normal mortality, sex-ratios, and reproductive rates of free-roaming horses in a herd of one-hundred and twenty horses, contraception in one mare for one year who would have otherwise foaled a reproductively viable female offspring will – on average – eliminate eight to twelve horses from the population over the next fourteen years. However, the author recommends contraception in mares ten years and older so that the genetic contribution of each mare in the herd remains intact. In this manner, one ten-year old mare blocked from contributing one viable female offspring might remove eight animals over the next fourteen years.

      The wild horses of the Outer Banks of North Carolina, Virginia, and Maryland have set the precedent for true art in the fertility management and adoption of free-roaming horses. Thousand Welcomes Farm in Chapel Hill, North Carolina, the Foundation for the Shackleford Horse in Beaufort, North Carolina, the National Parks Service at Assateague Island National Seashore Trust in Maryland, and the Chincoteague National Wildlife Refuge in Virginia are notable examples of farms and organizations who have proven their love and respect for historical America and free-roaming horses.

      The American Wild Horse Preservation Campaign is correct when it writes that “Solutions need to be implemented that will allow wild horses to be managed in the wild, securing a place for our wild herds in the American landscape. AWHPC is calling for fair and balanced management decisions that are based on accurate, scientific information and that take into account the interests of all parties, including the horses, public land ranchers, and the American public.” AWHPC supporters are against the use of hormones as contraceptives.

      I don’t know if all sides of the free-roaming horse debate can be placated, but I do agree with AWHPC when they say that ecotourism should also be given a chance to play a role in herd preservation. “Horse lovers, wildlife enthusiasts, as well as those with an interest in the history of the Old West, should be given the opportunity to enjoy wild horse excursions year-round. In addition to non-intrusive observation of wild horse behavior and herd dynamics, in-the-wild management itself could become part of a unique experience for visitors to herd management areas.”

      As an equine veterinarian who has spent his professional life protecting the publicly owned free-roaming horse, I understand the legitimate concerns of all interested parties, cattlemen included. However, I feel that we need to protect our public property first, while we weigh the proper stewardship, transport, and disposition of this revered part of our American spirit. I am not accusing our Interior Department and USDA land managers of mismanagement. My experience with the staff in these entities has been trusted and respectful. Most public servants and land managers tell me that they want to manage the free-roaming horse in the wild. They need the proper laws, proper policy, dedicated staff, time, and mandated funding to do it right and to study the effects of the management tools.

      The agriculture and veterinary medical colleges in states with free-roaming horse populations are a great mechanism of public oversight and could be recruited to assist these agencies and keep academic research and influence involved. After all, we trust the schools to teach and lead our young people, and since we already employ the staff and faculty, why not trust them to care for our free-roaming horses, too? It would be a perfect outdoor laboratory and a good use of public funds.

      Judiciously applied fertility control contraception will go a long way in maintaining horse populations at desired levels. However, when environmental study indicates that any given population of free-roaming horses needs to be artificially lowered by capture, I suggest the captives be remanded to the good and effective wild horse transition programs such as the prison inmate training programs and the New Mexico Mustang and Burro Association transition program. In this manner, free-roaming horses are gentled under a humane and controlled system before they are released into a domestic equine life.

      Many cattle growers would applaud effective free-roaming horse population control. Animal advocates should be pleased with preservation and limited captures. Wildlife managers would welcome the funding needed to appropriately manage all of our important wild species. Slaughter of free-roaming horses would not be necessary because the horses would either be returned to freedom after being examined and vaccinated, or gentled in a prison training program and sold to responsible owners. One in ten thousand horses is truly not amenable to gentling. Those few might be neutered and returned to their original herds. We can’t manage one million free-roaming horses, but we can manage enough to be meaningful. Should we tolerate the possibility of zero free-roaming horses?

      Even though emotion about the issue of free-roaming horse survival often takes us to places we do not want to go, we should allow reasoned passion from the heart to direct our efforts. We should protect all of our animal species from birth to death with intervention only when necessary and always as humanely as possible. If we choose to terminate animal life, we should do so humanely.

      It is time for solutions: not finger-pointing; not just words. It is time for action. Captures resulting in unreasonable numbers of abortions and death should stop until these problems are resolved. In the mean-time, funding for long-term study of all of our natural resources through the State University should be required. In this manner we can trust the university system in the ten states where free-roaming horses reside to report back to the taxpayer on best-solutions for management of all of our natural resources – including the free-roaming horse. Help save the free-roaming horse from destruction by contacting your elected representative and respectfully ask them to get involved in the management of free-roaming horses in-the-wild. After all, the horses belong to all of us, the land belongs to all of us, and the elected representatives who control our money work for all of us. These nobody’s horses are everybody’s horses.

      About the author: Dr. Don Höglund is an equine veterinarian and author of NOBODY’S HORSES: The Dramatic Rescue of the Wild Herd of White Sands. He has spent 25 years working with wild horses rounded-up in the western and eastern states. He served seven annual contracts with the BLM as a private contract veterinarian in the successful New Mexico prison inmate wild horse training programs organized by the then Congressman and current New Mexican Governor, Bill Richardson. http://www.nobodyshorses.com.

      Do not publish any part of this article without written permission from the author.

      • I am glad you posted this. Was just thinking about it because I reread it a few days ago from your book, Nobody’s Horses, and thought it should be presented.

      • This should be highlighted from above comment:

        “Other significant disadvantages with PZP vaccines exist. . . . . . .it is unknown if ovary destruction occurs as a result of the antibodies. ”

        (from your article, above – I thought it was an important fact)

  2. Well said Matt, well said

  3. GREAT job on this series, Matt!! I’ve learned a bunch, really! Even though I thought I already had a handle on it! I, too, would like to know more about the gelding process, so will look forward to that post as I do all of yours!!

    Again, you’re doing great things for the wild ones and many, many people out here appreciate it very, much!!


  4. In fairness there should be renewed discussion on the use of PZP-22 and the unintended consequences as revealed in the Princeton study released on Oct 26, 2010

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