FACT SHEET: The Truth about PZP

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PZP — The Pesticide

Porcine zona pellucida (PZP aka ZonaStat-H or Native PZP) is an EPA-registered pesticide derived from the ovaries of slaughtered pigs. PZP is approved for use on wild horses “in areas where they have become a nuisance ….” [20]

Some persons argue that, because PZP does not kill the mare, it is not really a “pesticide.” Actually, PZP does kill. As will be documented in this report, its use is associated with stillborn foals. In the long term, PZP will weaken a herd immunologically, which could swiftly lead to its extinction. So, yes, PZP is a real pesticide.

PZP — an Anti-Vaccine

While touted as a “vaccine,” PZP is actually a perversion of what a true vaccine is supposed to be. Instead of preventing disease, PZP causes disease — auto-immune disease. Thus, PZP is an anti-vaccine.

PZP’s Mode of Action as Stated in the Pesticide Registration Is a Disproved Hypothesis

The registrant of PZP advised the Environmental Protection Agency (EPA) that, based on information from the pesticide’s researcher-manufacturer, PZP works by generating antibodies that “block sperm attachment.” This representation of PZP as a sort of chemical condom was not fact but merely an untested hypothesis, postulated three decades ago. The old hypothesis was disproved by subsequent research. PZP’s manufacturer knew, or should have known, this. The manufacturer should also have been informed and up-to-date regarding the side effects and unintended consequences of PZP. Yet, the manufacturer continued to cite the disproved hypothesis and to deny that PZP has any adverse effects. [7 and 13]

PZP’s True Mode-of-Action

So how does PZP really work? PZP tricks the immune system into waging war on the ovaries. In a meta-analysis of ZP-type contraceptives, Kaur & Prabha (2014) reported that the infertility brought on by such products is ” … a consequence of ovarian dystrophy rather than inhibition of sperm-oocyte interaction.” Thus, PZP’s antibodies “work” not by blocking sperm attachment but by destroying the ovaries. Kaur & Prabha further disclosed that ” … histological examination of ovaries of immunized animals revealed the presence of atretic follicles with degenerating oocytes.” [5] [Atretic follicles are ovarian follicles in an undeveloped state due to immaturity, poor nutrition or systemic disease; manifested by prolonged anestrus.]

Kaur & Prabha’s review concluded that PZP’s antibodies induce ovarian dystrophy, oophoritis (inflammation of the ovaries), destruction of oocytes in all growing follicles, and depletion of resting follicles. The manufacturer of PZP as well as Bureau of Land Management (BLM) should have been aware of these and other findings about the pesticide. Yet they ignored or disregarded any information that was contrary to their personally-preferred but obsolete and false description of PZP’s mode-of-action.

PZP Manufacturer’s Own Research Found Markedly Depressed Estrogen Secretion

In a telling study published back in 1992, the manufacturer of Native PZP, along with colleagues, reported that ” … three consecutive years of PZP treatment may interfere with normal ovarian function as shown by markedly depressed oestrogen secretion.” [6] Thus, despite all the hype about PZP being non-hormonal, the manufacturer knew that ZonaStat-H has an adverse hormonal effect, causing significantly-lowered estrogen. Thus, PZP is an endocrine disruptor. [21] The plummeting estrogen-levels may also reflect the ovarian dystrophy and oophoritis now known to be caused by PZP. Despite personally discovering negative hormonal impacts 23 years ago, PZP’s manufacturer continued to cite misinformation regarding the product’s mode-of-action and endocrine-disruptor side-effects.

PZP Causes Ovarian Cysts

In their 2010 meta-analysis, Gray & Cameron cited a number of studies that found ” … alterations to ovarian function, oophoritis, and cyst formation with PZP treatment (Mahi-Brown et al.1988, Sehgal et al. 1989, Rhim et al. 1992, Stoops et al. 2006, Curtis et al. 2007).” [4] These findings support those of Kaur & Prabha while introducing yet another adverse effect: ovarian cysts. Gray & Cameron’s review also noted that increased irritability, aggression, and masculine behavior had been observed in females following PZP-treatment.

PZP → Endocrine Disruptor → Elevated Testosterone → Masculinizing Effects

Recall that PZP has endocrine-disrupting effects that result in lowered estrogen. Per the observed masculine behavior of treated mares, PZP seems to have a testosterone-elevating effect too. A deficit of estrogen alone would not necessarily manifest in the masculinization of treated females, but an excess of testosterone would. So, it appears that PZP disrupts at least two hormones: estrogen — by substantially lowering it — and testosterone — by substantially elevating it. Adverse effect: Unnatural behavior.

PZP → Ovarian Cysts → Elevated Testosterone → Masculinizing Effects

As discussed above, PZP correlates with abnormal masculine behavior on the part of treated females, a side-effect likely due to elevated testosterone. But in addition to the endocrine-disruption caused by PZP, there could be a second way for testosterone levels to become elevated. Recall that PZP causes ovarian cysts. An Internet search on “ovarian cysts and testosterone” yielded results for polycystic ovary syndrome (PCOS) in women. Interestingly, one of the symptoms of PCOS is high testosterone levels. [12 and 22] The connection between ovarian cysts and elevated testosterone suggests that the ovarian cysts caused by PZP could — either alone or in combination with PZP’s endocrine-disruptor effects — lead to high testosterone levels in treated females, as evidenced by their masculinized behavior.

PZP Causes Additional Adverse Effects

Gray & Cameron’s review also disclosed that, when PZP was administered to the females of a herd, males lost body condition while the oft-claimed improvement in female body condition did not hold up. Further, mares remained sexually active beyond the normal breeding season and had more estrus events.

PZP Selects for Weak Immune Function

Gray & Cameron’s analysis raised the possibility of PZP selecting for immuno-compromised individuals. Here’s why. Because PZP stimulates the immune system, it ironically works “best” — sterilizes faster — in mares that have strong immune-function. Such mares respond to the anti-vaccine and produce quantities of PZP antibodies that destroy their ovaries. But, conversely, PZP may not work at all in mares whose immune-function is weak or depressed. Those mares fail to respond to PZP. They keep getting pregnant and producing foals who, like their dam, suffer from weak immune-function. So, the PZP pesticide works against the very horses that Nature has best equipped for survival against disease while favoring and selecting for the immuno-compromised. Thus, a herd being treated with PZP is undergoing selective breeding for weak immunity, which puts the population at risk for disease — and ultimately, for extinction.

PZP Confers Dubious “Benefit” of Increased Longevity

Gray & Cameron also cited a study that found that “… PZP treated feral horse mares lived longer, resulting in a new age class (>25 years) not present before treatment ….” Exceptionally-long life is an ironic effect of PZP treatments. PZP’s manufacturer actually boasts about it, as if the anomaly were a good thing. However, Gray & Cameron questioned the supposed benefit of mares living much longer than their normal life expectancy. Indeed, such mares take up scarce slots within size-restricted populations. The ultra-elderly mares continue to consume resources for many years, but they no longer contribute to the gene-pool. It is detrimental to a population’s genetic viability to carry significant numbers of sterile herd-members way-beyond their normal life-span.

Research on Wildlife Contraceptives Revealed Stillbirths and Auto-Immune Oophoritis from PZP

There was an even earlier, definitive meta-analysis on wildlife contraceptives. Nettles (1997) reviewed 75 studies available at that time on the subject. Among his findings regarding PZP-use across different species, including horses, were: Stillbirths; altered ovarian structure and cyclicity; interference with normal ovarian function; permanent ovarian damage; and some cases of irreversible sterility due to auto-immune oophoritis, which suggested that PZP can be selective against a certain genotype in a population. [10] Many of these findings were confirmed by Kaur & Prabha as well as by Gray & Cameron. Please keep in mind these key findings: Stillbirths, and auto-immune oophoritis.

Recent Stillbirths Correlated with PZP

There is recent evidence confirming Nettles’ finding of a correlation between PZP treatments and subsequent stillbirths. In June 2015, Karen Sussman, President of the International Society for the Protection of Mustangs and Burros, reported that 7 mares previously treated with PZP at ISPMB, when taken off PZP, were able to get pregnant. However, 6 of those 7 mares — that is, 86 percent — produced foals that were stillborn. All other ISPMB mares that had not been injected with PZP successfully birthed healthy foals. Thus, given that environmental and other conditions were identical, the only variable was PZP. The dead foals have been sent to a university pathology department for autopsy. [18]

Autoimmune Oophoritis Induced by PZP

Research by the Rose-Cihakova-Caturegli Laboratory at Johns Hopkins Pathology found: “Automimmune oophoritis can be induced by immunization with testis and ovarian antigen murine human zona pellucida 3 peptide (pZP3) in adjuvant.” [16] Here again, is causation of autoimmune disease by a ZP-type product. This finding confirms other research cited herein.

Autoimmune Oophoritis and Risk of Other Autoimmune Diseases

A study by Varras et al. disclosed that, in humans, autoimmune oophoritis carries the risk of the patient developing other autoimmune diseases. [23] The correlation between autoimmune oophoritis and subsequent other autoimmune disorders weighs against injecting fillies and mares with PZP repeatedly and en masse.

Prolonged Breeding Season, Unusually-late Parturition Dates with PZP

Nettles’ meta-analysis on PZP disclosed other adverse effects: A prolonged breeding season and unusually-late parturition dates. (Parturition is the formal term for “giving birth.”) These findings have recently been confirmed, as is discussed below.

Parturition-Season Extends to Nearly Year-Round When a Herd Is Treated with PZP

A longitudinal study (Ransom et al. 2013) of three herds currently being managed by PZP — Little Book Cliffs, McCullough Peaks, and Pryor Mountain — found that the the parturition season lasted 341 days. [15] Ransom et al.’s finding of a nearly year-round birthing season supports the earlier finding by Nettles. Thus, during its period of potential reversibility, PZP’s effects wear off unpredictably. Out-of-season births put the life of both the mare and the foal in jeopardy. Nature designed the equine birthing-season to occur in Spring, not year-round, and certainly not in the dead of Winter.

Prolonged Delay in Recovery of Fertility

The same longitudinal study by Ransom et al. found that, after suspension of PZP, there was a delay lasting 411.3 days (1.13 years) per each year-of-treatment before mares recovered their fertility. What this means is that it takes that long, on average, for the ovaries to heal, to clear out all those cysts, and to regain some degree of normal hormonal function.

The question is: How is the delay in recovery-of-fertility addressed by BLM management practices? Answer: BLM ignores it. For instance, BLM currently administers PZP to Pryor Mountain’s fillies and mares starting at age 1½ — whom BLM artfully describes in the Environmental Assessment as fillies “becoming two year olds” — through age four. Thus, these fillies and mares receive intentional treatments for four consecutive years before being allowed the privilege of reproductive potentiality. Per Ransom et al.’s study, the Pryor Mountain fillies and mares would be expected to need 1,645.2 days (4.51 years) to regain reproductive capacity. But BLM gives the Pryor Mountain mares only 5 years off PZP before they are put back on it again — for the rest of their life. Thus, these fillies and mares might have just a six-month window — at best — in which to conceive. Due to the unpredictable timing of PZP’s wearing off, for some mares that window of fertility will close before they get a chance to produce a foal. Those mares’ genetic contribution will be zero.

As if the above scenario were not bad enough, PZP’s manufacturer conceded that it could take up to eight years to recover fertility after just three consecutive PZP treatments. [13]

Ransom Advises Proceeding with Caution regarding PZP

The Ransom et al. study warned: “Humans are increasingly attempting to manage the planet’s wildlife and habitats with new tools that are often not fully understood. The transient nature of the immunocontraceptive PZP can manifest into extraordinary persistence of infertility with repeated vaccinations, and ultimately can alter birth phenology in horses. This persistence may be of benefit for managing overabundant wildlife, but also suggests caution for use in small refugia or breeding facilities maintained for repatriation of rare species.” [15]

Because BLM keeps over 70 percent of the herds at levels below minimum-viable population (MVP), most herds qualify as “small refugia.” Pryor Mountain WHR is a small, isolated refuge, and its wild horses carry genes with rare alleles.

Ransom’s Exclusion of Seven Mares Evidences PZP’s Non-Effect on Immunocompromised Mares

In the “Data Collection” methodology section of the Ransom et al. report, the authors advised: “We omitted data for one female from the Little Book Cliffs and six females from McCullough Peaks because they produced offspring in every treatment year and thus were never effectively contracepted.”

This fact is important because it evidences PZP’s lack-of-efficacy on immunocompromised fillies and mares. To review: Because PZP activates the immune system, mares with naturally-low or depressed immune function do not “respond” to the treatment. It’s as if they had been injected with saline — their immune system is so weak that it does not react to the PZP by producing antibodies. The good news is such mares’ ovaries are saved from PZP’s destructive effects. The bad news is that these mares continue to become pregnant year after year, producing foals that will also tend to inherit low immune-function. Over time, the herd will become populated with more such low-immune horses because those with strong immunity get sterilized. Thus, PZP selects for horses with low immune function, which is bad for a herd in the long term. Even a routine infection could spread quickly and wipe out a population of horses with weak immune-function. If the goal is to preserve a herd, the use of PZP constitutes a worst management-practice.

BLM Was Fully-Aware of the Ransom Study but Suppressed the Findings

In their report, the authors of the Ransom et al. study gave a shout-out to BLM “for administrative and technical support throughout this project.” Thus, BLM was fully aware of the multi-year study while it was in progress and even lent support to it administratively and technically. Yet, in the case of the Pryor Mountain herd, BLM omitted this important report as a reference for the 2015 Environmental Assessment, which proposed intensifying the PZP “prescription.” Thus, BLM pretended that there was no such report and unethicallly suppressed it. Consequently, the public could not comment knowledgeably and appropriately on the continued use of — let alone the accelerated application of — PZP.

Three PZP Injections Can Trigger Sterility in Mares, or Just One Shot in Fillies Before Puberty

Disturbingly, another recent study on PZP (Knight & Rubenstein, 2014) found that ” … three or more consecutive years of treatment or administration of the first dose before sexual maturity may have triggered infertility in some mares. [9]

These findings are particularly troubling. They suggest that, actually, only two consecutive PZP-treatments may be reversible. Except, that is, in the case of fillies who have not yet reached puberty — they could be sterilized by just one injection. Recall the Pryor Mountain fillies, whose PZP treatments begin when they are just 1½ years old. They may not have reached puberty when they are initially treated. [1] And as we shall see later in this report, that first shot of PZP may not be their first shot of PZP.

Researchers Again Express Concerns about the Abnormal Life-Spans of Sterilized Mares

Knight & Rubenstein warned: “Inducing sterility, while relieving the mares from the energetic costs of lactation and reducing the stress from harem switching, may have unintended consequences on population dynamics by increasing longevity and eliminating the mares’ ability to contribute genetically.”

Knight & Rubenstein’s concerns support those of Gray & Cameron, who also questioned the supposed benefit of sterile mares’ extended life-spans. The abnormal numbers of aged, sterile mares count for census-purposes; but their presence disadvantages the younger horses, who become tageted for removal in order for BLM to achieve arbitrary management levels. Further, such mares no longer belong to the viable gene-pool.

PZP’s Destructive Antibodies Are Transmitted via the Placenta and Mother’s Milk

It gets worse. Sacco et al. reported that, per radioimmunoassay, PZP antibodies are transferred from mother to young via the placenta and milk. The transferred antibodies cross-react with and bind to the zonae pellucidae of female offspring, as demonstrated by immunofluorescent techniques. [17] These findings were disclosed in 1981. PZP’s manufacturer must have known about this dangerous effect, and certainly BLM should have investigated on its own whether there was any risk to the unborn or the nursing foal. Yet, the manufacturer continued to insist that there was no danger to the foal, whether born or unborn. [7 and 13] And in fact, BLM regularly administers PZP to pregnant and lactating mares, who transfer the destructive antibodies to their fetus, via the placenta, and to their foal, via mother’s milk.

Recall again the Pryor Mountain fillies. If their dams were injected with PZP while pregnant or nursing, such fillies will already have PZP antibodies cross-reacted with and bound to their zonae. Therefore, when those same fillies are injected at age 1½, it will be their second treatment, or potentially even their third. In fact, they could already have been sterilized in utero or while nursing, the treatment having been received prior to puberty, about which Knight & Rubenstein warned.

PZP Weakens Herd-Immunity, Posing Risk of Stochastic Events Leading to Herd-Extinction

To be self-sustaining, a herd needs to possess good immunity to withstand random catastrophes — known as stochastic events — such as contagious infections. There was such an event recently in Kazakhstan, where 120,000 endangered Saiga antelope — half the world’s population — died off suddenly and inexplicably within a two-week period. Scientists think a common bacterial infection was the cause of this mass-mortality event, but are unsure why the antelope were unable to fight off the disease immunologically. [14]

Imagine if such a catastrophe were to befall the Pryor Mountain horses, whose herd-immunity is being eroded by PZP. Note that the Saiga deaths involved antelope-mothers and their calves. If Pryor Mountain’s few fertile mares and their foals perished all of a sudden, that would leave just stallions and sterile old mares. The herd would be composed of the living dead, reproductively speaking, its rare alleles extinguished. BLM is failing to proactively manage the Pryor Mountain herd with stochastic events taken into consideration. That is malfeasance. PZP is a tool of immunological destruction, not of proper management.

PZP Continues the Use of Roundups and Removals

If the promise of PZP were true — if PZP really did eliminate the need to remove “excess” wild horses from the range — removals would have ended long ago in the Pryor Mountain Wild Horse Range, where PZP has been in use for approximately two decades. Yet removals are scheduled there with regularity every three years, the latest one in 2015.

Risks to Humans Who Administer PZP Injections

For staff and volunteers who inject wild horses with PZP, EPA’s Pesticide Fact Sheet advises that Personal Protective Equipment requirements include long sleeved shirt and long pants, gloves and shoes plus socks to mitigate occupational exposure. EPA specifically warns that pregnant women must not be involved in handling or injecting ZonaStat-H, and that all women should be aware that accidental self-injection may cause infertility. [20]

However, EPA’s Fact Sheet, the manufacturer’s training, and BLM’s operating procedures fail to inform pregnant women why it is so important that they strictly avoid PZP — because PZP’s antibodies cross the placenta and cross-react with and bind to an unborn female child’s own little zonae pellucidae. The baby-girl could be “anti-vaccinated” with PZP and even sterilized before birth.

EPA’s Fact Sheet, the manufacturer’s training, and BLM’s operating procedures fail to warn lactating women to avoid PZP and why — because PZP’s destructive antibodies would be passed along to a nursing female child via mother’s milk. The baby-girl could be “anti-vaccinated” with PZP and possibly sterilized simply from nursing.

EPA’s Fact Sheet, the manufacturer’s training, and BLM’s operating procedures fail to warn all women of the risk of ovarian dystrophy, oophoritis, ovarian cysts, and elevated testosterone-levels — in addition to infertility and, potentially, sterility — from unintentional self-injection.

EPA’s Fact Sheet, the manufacturer’s training, and BLM’s operating procedures fail to emphasize the magnitude of the risk — the PZP-in-question is a horse-size dose.

But Is There a Mandate to Practice Scientific Integrity?

Yes. The Department of the Interior’s (DOI) Code of Scientific and Scholarly Conduct applies to all staff members as well as to contractors, partners, permittees, and volunteers. The Code states: “Scholarly information considered in Departmental decision making must be robust, of the highest quality, and the result of as rigorous scientific and scholarly processes as can be achieved. Most importantly, it must be trustworthy.” [19]

BLM has ignored and suppressed independent scientific findings about PZP’s adverse effects and unintended consequences. Instead, BLM continues to rely almost exclusively on the manufacturer’s claims — shown and known to be false — regarding PZP’s safety for use on horses and for handling by humans. BLM is thus non-compliant with the Policy and malfeasant in its responsibilities to protect staff, volunteers, and the wild horses under its jurisdiction. BLM is also misleading and disinforming Congress and the American public about the PZP pesticide.

The manufacturer of PZP — a partner to BLM — misrepresented the pesticide as safe for use on animals by humans. The manufacturer knew or should have known that the former hypothesis regarding PZP’s mode-of-action had been disproved, and that PZP has dangerous side effects, safety-issues, and unintended consequences. Yet he hid and denied that information and failed to warn about PZP’s adverse effects. The manufacturer cited his own research as if it were definitive, and aggressively criticized independent researchers whose findings did not fully support his claims. Indeed, he recently submitted an Op-ed to The Salt Lake Tribune wherein he belittled the research of fellow scientists whose studies on PZP yielded results somewhat different from his own. [8] His accusations were so unreasonable that the scientists felt it necessary to submit an Op-ed in response to defend the integrity and validity of their work. [11] The manufacturer also disparaged members of the public — one of whom was a member of the Pennsylvania Game Commission — who expressed concerns about PZP. He dismissively accused them of “an attempt to mislead,” of “hyperbole,” of “knowingly manipulating information,” of “attempts to frighten people,” and of indulging in an “anti-intellectual approach to debates.” [7] By these actions, the manufacturer violated the DOI’s Code of Scientific and Scholarly Conduct.

PZP Manufacturer Misled Trainees into Believing that PZP Was Safe

BLM staff and volunteers receive their training from PZP’s manufacturer in how to handle and administer the pesticide. BLM is remiss in delegating the training to the manufacturer without verifying the adequacy of the instruction and the truthfulness of it. Two comments recorded recently in the media suggest that PZP’s manufacturer misled not just the public-at-large but those who received training therefrom in how to administer PZP.

First, the manufacturer has been quoted as saying that PZP is “so safe it is boring.” [3] Independent research shows otherwise — that PZP is a powerful hormone disruptor that could sterilize a female with just one injection. If trainees believe that PZP is boringly safe, they will be less likely to protect themselves adequately from this dangerous pesticide. Indeed, many of the trainees are women and, therefore, particularly at risk. Likewise, wild-horse advocates are lulled into complacency, trusting that PZP is harmless to the Pryor Mountain horses and their rare genetic alleles. Of course, none of that is true.

Second, a PZP supporter, who self-identified as a recent completer of the PZP-darting training program conducted by the manufacturer, said in a comment posted to a news article: “I just received my FDA certification to handle and administer Native PZP. Would you be so kind to provide a link to the study you keep referencing? To my knowledge, and those teaching the Native PZP certification class, there are no side effects of the PZP produced by Dr. Kirkpatrick and his team, which is Native PZP.” [2] Key words: “no side effects.” It is disturbing that a person who was, no doubt, motivated by a desire to help the horses has been disinformed regarding PZP’s safety-hazards to humans as well as to horses.

BLM Fails to Maintain Proper Supervision of the PZP Volunteers

The issue of safety is not the only concern. As BLM has admitted, volunteers darted the wrong mares on Pryor Mountain. These errors evidence that BLM has failed to maintain supervisory control over the volunteer-inoculators, allowing them to conduct the PZP-darting by themselves. The mistakes further evidence that the volunteers do not understand what is expected of them. Who can say whether other procedures were not complied with either. The fact that mares were darted who were ineligible for PZP per the then-current protocol, but who would be eligible under the proposed-but-not-yet-promulgated new “prescription,” suggests that the volunteers may have concluded — from BLM’s open contempt for the Constitution and disrespect for the NEPA process — that was okay for them to start darting otherwise-ineligible mares right away. Not surprisingly, BLM blames the volunteers for these mistakes, but probably has not informed them that they are being made to take the rap for management’s shortcomings.


PZP is appropriately categorized as a pesticide by the EPA. PZP “works” by tricking the immune system into attacking and destroying the ovaries. PZP has many adverse effects as well as unintended consequences. PZP presents safety-hazards to humans who handle it. PZP is a dangerous pesticide whose use is antithetical to the spirit and intent of the Wild and Free-Roaming Horses and Burros Act. BLM’s continuing to use PZP while ignoring and suppressing the evidence of its harmful effects constitutes malfeasance.


This report was completed by Marybeth Devlin on December 24, 2015. Copyright Marybeth Devlin and Protect Mustangs 2015.


1. EquiMed staff. (2010, March 13) Equine Reproductive Maturity in Mares and Stallions. Puberty in Equines. Retrieved from http://equimed.com/health-centers/reproductive-care/articles/equine-reproductive-maturity-in-mares-and-stallions

2. EWCS. (2015, November 10). Re: “Contraceptive could reduce taxpayer costs for wild horses.” Retrieved from http://wyomingpublicmedia.org/post/contraceptive-could-reduce-taxpayer-costs-wild-horses#comment-2352628323

3. Ferguson, Mike. (2015, June 4) “Police called as group protests wild horse contraceptives.” The Billings Gazette. Retrieved from http://billingsgazette.com/news/state-and-regional/montana/police-called-as-group-protests-wild-horse-contraceptives/article_81462303-e128-5ee8-a7ef-2c8b098450f6.html

4. Gray, M.E. and Cameron, E.Z. (2010) Does contraceptive treatment in wildlife result in side effects? A review of quantitative and anecdotal evidence. Reproduction 139, 45-55. Online publication date: 1-Jan-2010. Retrieved at http://www.reproduction-online.org/content/139/1/45.full

5. Kaur, Kiranjeet and Prabha, Vijay. (2014) “Immunocontraceptives: New Approaches to Fertility Control,” BioMed Research International, vol. 2014, Article ID 868196, 15 pages, 2014. doi:10.1155/2014/868196. Retrieved from http://downloads.hindawi.com/journals/bmri/2014/868196.pdf

6. Kirkpatrick, J. F., I. K. M. Liu, J. W. Turner, Jr., R. Naugle, and R. Keiper. 1992a. Long-term effects of porcine zonae pellucidae immunocontraception on ovarian function of feral horses (Equus caballus). J. Reprod. Fert. 94:437-444. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/1317449

7. Kirkpatrick, Jay. 2007. “Response to PA [Pennsylvania] Game Commission.” Posted on PNC’s Wildlife Forever Home Page. Retrieved from http://www.pzpinfo.org/home.html

8. Kirkpatrick, Jay F. (2015, May 16). Op-ed: Wild-horse contraceptives are based on sound science. The Salt Lake Tribune. Retrieved from http://www.sltrib.com/opinion/2517266-155/op-ed-wild-horse-contraceptives-are-based-on?fullpage=1

9. Knight, Colleen M., Rubenstein, Daniel I. 2014. The Effects of Porcine Zona Pellucida Immunocontraception on Health and Behavior of Feral Horses (Equus caballus). Princeton University Thesis, Ecology and Evolutionary Biology. Retrieved from http://dataspace.princeton.edu/jspui/handle/88435/dsp01vt150j42p

10. Nettles, Victor F. (1997) Potential consequences and problems with wildlife contraceptives. Reproduction, Fertility and Development 9(1) 137 – 144. Retrieved from http://www.publish.csiro.au/paper/R96054.htm Accessed full pdf text via purchase of a copy from Csiro Publishing.

11. Nuñez, Cassandra, Jim Adelman and Dan Rubenstein. (2015, July 3). Op-ed: Wild horse contraception not without unintended consequences. The Salt Lake Tribune. Retrieved from http://www.sltrib.com/opinion/2653298-155/op-ed-wild-horse-contraception-not-without

12. PCOS Foundation. (2015) What Causes PCOS? Retrieved from http://www.pcosfoundation.org/what-is-pcos

13. PNC, Inc. (Pity Not Cruelty). PZP FAQs. (2006) “Frequently Asked Questions on Immunocontraception.” (Special thanks to Jay Kirkpatrick and Rick Naugle for additions and corrections). Retrieved from http://www.pzpinfo.org/pzp_faqs.html

14. Raab. Lauren. (2015, May 31) “120,000 endangered saiga antelopes die mysteriously in Kazakhstan.” Los Angeles Times. Retrieved from http://www.latimes.com/science/sciencenow/la-sci-sn-saiga-antelope-die-off-20150531-story.html

15. Ransom JI, Hobbs NT, Bruemmer J (2013) Contraception Can Lead to Trophic Asynchrony between Birth Pulse and Resources. PLoS ONE 8(1): e54972. doi:10.1371/journal.pone.0054972. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/23383018

16. Rose-Cihakova-Caturegli Laboratory. (n.d.) Autoimmune Oophoritis. Johns Hopkins Pathology. Retrieved from http://pathology.jhu.edu/department/RCCLab/Oophoritis.cfm

17. Sacco AG, Subramanian MG, Yurewicz EC. (1981) Passage of zona antibodies via placenta and milk following active immunization of female mice with porcine zonae pellucidae. J Reprod Immunol. 1981 Dec;3(6):313-22. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/7328557

18. Sussman, Karen. (2015 June 6) “Suspicious Deaths with Use of Anti-Fertility Drugs.” International Society for the Protection of Mustangs and Burros. Retrieved from http://www.ispmb.org/BirthControlDeaths.html

19. United States Department of the Interior. Integrity of Scientific and Scholarly Activities Policy. Code of Conduct. Retrieved from https://www.doi.gov/scientificintegrity

20. United States Environmental Protection Agency. Office of Chemical Safety and Pollution Prevention. Pesticide Fact Sheet. Porcine Zona Pellucida (PZP). New Chemical. Nonfood Use. January 2012. Retrieved from

21. U.S. Department of Health and Human Services. National Institutes of Health. Endocrine Disruptors. Retrieved from http://www.niehs.nih.gov/health/topics/agents/endocrine/

22. U.S. National Library of Medicine. National Institutes of Health. Ovarian overproduction of androgens. Retrieved from https://www.nlm.nih.gov/medlineplus/ency/article/001165.htm

23. Varras M, Anastasiadis A, Panelos J, Balassi E, Demou A, Akrivis CH. (2013) Autoimmune oophoritis: Clinical presentation of an unusual clinical entity. OA Case Reports 2013 Jan 31;2(1):7. Retrieved from http://www.oapublishinglondon.com/article/369#

Pm PZP Darts


Protect Mustangs is a nonprofit organization who protects and preserves native and wild horses.

15 thoughts on “FACT SHEET: The Truth about PZP

  1. It is, without a shadow of a doubt, as to how much damage will be done to those horses with PZP. It doesn’t take much brains to realize how much destruction it will cause to the horses! We have uncaring, evil maniacs, and corrupt people who benefit, using this trash on helpless animals and the turn around and lie to the public. Well, I’ll take a comment from the movie “Planet of the Apes” when that person discovered what was buried partially in the ground— Damn you, Damn you all to Hell. That, sirs is/ are my steadfast opinion of you educated persons, if I may call you that!

  2. Wow That pretty much says it all. I don’t see how anyone could say PZP is a safe way to knock down the Wild Horse herds. Well unless your goal is to sterilize them out of existence and that seems to be the BLM’s goal. I don’t see how Sally Jewell can continue to let this agency destroy these animals! She either doesn’t know what’s going on or she doesn’t care either way The BLM needs to be stopped and if she can’t take care of that then we need someone who can! Time is running out for the Wild ones. We may have a few but they are not truly wild anymore because the BLM won’t keep their hands off them. Once you start altering their way of life you have altered their freedom and that can’t be returned no matter how hard you try.

  3. I am one of the few that have experienced PZP with my rescued horses. One was so old she died from it. The other was with foal. He was born reformed in his intestines. He died 6 weeks later. The mare went into deep depression. If we had not been with her she would have died too.

    No horse should be exposed to PZP FOR ANY REASON

  4. It is time to stop the use of PZP on our wild horses and burros. This is a potent pesticide with so many side effects and we have yet to see the adverse effects and damage this has already done to our population of wild horses. The research is clear and this will have permanent and severe damage on the internal makeup of our wild horses forever. Stop using our wild horses as guinea pigs. Stop the insanity. Our wild horses deserve the best care we can give them and freedom from abuse.

  5. With all these facts done by reputable and qualified people and not to be used by EPA standards I do not understand why the BLM keeps using it or is allowed to.
    The fact that women can not administer this without being protected by cautionary clothing or their ovaries will no longer be of use (in plain language ) should make it unusable on our wild horses. Also not to be around or used by pregnant women is pretty scarey. It affects the fetus. In horses it is used on pregnant mares. The fillies can not reproduce.
    This is so like what Dr Joseph Mengele might have done.
    For me I am reminded of the movie Frankenstein only this is worse as these wild horses are being altered while alive. BLM is overriding nature and when you overide nature you desrky the delicate balance that keeps our ecosystem in order.
    The PZP article must be read in detail. This is one tiny factor of it’s negative abilities to render our wild horses a death sentence by using PZP.
    The BLM must be held responsible for the extinction of our wild horses if this is not stopped. And what other things unknown yet will it do to human beings and other wild life. Scarey??? Damn right it is. No man should play God.

  6. After reading the information above I am convinced the BLM is out to rid us of our American rights to our American Heritage the Wild Horses. Why else would they go to such extremes. I can not respect the BLM and do not trust their judgment! Wait their judgment is not the problem it is their immoral act of destroy OUR horses. No one or group would do what they have done if they were not up to no good for the health and well being of OUR horses. The BLM lies where the horses are concerned. I ask why? For what gain? Is it money? What is it? I have my own idea as to why they do this and it isn’t pretty! Stop the terrorizing of these horses.

  7. Animal torture, lies, corruption..these are the words that come to mind when referring to the BLM. And just who are they beholden to? Apparently, they answer to their bosses at the pharmaceutical company. I have to wonder just how much they are compensated for torturing animals. The BLM mission statement, “to sustain the health, diversity and productivity of the public lands for the use and enjoyment of present and future generations”. Sorry President Truman, that idea has been replaced with vile, hellish experiments being performed on OUR wildlife! Hiding OUR wild horses away from the public, injecting mares with something derived from dead pig ovaries, having no idea what it really does to them, (aka: torture). I wonder what the BLM has planned for their next trick. My guess is “soylent green”.

  8. We have no idea how much damage will be done to the horses with PZP in their system. We all now know how much destruction it will cause to the horses! We have Moronic, greedy, selfish and corrupt people who control all that is going on, using this trash on helpless animals will solve nothing and it needs to stop. Take your stand.

  9. The government intent is clear for all to see, exterminate any wildlife that might possibly interfere with welfare rancher grazing of cattle and sheep on our public lands.

  10. This is an excellent report by Marybeth Devlin and should be read by everyone involved with the wild horses and burros. It is concise and factual and very incriminating to those who have opted for this “quick drug fix” approach which is, indeed, so antithetical to the true purpose and intent of the WFHBA. As you may know I am strongly pushing for Reserve Design and have a Go Fund Me to get this cherished yet terribly persecuted program back on the right track. The title of the GFM is Reserve Design for Wild Horses.

  11. The scientific evidence is very strong that PZP, this so-called contraceptive has long-term negative health effects both on individual treated mares and on the overall wild horse group survivability. This seems like a poorly disguised attempt to eradicate these horses, which aren’t present in excessive numbers, except only by the reckoning of the public land abusing livestock industry, which begrudges every blade of grass not consumed by their subsidized animals. The number of livestock grazing on public land should be greatly decreased – it’s bad for the land and unfair to farmers who utilize their own land and take care of it.

  12. Hi thanks for the article from Mary Beth.
    In New Zealand they have been using PZP to control possums (as pest control)
    It was approved over 10 years ago for use in containment. Because of the possum problem in NZ where as a non native the population have expanded. Originally those promoting its use wanted to PZP in GM carrots and potatoes growing around national parks etc. This could have allowed cross contamination of related weeds and crops (carrot seed from around the world are sourced in NZ)
    The source of the GM vegetables was the Hudson Institute and they were working in conjunction with research in NZ.
    It was stated that ‘others (risks)included the potential to jump to other animals or even humans.’
    For more about the antigen and its potential to shift between species http://press.gefree.org.nz/press/o08112004.htm

  13. Reno Gazette Journal just published a front-page article in their April 26th, 2016 paper entitled “Taking a Shot at Controlling Wild Horses” which seems to laud this “intelligent” effort to shrink the mustang herd of the Pine Nut Mountains. It gives a small quote from Anne Novak of Protect Mustangs at the end that calls into question the serious deleterious effects of PZP upon the wild horses. Shame on you, Reno Gazette Journal. You should do another front-page article and let Marybeth Devlin write it!

  14. I was an actual advocate for PZP figuring better than RU & holding pens, no more Wild Mustangs, just adopted, or down the auction tube to eventual slaughter. After reading this report of what PZP is and how it works, I am now totally against it. I wish the BLM would take all the $ it spends on this BC & sterilization process/experimentation & put it into buying up Lush acreage to move horses onto that are at risk of poor food & water qualities.

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