Sunday, January 10, 2016

The Mustang Management Contraceptive Primer


This is not a debate on which is the best method of controlling wild horse numbers. These are simply the facts. It is clear science is far from perfect but research and observation can serve to give us an idea, a general sense of something which can compel us to look for more answers and continue research, preferably as humanely and as compassionately as possible. This is also not a debate as to whether mustangs should be classified as a native species in North America, returned native species, indigenous or invasive. They are here, with limited resources, and they are our responsibility.


Porcine Zona Pellucida (PZP):  The compound PZP, which is short for Porcina Zona Pellucida, is derived from sow ovaries. When the pigs are slaughtered for the meat industry, the excess tissue not used for the food industry is either discarded or utilized for non-consumption purposes. Some tissue used for research and others for the preparation of drugs. Heparin is a potent anticoagulant given to almost every patient who has had surgery followed by an overnight stay in the hospital, to prevent the formation of blood clots. Heparin is derived from pig intestines. Insulin, given to diabetics, was originally made from cow, pig, and even whale pancreases. Currently there are still some available that contain animal products, although there are genetically modified human insulins and insulin analogs that are not animal based (

Pesticide ClassificationThe FDA classifies PZP as a pesticide simply because they do not have a category for contraception. Pesticides control the numbers of a populations be it insect or mammal, and because they can be quite destructive, pesticides are deemed as negative chemical compounds. PZP does not have any direct effect on any of the plants or animals other than the inoculated mares.

How it works: PZP works by stimulating the immune system of a mare to produce antibodies which migrate through the horse’s body to an oocyte (egg). When the mare ovulates, the antibodies immediately surround the egg, making it impenetrable to sperm. The egg cannot be fertilized and there is no foal. The reproductive behavior remains relatively normal, the mare goes into estrus and is covered by a stallion but there is no resulting offspring (Barber, Lee, Steffens, Ard, & Fayrer-Hosken, 2001).

Risks: PZP is not without risks. The currently long acting PZP-22 can last approximately 22 months. PZP is based on the immune system of the mares and this can cause variation in the efficacy and duration of the contraceptive effect. As in humans utilizing long term injectable contraception (Depo-Provera), the mare’s return to fertility is quite variable (Kirkpatrick et al., 2009). The reason PZP is not offered to humans is because the efficacy rate is not high enough.

“For contraceptive treatment to be an effective management tool, it usually needs to be reversible (Kirkpatrick & Turner 1991). A long term study of feral horses showed that PZP was reversible even when females were treated for several years (Kirkpatrick & Turner 2002). However some females appeared not to return to full fertility after long-term PZP treatment and similar side effects were seen with GNRH treatments in deer (e.g. Miller et al. 2000a). Consequently, most wildlife contraceptives are reversible, or have minimal impact after prolonged use.”  (Gray & Cameron, 2010).

Prolonged use has demonstrated that some mares will never return to fertility. Kirkpatrick, Liu, Turner, Naugle, and Keiper (1992) found that three factors determine the return to normal reproductive function: the amount of PZP administered, the number of antibodies produced by the mare, and ovarian dysfunction. Earlier studies also demonstrated damage to ovaries although the PZP preparation was crude in the earlier stages of development (Kirkpatrick et al., 1992).

Abscesses at the injection sites have been reported but these are temporary, and heal without complications.

PZP and Tuberculosis: Finally, there were some rumours floating around social media that PZP can cause tuberculosis. Although this may sound like science-fiction or the nefarious work of people against keeping the horses wild, there is some truth. The original method of getting PZP into the animals involved piggy-backing the molecule on a carrier molecule or adjuvant. Adjuvants are not biologically active but their presence can trigger an immune response. It may result in a false positive antibody response for tuberculosis. The animal doesn’t have the disease, and many human vaccines work this way by stimulating the body to form antibodies to something not biologically active.  The original choice for the adjuvant was a mycobacterium- the mycobacteria family are known to cause tuberculosis and many other diseases. Because the PZP was attached to an inactive mycobacterium, in some animals it cause a false-positive tuberculosis antibody response. They changed the adjuvant for the preparation of PZP so now there is no mycobacterium involved. Additionally, horses cannot contract or transmit tuberculosis (Lyda, Hall, & Kirkpatrick, 2005).

GONACON: This method works by injecting mares with a synthetic (man-made) chemical compound called GonaCon. This compound acts against a hormone called gonadotropin releasing hormone (GnRH). In the reproductive cycle of mammals, GnRH is produced by the anterior pituitary gland. This gland controls reproduction by stimulating the release of the follicle stimulating hormone and the luteinizing hormone (and others). ConaCon works by stopping the production of GnRH and subsequently the luteinizing hormone and without LH, there is no ovulation, no corpus luteum, and therefore, no egg/offspring (Speroff & Fritz, 2012). Like PZP, GonaCon can result in prolonged infertility (Ransom, 2014). The tables below represent one study each- these data are only reflecting the results one study and may not have generalisability to the entire population. The general consensus amongst zoos and researchers is that PZP is 90% effective when administered correctly.

In a study by Jason I. Ransom et al. (2014), the researchers found there were fewer behavioural differences in mares treated with GonaCon, compared to those treated with PZP. They modeled their GonaCon study after the PZP study and found fewer alterations in the wild mare's behavior. GonaCon is still a recent addition to the world of wildlife contraception and has potential as a potential management tool for equids. It shows promise but the long-term data is still unavailable.  

GonaCon can be given to males because the GnRH stimulates testosterone production in males. However, studies of stags treated with GonaCon resulted in antler deformity and other negative consequences. "In conclusion, the GnRH vaccination in male rusa deer resulted in the increase in GnRH antibody titer, which negatively correlated with blood testosterone. The decrease in blood testosterone might be involved in the lower semen quality and poor antler development" (Phraluk, O. et al, 2015). There is potential for use in stallions but we need more research in this area.

Because GonaCon a works systemically, not targeting the reproductive tract as specifically as PZP, the potential for side effects increases. The closer to the intended target a medication or treatment is administered, the more effective, the lower the dose, and adverse drugs reaction are substantially decreased. The Global Library of Women's Health states: "In non-reproductive tissues, GnRH has been reported to modulate neuronal migration, visual processing, digestive tract function, and immune T cell chemotaxis. Studies in endometrial, ovarian, and prostate tumor cell lines have implicated GnRH in mediating cell growth, angiogenesis, invasion, and metastasis." (
Table 1. (Represents data from one study)
Infertility over three-years with equine contraception (Kilian et al., 2006).

Infertility after one year
Infertility after two years
Infertility after three years
Spay- Vac PZP
Copper IUD
*The assumption is the IUD’s had been expelled in the mare who became pregnant

Table 2. (Represents data from another study)

Foaling rates at three horse management areas after PZP treatment  (Ransom, J.,et al, 2011).

Foaling Rates
Type of contraception


Little Book Cliffs
PZP in liquid form requires annual boosters

PZP in pelleted form- designed to last two years

Pryor Mountain
PZP in liquid form requires annual boosters

Any time we begin to tamper with Mother Nature- it places us at risk. Treating mares with immunocontraception and/or Gonacon can have negative consequences on the familial structures of a highly social animal. Treating mares with these two compounds can result in mare giving birth to foal during time of limited resources. A study determined that there are differences in parturition times for mare treated compared with mares that were not treated. Foals born during more normal times for wild horses have a higher survival rate. (J. I. Ransom, Hobbs, & Bruemmer, 2013).  

However, all negative consequences of injectable contraception pale in comparison to the disruption of the social structure during round-ups. Separation of mares from stallions and their offspring occurs during round-ups and culling. Family bands are broken up and the horses face the terrible loss of their freedom. A balance must be found and the benefits and the negative outcomes must be weighed. There is a chance a mare treated with the above methods, may never foal again- but that mare remains free. There is always the potential she may resume ovulation however, she is will not spend her life in a holding pen. A study by Turner, Liu, Flanagan, and Rutberg (2007) indicated one mare out of sixteen in the study, did not have a normal return to fertility. Is a chance of infertility worth the price of freedom?

OVARECTOMY: This is removal of a mare’s ovaries. This method is irreversible, the mare will never breed again. Behaviorally, it reverts a mare to a non-breeding state. There is not a lot of data and/or research regarding this permanent method in the wild horse population which is why research proposals are being requested by government agencies The risks are the this surgical procedure are post-operative infection and possible abortion of a fetus if the mare is gravid at the time of the procedure.  Additionally there is the loss of that mare's potential contribution to the genetic diversity of that herd management area.  This loss is not well defined and may vary based of the genetic variation in a herd management area. Mares should be monitored for three weeks post-operative (optimally), before being returned to the range to live out their lives. The upfront cost is high, but the end result is permanent. It has been proposed for mare over a certain age, selected by the currently proposed studies, as means to control the populations which would allow the mares to live free without foaling year after year (Speroff & Fritz, 2012, National Research Council, 2013).

CHEMICAL STERILIZATION/VASECTOMY/NEUTERING:  These methods, performed on stallions, are permanent means of limiting wild horse populations. In chemical sterilization, stallions have a solution injected into the testes which causes necrosis and eventual tissue death of the testes. It is painful and carries a high risk of infection but is very cost-effective (Zhanwei, 1989),  Orchiectomy (removal of testes: aka gelding/neutering) is another permanent method which removes the testes of a stallion. Like chemical sterilization, ‘gelding’ causes behavioral changes and the stallions become less aggressive, there is less fighting and they cannot reproduce. Vasectomy involves severing the vas deferens of a stallion so that the communication between the testes and the penis is removed. The sperm are not able to be passed through the urethra during copulation. Behavior remains the same because the testes are still present and producing testosterone. Infection rates are much lower in vasectomies, the procedure is less painful but it requires a delicate touch, it may not be 100% effective, and the horse must be anesthetized or sedated (Speroff & Fritz, 2012, National Research Council, 2013).

ROUND-UPS: This method involves gathering the horses by use of helicopters, men on horses, ATV, trucks, and bait trapping. The bait-trapping is the least likely to cause physical harm but none of these methods are without significant risks. Helicopter round-ups have the highest incidences of morbidity and mortality.  The horses are gathered, separated and either returned to the horse management area, or they are removed to a holding facility for potential adoption.

RESERVE DESIGN:  This method of mustang management has merit in a perfect world. The theory is to find a place for the horses to live. A place that has natural horse-predators (may prove to be difficult to find), natural barriers to prevent migration in/out of the area, and neighbours sympathetic to mustangs.  The area has to have sufficient food, water, shelter, and other necessities for survival year round. Funds for this preserve may be obtained through ‘eco-tourism’ and public education regarding the wild horses are included in this plan.  Reserve design is a wonderful idea but it does not have practical application. The land that would be needed is not available, at least not at this time, for the numbers necessary.  The monitoring of these herds and the management of these herds must be carried out as well and the necessary people to oversee each area. The goal of 'reserve design' is to be self-sufficient in which the wild horses achieve homoeostasis with regards to population growth. Because the predation is very low, and we cannot safely import predators, this will prove to be challenging. Incorporating natural predators to assist in controlling the wild horse population can have deleterious effects on the domestic population of horses and of livestock. Historically, once a predator begins to hunt within the domestic population, the end result usually has rather negative consequences for the predator.

SELF-REGULATION: This involves leaving the mustangs alone to establish population equilibrium without any interference. There is no scientific evidence that this methods works and history has shown us wild horses do not fare well with a hands-off approach. Unfortunately the resources the mustangs have within the management areas are limited and the horses will suffer in one way or another if left unmanaged. There are three factors that determine a population’s ability to grow. They are: available resources, predation, and disease. Many of the wild horses at Cold Creek starved to death or had to be humanely euthanized because resources became compromised.  The resources may be limited due to naturally occurring factors such as drought or fire, or they may face competition from livestock grazing on the same land. Regardless of the mechanism, reduced resources will cause competition, and result starvation or disease in the wild horse population. Occasionally a mountain lion will take a foal or an older/injured horse but cougars are not primary predators of the mustangs . Wolves generally do not live in the horse management areas with any regularity. Disease is a concern with any animal left in overcrowded situations. Chronic Wasting Disease is a disease that began in the mule deer population, and has spread to other cervids (herbivorous even-toed mammals in which the male carries antlers). The current deer populations are substantially larger than the available resources in the north-east United States, and this disease has now been identified in white-tailed deer of the Adirondack Mountains. Similar to Bovine Spongiform Encephalopathy (BSE), or Mad Cow Disease, this disease is affecting all cervids, not just the deer (Chronic Wasting Disease Alliance, 2016).  If the populations of wild horses were left alone, they would increase to the point of starvation and disease. It would only be a matter of time before a disease mutated in that population and spread to the domestic horse population with devastating consequences.

DECLARING THE MUSTANG “ENDANGERED”: This method is to first declare the mustang a ‘native wild species’ and then have it declared endangered- which may prove difficult. The mustang is the same species as domestic horses. They fall within the same genera: Equus and species: caballus. They have no genetic markers or any other characteristic that differentiate them from their domestic counterparts.  There has been proposed theories that they behave differently but this was not enough for the United States Fish and Wildlife to declare mustangs as a separate subspecies.  The wild horses are identical genetically, physiologically, and behaviorally to the horses in your paddock.  There are currently 40-50,000 presumably wild horses in captivity at BLM holding facilities. There are an estimated 20-40,000 living wild on horse management areas, and several thousand more domesticated mustangs living with people. These numbers alone are not sufficient for endangered species status, or even threatened species if USFW was willing to grant them subspecies status. 

Proponents of making mustangs endangered believe that once they achieve the endangered species status, the mustangs would be granted the ultimate protection. However, advocacy groups would no longer have a say in their conservation; they would be managed by Fish and Wildlife. The now ‘endangered’ mustangs may be moved to locations to protect their numbers and they may very well lose their freedom if they were to ever to gain protected status. Our descendants would not be able to see these 'endangered mustangs' living free; they would only see them in zoos and protected reserves.

The belief that mustangs are a separate species or a separate animal from domestic horses is the first hurdle to overcome with this method. However, that has proven to be impossible. They are not separate; they are the same species just as a miniature horse and a Clydesdale are the same species. They are horses that have returned to the wild and been wildly successful at surviving and reproducing.   

Managing them to extinction” is a catch-phrase often used to describe the situation of the wild horses. Mustangs will never become extinct because they aren't recognized as a separate sub-species of the modern horse. However, there is a very good chance our children and their descendants will never see a free roaming mustang, and that would be the greatest tragedy of all. Regardless of their origin, regardless of their heritage, the mustangs are our responsibility. We need science to save the mustangs, scientists with the necessary credentials and expertise in wildlife management, ecology, contraceptive experts, and equine ethology all working to establish the best and most humane method of managing wild horses. Each management area is unique and each management area needs its own method of achieving appropriate and healthy numbers. The phrase "managing them to eradication" is more accurate, less sensational rhetoric and implies the same message without any controversy regarding the species/subspecies status of the wild mustangs.  


Meredith Hudes-Lowder is a Nurse Practitioner in Women’s Health and an expert on contraception. Additionally, she has a bachelor’s of biology with a concentration in environmental education.  She will graduate in May 2016 with a Doctorate of Nursing Practice. She runs the largest exclusively wild horse photography site on Facebook with Karen McLain- they have almost half a million fans. Bruce Lowder was consulted for this blog. He is a wildlife expert, naturalist, and worked for U.S. Fish and Game.


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Gray, M. E., & Cameron, E. Z. (2010). Does contraceptive treatment in wildlife result in side effects? A review of quantitative and anecdotal evidence. Reproduction, 139(1), 45-55. doi:10.1530/rep-08-0456

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Ransom, J. I., Powers, J. G., Garbe, H. M., Oehler Sr, M. W., Nett, T. M., & Baker, D. L. (2014). Behavior of feral horses in response to culling and GnRH immunocontraception. Applied Animal Behaviour Science, 157, 81-92. doi:

Speroff, L., & Fritz, M. (2012). Clinical Gynecologic Endocrinology and Infertility (8th ed.). New Yok: Lippincott Williams & Wilkins.

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Zhanwei, S. (1989). Chemical castration of horses and mules by injecting testis with iodine tincture. Journal of Liaoning Animal Husbandry and Veterinary Medicine.

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Barber, M. R., Lee, S. M., Steffens, W. L., Ard, M., & Fayrer-Hosken, R. A. (2001). 
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Gray, M. E., Thain, D. S., Cameron, E. Z., & Miller, L. A. (2010). Multi-year fertility reduction in free-roaming feral horses with single-injection immunocontraceptive formulations. Wildlife Research, 37(6), 475-481. doi:10.1071/wr09175
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Ransom, J. I., Powers, J. G., Garbe, H. M., Oehler Sr, M. W., Nett, T. M., & Baker, D. L. (2014). Behavior of feral horses in response to culling and GnRH immunocontraception. Applied Animal Behaviour Science, 157, 81-92. doi:

Schulman, M. L., Botha, A. E., Muenscher, S. B., Annandale, C. H., Guthrie, A. J., & Bertschinger, H. J. (2013). Reversibility of the effects of GnRH-vaccination used to suppress reproductive function in mares. Equine Veterinary Journal, 45(1), 111-113. doi:10.1111/j.2042-3306.2012.00577.x

Speroff, L., & Fritz, M. (2012). Clinical Gynecologic Endocrinology and Infertility (8th ed.). New Yok: Lippincott Williams & Wilkins.

Turner, J. W., Liu, I. K. M., Flanagan, D. R., & Rutberg, A. T. (2007). Immunocontraception in wild horses: One inoculation provides two years of infertility. Journal of Wildlife Management, 71(2), 662-667. doi:10.2193/2005-779


  1. This is an excellent and well cited primer. Thank you for the explanation and research that went into it.

  2. Excellent. A must read for anyone swayed into hysterics about "disappearing" wild horses.


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