Response to BLM’s Dismissal of PZP Fertility Control as a Viable Population Management Tool

(August 2, 2021) The BLM continues to say that the use of PZP fertility control in wild herds is impractical due to the need for annual booster vaccines. The agency also continues to falsely assert that delivering the vaccine to wild horses requires helicopter roundups and holding of horses, thereby dismissing other practical application methods that are already in use in federally-protected herds. The BLM also continues to point to a one-shot sterilant that is in development as a reason not to implement PZP on a wide scale now.  However, as the information below details, that technology is still in a preliminary research phase and has significant hurdles to cross before it could be deployed at scale in wild herds. 

BLM is right about one thing: PZP doesn’t work if you don’t use it. Please read on for a detailed response to the excuses the agency makes to avoid implementing this humane and readily available population management technology now. 

Practicality of Darting

Implementing a fertility control program, even in large herds, is both possible and practical using a number of proven strategies to identify and treat WHBs. Currently, the BLM’s approach is that “you have to go to the horse,” when in fact, the opposite is true: “you let the horse come to you.” The key is to gather field data on the characteristics of the herd, their movement patterns and the topography of the land they inhabit to determine the best method to use for administering treatments. For example, seasonal fly-overs can be conducted to identify and create a map of areas where horses are located.

One of the first factors to assess is whether the herd is limited by food or water, such as during a drought. By tracking herd movements to certain water sources, the herd’s future movements can be predicted. Darters can then stand some short distance from those water sources, utilizing blinds as necessary, and dart the mares at or as they leave the water source. If necessary to keep the horses at the water source for a longer period of time, hay can be provided to encourage the herd to stay. Similarly, herds that are food-limited can be baited with hay, grain, or some other attractant instead of water. The BLM has acknowledged this technique: “[t]o increase the success rate of the darting and to insure [sic] proper placement of the vaccine, darting should occur along travel corridors or at water sources. If necessary, bait stations using hay or salt may be utilized to draw the horses into specific areas for treatment.” However, the BLM has notably failed to utilize this method in its own fertility control program, relying almost entirely on helicopter roundups for fertility control application.

Additionally, WHBs can be baited and trapped to receive fertility control, either by hand injection or dart. Darters can take advantage of existing allotment fencing to bait, trap, and enclose herds within an existing pasture. These WHBs can remain in the pasture for the weeks in between primer and booster doses as an efficient way to ensure that they receive their second treatment. Each of these methods is implemented without the use or expense of helicopters.

Any fertility control program involves photographically documenting wild horse bands and keeping vaccination records to determine who has already received a treatment, when that treatment was administered, and if the individual is due for a booster dose. While it is currently possible to identify horses by their markings, color, and social affiliation, using a database to manually confirm the identity of individuals can sometimes be a time-consuming process. To alleviate this, the AWHC has partnered with Wild Me to develop an algorithm to identify individuals based on photographs. Wild Me technology has been developed and utilized successfully to identify members of 53 species, including right whales, manta rays, zebras, and lynx. Having completed a successful proof of concept, this technology is likely to completely revolutionize the fertility control process in the near future.

Reduced Need for Annual Treatments

The reversibility of immunocontraceptives is determined by the reduction in antibody titers. Like efficacy, reversibility can vary depending on the specific treatment protocol followed. Most notably, though, is the fact that the reversibility of PZP may decrease with consecutive treatments. Kirkpatrick & Turner (2002) noted 100% reversibility in mares treated for four consecutive years and five consecutive years, though the time to first parturition after treatment ranged from one to eight years. Ransom (2012) found that the time to parturition increased by 411 days with each consecutive year of treatment with liquid PZP. Consequently, the frequency of treatments can likely be relaxed after a certain number of consecutive years of treatment without immediately reducing the rate of infertility. 

Oocyte Growth Factor Vaccine

Recent developments on the Oocyte Growth Factor (“OGF”) vaccine indicate that the immunocontraceptive could be a safe and effective sterilization option. Two growth factors, Bone Morphogenetic Protein-15 (“BMP-15”) and Growth Differentiation Factor-9 (“GDF-9”), are known to regulate follicular growth in several mammalian species. Though the mechanism is not entirely understood, immunization against these growth factors is expected to either prevent ovulation, accelerate depletion of oocyte reserves, or both. Because females are born with a finite number of oocytes; depletion of these reserves results in permanent sterility.

Launching off of the research conducted by Davis et al. (2018), the BLM is currently studying an OGF vaccine for use in wild horses. The ongoing study made several methodological changes to test the hypothesis that immunizing against these growth factors could result in permanent sterility—or, at least, prolonged infertility—with a single dose. The changes include (1) combining both BMP-15 and GDF-9 into a single vaccine, (2) using AdjuVac instead of Seppic MontanideTM Pet Gel A, and (3) encapsulating the proteins in liposomes. While the study has demonstrated promising results, the BLM admits “it may not be possible to determine whether the one-dose version of the oocyte growth factor vaccine would cause permanent sterility” given the study’s limited, three-year duration. Furthermore, decreasing ovulation rates and follicular development may alter the normal estrus cycle of a mare and, as a result, social behavior and herd stability. Thus, the ongoing research into the OGF vaccine, as in any prospective fertility control study, should examine the impacts on behavior and social structure in a free-roaming, field setting.

1. This strategy has been successfully employed by the BLM Vale District in darting several very remote and wild herds. Presentation by Shaney Rockefeller & Eric Youngberg, High Desert Strategies, Inc., Vale District BLM,…. To develop the program, the BLM wild horse specialist recruited a hunting expert to devise a darting strategy. The expert has since formed a non-profit to assist in remote fertility control darting of wild horse populations. See High Desert Strategy, (last accessed Apr. 27, 2021).

2.  See, e.g., J.W. Turner et al., Remotely Delivered Immunocontraception in Free-Roaming Feral Burros, 107 J. Reproduction & Fertility 31, 32 (1996).

3. U.S. Bureau of Land Mgmt., IM No. 2009-090, Population-Level Fertility Control Field Trials: Herd Management Area Selection, Vaccine Application, Monitoring and Reporting Requirements (2009),

4. Kathleen A. Carey et al., Efficacy of Dart-Delivered PZP-22 in Wild Horses in Baited Traps in New Mexico, USA, 46 Wildlife Research 714, 715 (2019).

5. Conservation Meets Machine Learning, WildMe, (last accessed Apr. 8, 2021); Platforms, WildMe, (last accessed Apr. 8, 2021).

6.  National Research Council, Using Science to Improve the BLM Wild Horse and Burro Program: A Way Forward 103 (2013) [hereinafter NAS Report].

7.  Id.

8. J.F. Kirkpatrick & A. Turner, Reversibility of Action and Safety During Pregnancy of Immunization Against Porcine Zona Pellucida in Wild Mares (Equus caballus), 60 Reprod. Supplement 197 (2002); National Research Council, supra note 6, at 104. At the time the NAS report was written, none of the mares treated for seven consecutive years had reversed over the seven years of monitoring. National Research Council, supra note 6, at 104. Ransom (2012) replicated the finding that mares returned to parturition one to eight years after cessation of treatment. Jason Ian Ransom, Population Ecology of Feral Horses in an Area of Fertility Control Management 21 (2012) (dissertation).

 9. Ransom,  supra note 8, at 22.

10.  E.g., Ransom et al., Foaling Rates in Feral Horses Treated With the Immunocontraceptive Porcine Zona Pellucida, 35 Wildlife Soc’y Bulletin 343, 347 (2011).

11. E.g., Kelli A. Davis et al., Effects of Immunization Against Bone Morphogenetic Protein-15 and Growth Differentiation Factor-9 on Ovarian Function in Mares, 192 Animal Reproduction Sci. 69, 69–70 (2018).

12. E.g., id. at 70.

13. Bureau of Land Mgmt., DOI-BLM-NV-0000-2020-0001-EA, Oocyte Growth Factor Vaccine Study 6 (2020) [hereinafter BLM OGF EA]. In 2016, researchers analyzed the physiological effects in mares resulting from immunization against BMP-15 and GDF-9, separately, using a four-dose methodology. Davis et al., supra note 11, at 70 (mares vaccinated at weeks 0, 6, 12, and 18 relative to the time of the first vaccination). The study did not test the efficacy of the vaccines in preventing pregnancy, but it did find that the mares vaccinated against BMP-15 showed increased titers consistently above the antibody threshold value following the second dose, fewer ovulations, smaller follicles, and decreased estrous behavior. Id. at 72–74. Of the ovulations observed, 92% occurred after the final dose, which appeared to correspond with decreasing antibody titers. Id. at 72, 74. Mares vaccinated against GDF-9 showed less robust antibody titer results; however, other studies indicate there may be a one-year delayed response. Id. While there was no observed reduction in ovulations, mares in this group did exhibit abnormal follicles and a decrease in the total number of days in estrus after the second dose. Id. at 72–73. The unchanged ovulations and abnormal follicles suggest that the GDF-9 vaccine functions by means of accelerated depletion of the oocyte reserves. Id. at 74.

14. See BLM OGF EA, supra note 13, at 16.

15. BLM OGF EA, supra note 13, at 6; Bureau of Land Mgmt., DOI-BLM-NV-0000-2020-0001-EA, Oocyte Growth Factor Vaccine Study: Appendix A 7 (2020) [hereinafter BLM OGF EA App. A].

16. BLM OGF EA, supra note 13, at 16; BLM OGF EA App. A, supra note 15, at 5. All ten mares in the study were infertile in year one (no ovulations; follicles no greater than 12mm), and 90% were infertile in year two. BLM OGF EA, supra note 13, at 16; BLM OGF EA App. A, supra note 15, at 5. Normal follicle size for ovulation is 35–50mm. Davis et al., supra note 11, at 74.