BLM's Dismissal of PZP Fertility Control: A Critical Response

Introduction

The Bureau of Land Management (BLM) has dismissed the use of PZP fertility control in wild horse herds, citing impracticality due to the need for annual booster vaccines. This article challenges the BLM's stance, highlighting practical application methods already in use and ongoing research into alternative fertility control technologies. The American Wild Horse Conservation (formerly American Wild Horse Campaign) advocates for humane and effective population management solutions.

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 wild horses and burros (WHBs). The BLM's approach suggests that "you have to go to the horse," but in reality, "you let the horse come to you." Gathering field data on herd characteristics, movement patterns, and land topography helps determine the best method for administering treatments. Seasonal fly-overs can identify and map areas where horses are located.

Assessing whether the herd is limited by food or water, such as during a drought, is crucial. By tracking herd movements to water sources, future movements can be predicted. Darters can then position themselves near these sources, using blinds if necessary, to dart mares as they approach or leave. Providing hay can encourage herds to stay longer at water sources. Similarly, food-limited herds can be baited with hay, grain, or other attractants. The BLM acknowledges this technique but has failed to utilize it, relying instead on helicopter roundups.

WHBs can also be baited and trapped for fertility control, either by hand injection or dart. Existing allotment fencing can be used to bait, trap, and enclose herds within a pasture, allowing them to remain there between primer and booster doses. These methods eliminate the need for helicopters.

Fertility control programs involve photographing wild horse bands and maintaining vaccination records to track treatments. While horses can be identified by markings, color, and social affiliation, manually confirming identities can be time-consuming. To address this, the AWHC has partnered with Wild Me to develop an algorithm for identifying individuals based on photographs. This technology, used successfully for 53 species, is set to revolutionize fertility control.

Reduced Need for Annual Treatments

The reversibility of immunocontraceptives depends on antibody titer reduction. Reversibility and efficacy vary with treatment protocols. Notably, PZP's reversibility may decrease with consecutive treatments. Kirkpatrick & Turner (2002) noted 100% reversibility in mares treated for four to five consecutive years, though time to first parturition ranged from one to eight years. Ransom (2012) found that time to parturition increased by 411 days with each consecutive year of liquid PZP treatment. Consequently, treatment frequency can likely be relaxed after several consecutive years without reducing infertility rates.

Oocyte Growth Factor Vaccine

Recent developments on the Oocyte Growth Factor (OGF) vaccine suggest it could be a safe and effective sterilization option. Bone Morphogenetic Protein-15 (BMP-15) and Growth Differentiation Factor-9 (GDF-9) regulate follicular growth in mammals. Immunization against these factors may prevent ovulation, accelerate oocyte reserve depletion, or both, leading to permanent sterility.

Building on Davis et al. (2018) research, the BLM is studying an OGF vaccine for wild horses. The study combines BMP-15 and GDF-9 into a single vaccine, uses AdjuVac instead of Seppic MontanideTM Pet Gel A, and encapsulates proteins in liposomes. While promising, the BLM admits the study's limited duration may not confirm permanent sterility. Decreased ovulation rates and follicular development could alter mares' estrus cycles, affecting social behavior and herd stability. Ongoing research should examine these impacts in free-roaming settings.

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, link. 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, link (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), link.

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, link (last accessed Apr. 8, 2021); Platforms, WildMe, link (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.