Eyewitness Report: NAS Webinar on SpayVac

On Thursday, May 3, 2012, the National Academy of Sciences (NAS) organized a public webinar for the panel that is conducting a scientific review of the Bureau of Land Management's (BLM's) Wild Horse and Burro Program.

The topic of the webinar was SpayVac, an immunocontraception vaccine. It was presented by Dr. Mark Fraker (pictured at left) of TerraMar Research Ltd., the company that conducts research and development of SpayVac for wildlife application. SpayVac is a formulation of PZP (porcine zona pellucida). Native PZP has been utilized successfully for over 20 years on wild horses and other wildlife species. SpayVac is a special formulation of PZP that was developed to be a single-treatment, long-lasting fertility control vaccine. It was first utilized in gray seals on Sable Island of Canada's west coast, and has since been utilized in several wildlife species, including deer. It was tested on wild horses in a study on 12 mares in Nevada -- more on this study below. Dr. Fraker indicated that his company views the BLM as a major potential customer for SpayVac. His presentation painted a very positive picture of SpayVac, but questioning from the panel raised issues not initially disclosed, as well as serious concerns.

Permanent Infertility?

Fraker stated that long-term studies have not been conducted to determine whether or not a single application of SpayVac is a permanent sterilant. He did note, however, that a 10-year follow up of the treated gray seal population was conducted, and only 10 percent had returned to fertility a decade after application of the vaccine.  In the Nevada wild horse study, just 2 of 12 mares returned to fertility after four years. Further study of those mares was not conducted -- so the long-term effects of the drug are unknown. Dr. Irwin Liu, the NAS panel’s consultant on equine reproductive biology and physiology, questioned Dr. Fraker about this lack of return to fertility. Referencing the one published paper that included findings of limited research on SpayVac in horses, Dr. Liu asked about the observation that the vaccine had a self-boostering effect (which is the automatic renewal of the infertility effect potentially cause ongoing infertility). "That worries me," he said, "because it would be perpetual. It would go on forever as long as the mare is cycling." Dr. Liu suggested that it is critical to know if and when mares returned to fertility when using this drug. Dr. Fraker responded that the company would love to follow the animals longer, but that doing so would take money as well as time.

Side effects, including increased risk of i\nfection and behavioral impacts?

Given Dr. Liu's extensive experience with the issue of fertility control, he again returned to reported results in the published paper, which showed excessive reproductive tract edema occurring in mares after vaccination with the SpayVac vaccine. The research paper noted that 100% of the 12 mares had excessive edema three years after the SpayVac drug had been administered. Dr. Lui noted that, while excessive edema could have a contraceptive effect by preventing survival of an embryo, it could also provide a very good environment for bacterial growth, thus potentially making the mares susceptible touterine infections. Dr. Liu asked why would the mares show persistent edema year after year in the study. Dr. David Thain, an NAS panel member (whose appointment AWHPC opposed) and co-author of the paper, answered that he did not know.  The mares appeared to be more frequently in heat than they would have expected, he stated, acknowledging that in the future, he would conduct such a study differently. Dr. Liu continued this line of questioning. "Yes, but why would they all be in estrus at one time?" he asked, noting that, perhaps, the mares did not ovulate at all. "That would make a very nice contraceptive effect, but you might have other effects too if they did fail to ovulate." He concluded by noting that the only published paper on SpayVac in horses, the paper on which Dr. Thain is a co-author, was "somewhat speculative" and more scientific information was needed.

Moving Forward

Dr. Fraker noted that the timeline for moving forward with registration of SpayVac with the Environmental Protection Agency (EPA) will depend on having a clearly-defined market that would allow them to fund the effort. He predicted that once they decided to move forward, EPA registration could be in place within a year or so, allowing them to utilize the vaccine on a broad scale. Panel member Cheryl Asa asked about the availability of SpayVac for use in wild horses. “Could the company meet the demand if the BLM started using it widely?” she asked. Dr. Fraker replied with a definitive yes, noting that the vaccine would be manufactured within the company.


Unlike with native PZP, which has been extensively studied and utilized in wildlife species, including wild horses, for over two decades, the use of SpayVac in wild horses raises many unanswered questions. Based upon information currently available, it appears that SpayVac may have unintended, and potentially dangerous, side effects – including permanent sterilization – that make it an undesirable and dangerous tool for wild horse management, particularly in the hands of the BLM. The information provided and questions raised during the NAS webinar further strengthened AWHPC's opposition to using SpayVac as an alternative fertility control drug on wild horses and burros. Not enough information is known to justify its use in wild herds, particularly when the long-term viability of wild horse and burro populations is at stake.