Dan Baker is not like an expectant dad waiting to find out if it’s a boy or a girl. He’s the opposite, hoping to hear that all the pregnancy tests come back negative. Baker, a research biologist at Colorado State University’s animal reproduction and biotechnology laboratory, is the man in charge of an experimental contraception program in the wild horse herd at Theodore Roosevelt National Park. There is hope that Baker’s work is productive — not reproductive.
Waiting for results
Within a month, he’ll know if he’s onto something that will have implications far beyond this singular herd in this one park — or if he’s back to square one.
The samples are in, and tests will be run soon. He hesitates to even guess at the results. “It’s totally unknown. It could be anything between no effects all the way to permanent sterilization. This question has never been answered,” Baker said.
If his experiment works, it could be a new way to control the park’s constantly expanding wild horse herd and possibly the thousands of wild horses on Bureau of Land Management land. The method also could have uses in the control of unmanaged wild dog populations in Third World countries, or simply to suppress fertility in domestic horses, dogs and cats.
Baker’s work in what he calls a perfect — not to mention beautiful — outdoors laboratory dates back to 2009. “It’s such a great natural lab out there. The area the horses are confined in is large, but not too large. It’s great landscape, and we can find them most of the time,” he said.
In 2009, during the park’s scheduled wild horse roundup and herd reduction, Baker vaccinated 28 wild horses with GonaCon, a vaccine that has been used to suppress pregnancy in captive animals, not free-roaming wild ones such as those in the park. The results were poor. Half the vaccinated mares became pregnant and, within three years, they all did. What they’ve since learned is that, even though the park’s wild horses are in excellent physical condition, with good forage, they carry a big parasite load, which may have prevented the kind of antibody response needed to suppress pregnancy.
“Real world horses get injured, or have fence cuts, and their immune systems go toward those things rather than suppressing the hormones that control reproduction,” Baker said.
Last year, the park conducted another wild horse roundup and that’s when Baker’s research took a step further. The same 28 mares were revaccinated to learn whether a second booster of the same drug would achieve a higher antibody response and improve contraception.
Last month, volunteers collected fecal samples dropped on park ground by as many of the 28 vaccinated mares as could be located.
By measuring the feces for estradiol, a hormone excreted by a fetus, Baker’s lab team will soon know if the revaccination was successful.
“As the fetus matures, the concentration of estradiol gets higher and higher. If it’s 10 (nanograms per gram), they’re not pregnant. If it’s 100, they are. In a couple of weeks, after we’ve looked, if everything’s really high, the study’s over,” Baker said.
The proof will be in the lab, but the mares will also be observed in the spring to verify the actual foaling rate.
Park waiting, too
Blake McCann is the park’s wildlife biologist, a man who prizes science and wildlife equally. McCann’s hopeful the revaccination works, too, but for reasons that have more to do with the horses, than the science itself. He’d like to see the park bring to an end the longstanding practice of controlling the wild horse population with controversial helicopter-driven roundups and transport to public livestock sales barns. Instead, if the revaccination controls pregnancy by even 50 percent, McCann said becomes more feasible to also lure the wild horses into a makeshift corral in their own environment and remove small select numbers for sale right there.
That practice would be much less traumatic all around for humans and horses, he said. He plans to conduct a corral trap this year to start learning how and to manage the 142 wild horses currently in the park, a number well above the 40 to 90 population considered ideal.
Some doses of the second vaccine were delivered by dart, which was acceptable for the experiment. “For research, yes, but to use that as a management tool, we would have to go into an environmental impact statement. Darting animals is not part of our management plan,” McCann said.
Whether through contraception or smaller removals from the temporary corrals, McCann said he does not want to see wild horse numbers return to the all-time high of 200 that were there last year when 103 were culled and sold at Wishek Livestock. “I don’t want to get to 200 again and do another helicopter roundup. With the corral trapping, we can remove a dozen or so every year and get the young mares out before they become reproductively active,” he said.
That said, McCann said he’s hoping Baker’s work is productive, not reproductive, as it were. “I would like to see the vaccine be a viable tool. We always have to be adaptable as a situation unfolds. I’m hopeful it’s effective,” he said.
Source: Bismarck Tribune by Lauren Donovan
Largely unchecked by natural predators, many wild horse populations grow at rates of 18–25 percent per year. This unregulated growth can overtax vegetation and affect herd health as well as native wildlife populations. The Wild Free-Roaming Horses and Burros Act of 1971 requires the U.S. Bureau of Land Management (BLM) and USDA Forest Service to manage these free-roaming herds for a “thriving natural ecological balance.”
To ensure the sustainability and health of both horse herds and the public lands they roam, and to reduce the number of animals requiring either adoption or long-term care in holding areas, managers began to explore fertility control as an alternative management technique.
From 1978 into the 1980s, the BLM worked through a series of research contracts focusing primarily on development of a chemosterilant for wild stallions. In the early 1990s, research turned to silicone implants in mares in an effort to achieve fertility control. Although both routes produced fertility control, they had too many drawbacks and were eventually abandoned.
In light of these problems and the continuing need for some form of contraception, in 1991 researchers identified the desired characteristics for an ideal wild horse fertility control agent.
Specifically, the agent should:
How PZP works
In order for sperm to attach to the ovum and fertilize the egg, there must be complementary proteins on both the surface of the sperm and the zona pellucida (ZP) of the ovum. PZP acts as a foreign protein against which the treated mare produces antibodies (thus, the PZP fertility control agent is actually a vaccine). These antibodies attach to the mare’s zonae sperm receptors on the ovum and block fertilization. Domestic pig ovaries (obtained from slaughterhouses) are minced and the PZP is obtained from screening filtration. An adjuvant is mixed with the PZP to enhance its effectiveness when it is injected into mares intramuscularly. Once injected, it causes an immune response, making the mare infertile. Over time, the antibody titers fall and fertility returns. With the liquid vaccine, a booster injection can be given at 10 months to raise the titers back to the infertile range. This can be done each year for at least 4 years, after which time the effects may be more likely to become permanent. For this reason, current individual-level field trials involve only 1–4 years of treatment.
This list of needs would drive much of the U.S. research on wild horse contraception during the 1990s, including research funded by both the BLM and the U.S. Geological Survey (USGS). To meet the stated criteria, a National Park Service research team on Assateague Island National Seashore turned to an immunocontraceptive agent, porcine zona pellucida (PZP), which had been reported to block fertilization in dogs, rabbits, and primates. Experimental PZP application on the wild horses of Assateague Island commenced in 1988, resulting in promising reductions in the pregnancy rates of mares: by 1994, population growth began to stabilize solely through the use of PZP immunocontraception.
The Outlook for PZP
The PZP agent appears to meet most of the safety concerns of the BLM. The fact that PZP is a glycoprotein suggests that it should be digested before it can enter the food chain. Its effects passively wear off with time if annual injections are terminated; normal reproduction can be resumed following at least 4 years of use, and perhaps more. It does no harm if injected into mares that are already pregnant—they carry foals to term. Initial research suggests that PZP does not affect ovarian function or hormonal health. Life span seems to increase with improved health of treated mares, apparently due to the absence of stresses from pregnancy and lactation. Treated mares can live 5–10 years longer than untreated mares that continue to get pregnant and produce young. An initial study suggested that harem behaviors are not influenced, and more in-depth investigations are currently underway. There appear to be no generational effects; offspring of treated mares are able to reproduce normally. Finally, at least some forms of PZP may be 90% effective in blocking fertility in mares (see Wild Horse Resources, Fertility Control in Mares).
PZP has been successfully applied to control fertility in several small populations of wild horses on eastern barrier islands since the early 1990s. Population-level field trials of an injectable, time-release, pellet form of PZP that will allow almost 2 years of fertility control with a single treatment are currently underway in many western herds. The Assateague team also developed noninvasive methods to assess the pregnancy rates of, and detect ovulation in, free-ranging treated and nontreated, individually recognizable mares by analyzing reproductive steroid metabolites in their feces and urine.
PZP Field TrialsThe USGS, BLM, and USDA Animal and Plant Health Inspection Service (APHIS) have essentially completed individual-level field trials of PZP in free-roaming wild horses at three locations: Pryor Mountain Wild Horse Range, Montana and Wyoming; McCullough Peaks Herd Management Area, Wyoming; and Little Book Cliffs Wild Horse Range, Colorado. Application of PZP began in 2001 at Pryor Mountain, in 2002 at Little Book Cliffs, and in 2004 at McCullough Peaks. Results of this work in terms of horse behavior are reported in Ransom et al. (2010), and efficacy results are detailed in Ransom et al. (2011). In addition, in 2011 the USGS, BLM, and the Animal and Plant Health Inspection Service (APHIS; Department of Agriculture) began studies on the safety and efficacy of SpayVac®, a form of PZP made using a proprietary technology developed by Immunovaccine Inc. (Halifax, Nova Scotia, Canada).
Source: USGS, Fort Collins Science Center