dead end, says Boyles Griffin. “It’s simple arithmetic; it’s not sustainable,” she says. “We have a solution: You have to gather only as many as you can adopt, and then you have to contracept like crazy.” Such an approach, she argues, pointing to a report in the Journal of Zoo and Wildlife Medicine, could potentially save taxpayers tens of millions over a decade.
As fewer people adopt horses, the BLM has been forced to slow the pace of roundups. And soon-to-be-published HSUS research may help persuade the agency to start relying more on PZP-22. The vaccine, designed to be administered every other year, is being used on a trial basis at two BLM herd management areas, Sand Wash Basin in Colorado and Cedar Mountains in Utah. (Click here for a map.) At Cedar Mountains, where 600 horses now roam an area the BLM believes can support 200 to 400, PZP-22 held foaling rates below 5 percent in 2013, and preliminary data shows it is expected to significantly reduce reproduction this year from the 60 percent observed in untreated mares. (Results from Sand Wash won’t be available until 2015.)
The Cedars data bolsters a 2013 recommendation by the National Academy of Sciences that the agency start using contraception immediately, beginning with the best option available—PZP. The Cedars results, due to be published in 2015, form the strongest argument so far that the BLM should be using contraception in the majority of its herd management areas, expanding from the seven BLM sites where PZP or PZP-22 is currently employed.
“Cedars is the big one”—the evidence wild horse managers have been waiting to see, says Allen Rutberg, a Tufts University professor and PZP researcher working with The HSUS on the trials.
Joan Guilfoyle, chief of the BLM’s wild horse and burro division, won’t commit to vastly increasing use of PZP but says the agency is looking for a way to follow the NAS recommendations and provide proven contraceptives to all its horse management areas. “PZP and PZP-22 are so far the most effective tools we have. We know they do reduce reproduction,” says Guilfoyle. “We are asking our people to look at where PZP can be used.”
Whether PZP-22 is widely adopted, says Guilfoyle, will depend on the decisions of individual managers, such as Warr and Jerome Fox. Fox, the wild horse and burro specialist at Sand Wash Basin, says he knows his BLM colleagues are closely following the trials. “There’s a lot of attention being paid to what’s happening at Sand Wash and Cedar Mountains,” he says. “Favorable results or negative results are going to have a huge impact on the horse program.”
PZP has been around for a long time—since 1988, when it was first used on wild horses at Assateague Island National Seashore, off the coast of Maryland. Jay Kirkpatrick, of the Science and Conservation Center in Billings, Montana, spent season after season wading through the marshes and walking the barrier beaches, struggling to get within 40 yards of the island’s horses so he could dart them with the vaccine. After just two years, PZP had stabilized the Assateague population—foals were no longer being born. But the vaccine had an unintended consequence. The mares who received it became healthier, heavier and better able to survive the winter. Where they used to die at the age of about 7, they began to live 21 years or more. Because of this, it took many more years for the populations to decline significantly. By 2013, however, the number of wild horses on Assateague was down by 40 percent, to 100 from 175. PZP had been proven effective.
“The management plan on Assateague is pure genius,” says Kirkpatrick, praising the openness of the National Park Service toward PZP. “And slowly but surely various horse herds are moving toward it.” Those herds include horses on the lands of the Navajo tribe, which has turned to Kirkpatrick for help.
The BLM, however, much to Kirkpatrick’s frustration, has hesitated to adopt the original form of PZP. To work, the drug has to be administered to mares twice the first year—with an initial vaccination and then a booster—and once every 