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 year after that. The agency didn’t see this as practical. It also wanted proof the contraceptive would work out West, on larger herds that might not be approachable, and on land that was millions of acres, not a small island. PZP-22 promised to address the first concern because it was a longer-acting version of the drug—boosters suspended in the initial vaccine in time-release capsules are delivered after one, three and 12 months. To address the second concern, The HSUS used a $1.7 million grant from the Annenberg Foundation to try out PZP-22 at Cedar Mountains and Sand Wash Basin starting in 2008. The trials are aiming to replicate results of a 2007 study in the BLM’s Clan Alpine, Nevada, management area that found that PZP-22 lowered the foaling rate of individual wild horses to 5 percent the first year after it was administered and 15 percent the second year (untreated horses in that area had foaling rates of 54 percent and 59 percent during those years).
If PZP-22 can keep annual population growth to 10 percent—which Rutberg, the professor and PZP researcher, believes it can—the contraceptive will be worthwhile to use at Sand Wash, Fox says. He’d only have to dart every two to three years and he could stop large roundups, selectively gathering only younger animals likely to be adopted. At Cedars, Warr says that even if the results are not quite that good, he would propose removing 200 horses and then, with the herd down to 400, try to treat all the mares.
Except when they’re thirsty, the Cedars horses aren’t easy to reach. Finding the herd requires long hikes or horseback rides. Kayla Grams, HSUS head field technician, says she spends a lot of her time sneaking up on horses, but the closest she’s ever been able to get to these animals, who’ve been shot at, is 50 yards. The horses can hear well and are alert to smells and to alarm signals given by other wildlife, like pronghorns, and by each other. “It’s sort of like hunting. I usually want to be downwind. I use vegetation to hide. I’ll sneak down a gully; I’l