Vaccine could reduce wild horse overpopulation

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A recently registered immunocontraceptive vaccine could help stabilize or reduce populations of wild horses and burros.

wild horses
Wild horses in Idaho (Courtesy of the Bureau of Land Management)

Earlier this year, the Environmental Protection Agency granted the Humane Society of the United States registration of an injectable porcine zona pellucida product, ZonaStat-H, as a restricted-use pesticide to control populations of wild and feral horses and burros. Under provisions of the Federal Insecticide, Fungicide, and Rodenticide Act, the EPA can consider nonhuman animals to be pests if they harm human or environmental health.

Holly Hazard, senior vice president for programs and innovation for the HSUS, hopes use of ZonaStat-H will lessen the need to round up herds of wild horses and burros, reducing stress on the animals and giving them more natural lives. She also expects that the immunocontraceptive will reduce costs to the Bureau of Land Management, which would spend up to $200 each year per mare on ZonaStat-H and a slow-release contraceptive instead of more than $100,000 on each herd gathered and more than $500 annually for care of each horse.

"Treating those animals on the range and releasing them is a bargain," she said.

An EPA memorandum published in July 2010 indicates that vaccination of mares on Assateague Island National Seashore, off the eastern coasts of Maryland and Virginia, was associated with increased lifespan and improved body condition scores, compared with findings for mares that endured the stresses of gestation and lactation. It also said that studies showed fertility didn't return in mares that had been treated for more than seven years.

Hazard said mares that avoid the stress of yearly foaling also live healthier, longer lives. The HSUS hopes the vaccine can be used to gradually reduce horse and burro overpopulation until the BLM needs to gather only as many horses as it can reasonably expect to remove from rangelands through adoption, about 4,000 animals annually.

In response to questions about the vaccine, BLM officials provided a statement in early March that the vaccine was being used according to individual herd circumstances. Because it is effective for one year, the agency still will likely need to gather horses for adoption.

"The ZonaStat-H vaccine will be used where BLM feels it can help make a difference controlling population growth rates and where it is anticipated that the horses can be approached for darting," the response states. "In other areas, BLM uses the longer-lasting pelleted PZP-22 product that is hand-injected after the animals are captured. PZP-22 seems to offer about two years of efficacy."

ZonaStat-H can be delivered by hand, jab stick, or syringe dart, label information states.

BLM information also indicates the wild horse and burro herd populations grow about 20 percent yearly, and the bureau removes thousands of horses annually to protect rangelands. The Western rangeland has about 38,500 free-roaming horses and burros, about 12,000 more than can exist "in balance with other public rangeland resources and uses."

Wild horse adoptions have declined in recent years, and more than 47,000 horses and burros were held in corrals and pastures in February 2012, BLM information states.

The HSUS first applied for registration of the porcine zona pellucida vaccine as a contraceptive for horse and burro population control in September 2009. The EPA's November 2011 proposal to grant the HSUS application states that, like deer and geese, "horses may be pests in some situations."

The product is the second immunocontraceptive vaccine registered with the EPA. In 2009, the agency granted a Department of Agriculture request for registration of GonaCon, a mammalian gonadotropin–releasing hormone vaccine used to control deer populations. The agency has also registered the nicarbazin-based OvoControl contraceptive pesticide that is used to control pest bird populations.

In describing the need for the vaccine, the EPA cited in its proposal the effects of federal protection for horses, the lack of natural predators for wild horses in the U.S., increasing herd sizes, and ongoing efforts to relieve overpopulation.

"With high population levels and the inability to sell or adopt out all captured wild horses and burros, the BLM has expressed that there is an explicit need to manage wild horse and burro populations because uncontrolled populations may lead to adverse environmental effects such as degradation of wildlife and native vegetation habitat," the proposal states. "Additionally, these populations may lead to conflicts with other rangeland uses such as cattle grazing and recreation."

Hazard said in early March that the HSUS was working with the BLM toward using the vaccine on herds in the spring, and the vaccine has already had its first use outside of research.

"We just inoculated a herd last week in Utah that we've been working with for a number of years," she said.

An efficacy summary included with the EPA documents indicates that, when ZonaStat-H was tested on mares at Assateague Island National Seashore, zero population growth was reached in two years, and the population declined over 11 years from 175 horses to 135.