Nov 12, 2015

Contraceptives providing cheaper way to manage wild horse herds

More than 58,000 wild horses live on 31 million acres of public land and about 45,000 live in long-term holding facilities — costing taxpayers millions of dollars per year, according to the Bureau of Land Management. In the wild, they compete with livestock and wildlife for forage; and if herds grow too large, they can ruin the environment.

To keep herds in check, there are two options: round them up or shoot the mares with a dart loaded with a contraceptive vaccine.

PZP fertility control is used on smaller wild horse herds, such as those in the Pryor Mountains. Cody News Co. file photo by Gib Mathers
Jay Kirkpatrick, Kimberly Frank and Robin Lyda of the Science and Conservation Center in Billings gave a presentation on vaccination and held a question and answer forum with about 30 members of the public at the Park County Library on Nov. 5.

The Science and Conservation Center produced the federally approved Porcine Zona Pellucida Contraceptive Vaccine, or more commonly known as PZP. They are a 501c3 nonprofit on a mission to apply non-lethal methods of wildlife population control.

“We have trouble separating symptoms of the problem from the problem,” said Kirkpatrick, the center’s reproductive physiologist who helped develop the vaccine. “Those are symptoms of a problem — the problem is reproduction. You can remove horses until the cows come home, but you won’t solve the problem, you are addressing symptoms.”

The Wild Horse and Burro Act requires population control, but laws and public opinion restrict what can be done to control herd populations, Kirkpatrick said. And this comes with a hefty price tag.

Each year, wild horse long-term and short-term holding facilities have a combined cost of $49 million, according to the BLM.

“Those are symptoms of a problem — the problem is reproduction,” Kirkpatrick says of horse overpopulation. “You can remove horses until the cows come home, but you won’t solve the problem, you are addressing symptoms.”

A big problem vaccination faces is lack of understanding, said Lyda, chief scientist for the center.

“If someone doesn’t understand something, they say it is not so,” Lyda said. “Then there is politics, influence from money being given to people gathering horses ... the majority of the problem is basic governmental, not wanting to support wildlife contraception.”

Wild horses are often rounded up and placed into long-term holding facilities because wild horse herds are required to be kept within a sustainable size so that the herd can remain healthy.

“Nothing is more genetically devastating than a round up and removal,” Kirkpatrick said.

Roundups aren’t perfect. Sometimes horses are injured or die in the process. They also are not a permanent solution, the wild horses will continue to breed.

But, when breeding is managed so that horse births and deaths were equal, then round ups are not needed.

The center makes about 150-200 doses of the vaccine per week and charges about 60 percent of what it costs to make, Lyda said.

The center also does quality control for the vaccine, trains personnel how to use it and keeps a database for all animals treated by the drug.


The way PZP works is no different from a standard vaccination that’s commonly used on humans, pets and livestock.

“But, it is safer and more efficient,” Kirkpatrick said. “It beats the Dickens out of flu vaccine.”

In this case, the vaccination prevents fertilization from occurring for anywhere from a year on up to 10 years, depending on how the mare reacts to the vaccine, Kirkpatrick said. The average timeframe is 4 years.

“Boosters are needed because some mares respond differently,” Kirkpatrick said. “Our rule of thumb is the first 3 years is to prime them, boost them and boost them and then you can start skipping years.”


Eventually, the vaccine wears off and the mare is able to be fertilized. This is the same as when a tetanus shot has worn off and another is needed.

The only change PZP causes is it extends the breeding season by two weeks, Kirkpatrick said. But, some mares will breed late no matter if they were or were not treated.

Sterilization is not good for herd health since only a few horses would breed and that does not promote a long-lasting and healthy herd.

The cost for removing wild horses varies by location and herd size. In the Pryors, a helicopter round up cost $2,165 per horse and water or bait trapping cost $1,400 per horse.

Meanwhile, it cost about $106 per horse for a contraceptive application with a vaccination dart.

Not only was the vaccination cheaper and hands-off, it also:

• has a 95 percent efficacy rate
• is reversible
• does not impact current pregnancy
• does not alter social behavior
• does not effect ovarian function

Application of the vaccine on the wild horse herd on Assateague Island from 1994-2004 reduced the herd size from 175 to 99 without using round ups. There was also a decrease in adult and foal mortality and improved body condition scores.

“Self-regulating is delusional,” Kirkpatrick said. “They self-regulate because so many die, there is nothing for them to eat. They destroy the range and then they starve to death.”

Wild horse herds are required to be preserved because they are regarded as a cultural and historical resource, Frank said

“Self-regulating is delusional,” Kirkpatrick said of wild horse populations. “They self-regulate because so many die, there is nothing for them to eat. They destroy the range and then they starve to death.”

On Assateague Island, legislation required the herd be kept at a sustainable size. But, the National Park Service did not want the horses to be touched, rounded up, or removed, Frank said.

So, the vaccine was applied with a dart gun.

“The public loves it because there is nothing horrible happening to the horses,” Frank said. “But, we are getting smarter horses, they are learning how far 20, 30, 40 yards is. It gets more difficult.”

The dart gun method would be harder to do with larger herds, but not impossible. Shooters would need to identify the horses and vaccinate 65 percent of the mares. One way of making this easier would be to bait a few horses into an area and do a few at a time, Kirkpatrick said. But, they would need held for a couple of weeks after for a booster shot and that gets complicated too.


From 2004-13, the average age of death for the Assateague wild horses increased from 17 years old to 26 years old. Prior to any management practices in 1994, the average age at death was less than 7 years old.

While round ups provide an immediate herd size reduction, vaccinations take a few years to impact herd size since it requires waiting for older horses to die.

In the McCullough Peaks, the herd growth rate was 14.88 percent per year before contraceptives were applied from 1971-2011. After contraceptives were applied, the growth rate dropped to 4.23 percent per year in 2012-14. Then in 2014, the herd did not increase in size.

When round ups began on the McCullough Peaks herd, the herd went from an annual growth rate of 15 percent in 18 months to about 50 percent. Part of the increased growth was because of increased foal survival, Kirkpatrick said. With contraceptives, foal survival rate was around 85 percent.

Of course, projects of this magnitude require the help of volunteers, known as non-governmental organization partnerships, such the partnership between the Cody BLM office and FOAL.

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