Volunteers quietly track throngs of wild horses through the rugged mountain passes of western Alberta. Armed with dart guns, they search for the mares among the herd before silently taking aim.

For the past 4½ years, teams of trackers have canvassed a 490-square-kilometre area in the foothills near Sundre, Alta., injecting horses with a contraceptive vaccine called ZonaStat-H.

The drug has been touted as a way to control the growing wild horse population, without capturing or destroying the animals. Early results of the research project show promise, but its future is uncertain.

"The vaccine was effective," said Bob Henderson, president of the Wild Horses of Alberta Society.

"The idea that it could be used as population control is scientifically sound but there are other difficulties we would have to overcome to instigate it throughout the province."

No one is willing to take responsibility when it comes to the government. - Bob Henderson

The project is part of a five-year agreement signed in November 2014 between the Alberta government and the Wild Horses of Alberta Society.

The society hopes to continue its research on population control but there is no ongoing commitment from the government.

"At the end of September, our MOU [memorandum of understanding] comes to an end. We haven't been offered a chance to extend it," Henderson said.

"You come up with good ideas and then it's left in limbo and it's really frustrating because there are better ways to manage our horses than with culls."

In the past, the Alberta government has said the horses don't deserve special protections because they are feral, not wild — descended from escaped or released domestic horses used for logging and mining operations approximately a century ago.

As of February 2019, there were 1,679 feral horses counted in the Alberta foothills of the Rocky Mountains, according to Alberta Environment and Parks.

Alberta's history of managing the wild horse population by trapping and culling dates back to the 1950s and remains the only strategy still being used. The horses are sold at auction and, with little interest from private owners, are often sent to slaughter.

The last government cull took place in 2015 — the same year that the Wild Horses of Alberta Society began doing its contraception research project.

ZonaStat-H, which acts like a vaccine, can prevent pregnancy for up to three years before the effects begin to wear off. (Wild Horses of Alberta Society)

When the agreement was signed, the province had said the strategy could be adopted in other areas of Alberta where wild horses roam.

But Henderson said government officials are no longer interested in testing the drugs viability beyond the borders of the Sundre Equine zone.

"We just finished a meeting in July with the government to go over where we are and what we've done and there is a general consensus," Henderson said.

"No one is willing to take responsibility when it comes to the government."

The drug won't harm a fetus if a mare happens to be pregnant when vaccinated. (Wild Horses of Alberta Society)

ZonaStat-H, also known as PZP, has been touted as a humane alternative to culling.The drug, which acts as a vaccine, can prevent fertilization with up to 90 per cent effectiveness.

It produces antibodies around the mare's eggs, blocking sperm from fertilizing them and preventing pregnancy.

The effects of the contraceptive are temporary, wearing off in time, and the drug won't harm a fetus if a horse is pregnant when it is vaccinated. Only animals that have been given doses for five or more consecutive years would be at risk of sterility.

It has been used extensively in Mexico and the United States to control populations of wild donkeys and horses.

But it has rarely been tested in Canada, where the drug is not licenced for use and is classified as a restricted-use pesticide. Special authorization is required to import it.

Rugged country

When properly administered, the drug is effective for up to three years, Henderson said. Full efficacy requires that mares be injected with a booster shot following the initial inoculation, he added.

Between 2015 and 2017, study volunteers darted approximately 90 horses. Of those horses, 23 also received the booster shot.

The volunteers travelled in groups of three, armed with small dart guns filled with ZonaStat-H. They were careful to leave the herds undisturbed during the springtime foaling season.

Standing about 25 to 50 metres away, they would shoot the drug into the animal's hips or hindquarter.

Each horse is photographed and details of the innoculation are entered into a database. The GPS location of each interaction with the horses is carefully cataloged so they can be tracked through the dense wilderness.

You have to put on a lot of miles in order to find those horses​​​​​​. - Bob Henderson

The study, funded entirely by the non-profit society, encountered many challenges, Henderson said. The rough, often impassable terrain made the horses especially difficult to track.

"The horses have a huge range so you can identify them in one location one day and then they'll be miles away, roaming as horses do, and you can't just locate them because of the country or lack of access to it," he said.

"It's a beautiful country but it is rugged and because most of the logging roads have all been reclaimed, access was generally through an ATV, on foot or on horseback."

In order for the study to work on a larger scale, the strategy would need more manpower, Henderson said.

Researchers believe it would take five years of active treatment to stabilize the foothills population.

"You have to put on a lot of miles in order to find those horses and get close enough to apply the vaccine to them," Henderson said.

"We're all volunteers and we only have so much time. We would get out as often as we could. The number of hours is in the thousands.

"A bigger organization than us could have people out in the field and apply this vaccine more extensively."

The province considers the horses to be feral animals, not wild ones, that can be destructive to the ecosystem. (Wild Horses of Alberta Society)

In a statement emailed to CBC News, Alberta Environment and Parks said it will review the viability of the project when the society submits its final report to the ministry this fall.

"Eighty-seven feral horses were darted over five years through the pilot. The contraceptive is effective for one breeding season," reads the statement.

"Alberta Environment and Parks appreciates the efforts of WHOAS's volunteers over the past five years."

Henderson said the society is working with academic researchers to carry on with their project.

Without an alternative strategy in place, Henderson fears the province will once again begin capturing the animals.

The horses aren't pests, he said, but a valuable part of the Alberta ecosystem.

"No one can tell you what sort of impact the horses might be having on the environment out there because the ecosystems are changing so drastically from logging and oil and gas," Henderson said.

"It all boils down to this. The horses have been part of our culture forever … If we lose these symbols of our freedom and the spirit of who we are, we'll be emptier as a people."