Free-roaming horses are a quintessential image of the rugged and unbroken American west, but they also serve as an illustration of the complicated challenges western states are facing as population growth and climate change increasingly stress the environment.
First introduced by Spanish explorers, horses have roamed what is now the western United States for more than 400 years. In recent decades, however, managing those herds has become a major dilemma.
Dr. John Turner, a professor in the College of Medicine and Life Sciences, has studied wild horses for more than four decades and is one of the nation’s leading experts in controlling their populations through contraception.
“Human population has increased to the point where it’s compressing the available space for wildlife,” said Dr. John Turner, a professor in The University of Toledo College of Medicine and Life Sciences. “Once you have wildlife limited to a given space, you’ve got to figure out a way to regulate the population. In the last 20 years, the western rangeland situation has gotten much, much worse because of the drought. The habitat is not able to come back, so the need is greater than ever.”
Turner has been studying wild horses for more than four decades and is one of the nation’s leading experts in controlling their populations through contraception.
In the mid-1980s, Turner and research partner Dr. Jay Kirkpatrick of Eastern Montana College reported the first field studies in wildlife contraception. They subsequently played an instrumental role in showing that an immunocontraceptive vaccine called porcine zona pellucida, or PZP, was effective at preventing pregnancy in several species, including deer, elephants and free-roaming wild horses. The two investigators worked closely together until Kirkpatrick passed away in 2015.
Turner more recently headed a team that showed it was possible to create an extended-release version of the PZP vaccine that would reduce the number of shots required and lengthen the period of infertility.
The PZP vaccine, often delivered via a dart, is an effective contraceptive method used by the U.S. Bureau of Land Management, which is tasked with managing the wild horse population on millions of acres of public land.
Now, with horse populations on the rise and concerns growing about the increasingly limited grazing land that horses often share with commercial livestock, Turner is turning his sights on further improving fertility control measures.
“The best solution is something that can be delivered once and not have to be delivered again for multiple years. The longer a vaccine lasts the better,” he said. “Not only do we want a longer-lasting vaccine, but we also want to increase the ease of preparation. As we’ve been marching into the field to address those things, one of the things we’ve wanted to do for quite a while was to improve the adjuvant.”
Adjuvants are a key component in vaccines that help amplify the body’s immune response following inoculation, making a vaccine more effective.
Last month Turner and his collaborators published a study in the journal Veterinary Immunology and Immunopathology that evaluated a pair of new adjuvants against the 1940s-era Freund’s adjuvants currently used in PZP vaccines.
“Freund’s is a classic adjuvant, but there have been a huge number of new adjuvants developed over the past 20 years,” Turner said. “We believe we can significantly improve the safety, shelf life, effectiveness and durability of the vaccine by reformulating it with a different, current-generation adjuvant.”
PZP vaccines in use now are prepared just before being administered, which creates logistical challenges. The adjuvant also can occasionally cause injection-site reactions.
The results of the recently published study were promising, with one new adjuvant allowing vaccine preparation days before injection and yielding a strong immune response without injection-site effects.
The timing of Turner’s research appears to be particularly good, as the federal government is beginning to put more emphasis on contraceptive control measures.
A 1971 law meant to preserve then-dwindling horse population prohibits the federal government from killing wild horses and burros. However, as populations increased, the federal government has struggled to keep up.
BLM currently says there are more than 80,000 horses on federal land alone — about three times what it considers the appropriate level — with herds able to double in size every four years.
Despite the availability of contraceptive vaccines, the primary method of population control has been rounding up and removing horses. In the 2022 fiscal year alone, more than 20,000 animals were removed.
However, with Congress recently dictating that BLM must use a portion of its funding to bolster contraception programs, there may be a shift coming. Last year, BLM used fertility control measures on more than 1,600 horses, by far the most in the last decade
“I think that’s a major step forward,” Turner said. “It’s going to be a slow process, but it’s still a major shift from the previous behavior and it’s one of the reasons why we’re really interested in getting this off the ground.”
Turner plans to continue studying other adjuvants while also looking to maximize the controlled release capacity to develop not only a more effective vaccine but one that will extend the duration needed between booster doses.
“Simply rounding up horses is not sustainable,” Turner said. “The population is growing too fast and with climate change and drought, the habitat is not able to come back. We’ve got to figure out a way to regulate the population and longer-lasting fertility control is going to be a big part of that. This is just the front end of that development, but we think it’s very promising.”
