A study of the long-acting immunocontraceptive vaccine Gonacan-Equine in free-living wild horses has shown its effectiveness increased after a repeated dose, showing the ability to suppress breeding for four years.
The only detectable adverse side-effect was intramuscular swelling at the vaccination site, which occurred in 62% of the 55 horses used in the research, regardless of whether it was a repeat injection or not.
The researchers said their work had demonstrated that GonaCon could be an effective immunocontraceptive for free-ranging horses, particularly when the primary vaccination was followed by re-immunization four years later.
The vaccine, which temporarily interrupts ovulation and can be delivered by dart, was a safe and effective vaccine without significant side effects, they concluded.
It was safe for pregnant mares and newborns, they added, and did not result in any harmful behavioral side-effects during the foaling/breeding season.
However, long-term effective management of free-ranging horses using the vaccine would require repeat immunization in order to stabilize the growth rate of populations.
They said their findings indicated that the initial inoculation may provide only modest suppression of fertility, but re-immunization over time will result in greater reduction in population growth rates.
Future research will look to define the most effective revaccination schedule, the duration of that effectiveness, and the return to fertility following treatment.
Choices would ultimately have to be made on the age to treat horses, and the frequency of treatment needed to maintain the desired population, age structure, and genetic diversity, they said.
“Decisions on the most beneficial tactics will depend on overarching management goals and long-term objectives for the population,” Colorado State University researcher Dan Baker and his colleagues wrote in the open-access journal PLOS ONE.
The vaccine has been studied by researchers at the university for more than eight years, amid hopes it could help solve some of the conflicts seen over wild horse management.
As the researchers noted in their paper: “Controlling the abundance of wildlife species that are simultaneously protected, abundant, competitive for resources, and in conflict with some stakeholders but beloved by others, is a daunting challenge.”
Baker, who is co-principal investigator on the project and an affiliate faculty member in the Department of Biomedical Sciences, said: “This is not just an issue in the western United States, it’s a worldwide problem.
“And what we’ve learned is very encouraging and could provide a useful management tool for suppressing population growth rates in free-ranging horses and possibly other wild ungulate species.”
In fall of 2013, the research team gave booster shots of GonaCon to a group of mares in Theodore Roosevelt National Park, marking the first time revaccination with the immunoconceptive had ever been studied in free-roaming horses.
While the initial vaccine was only about 50 percent effective in suppressing fertility, administering the booster vaccine resulted in 85 to 100 percent sustained infertility for four years in treated mares.
Professor Terry Nett, co-principal investigator on the project, said there were two primary methods for managing wild horses on the range – surgical castration, and immunization with another contraceptive vaccine, porcine zona pellucida (PZP).
“Surgical castration requires catching animals individually, anesthetizing them, and having a veterinarian perform surgery, often under less-than-ideal conditions,” he says.
“This is an expensive, labor-intensive approach to reducing fertility and can be dangerous, particularly for mares.
“The PZP vaccine, which targets the covering of the egg and prevents sperm from fertilizing it, is quite effective with around a 90 percent reduction rate in fertility and it can be delivered by dart, but it needs to be administered each year to maintain effectiveness.
“And, since PZP does not suppress sexual behavior, immunized mares continue to display sexual receptivity for an entire breeding season, which has a major impact on herd dynamics.
“Our approach, GonaCon, is a vaccine that targets the brain hormone gonadotropin-releasing hormone (GnRH), which is responsible for stimulating reproduction in both females and males.
“It can be administered by dart and prevents ovulation and displays of sexual receptivity from occurring so that infertility is achieved and normal herd dynamics are maintained.
“We found that an initial vaccination against GnRH with GonaCon leads to suppression of fertility in only about half of the mares, but near complete infertility is achieved for up to four years following the administration of a booster immunization. This vaccine has the potential to lead to sterility in stallions as well.”
Professor Jason Bruemmer, a collaborator on the project, said work to date showed that GonaCon was an effective tool for managing wild horse populations.
“Because it is a product designed and produced to target an action by a very specific hormone (GnRH), there are no side effects.
“We have demonstrated that the booster vaccine leads to almost complete infertility for approximately four years.
“This allows those managing horse numbers on public lands to decide if or when to reimmunize in order to efficiently control the population by maintaining the herds and slowing or stopping growth.
“As Terry Nett mentioned, an important feature of this project is that the behavior of the herd as a result of the GonaCon vaccine remains unchanged and the healthy dynamics remain intact, unlike PZP-treated herds.”
Baker says additional research is needed to complete the study’s objectives.
“We now need to define the duration of effectiveness and determine if long-term or permanent infertility is a possible outcome. We also need to investigate the optimum revaccination schedule to maintain infertility and the safety of repeat vaccinations.
“Ultimately, there is an urgent need to extend the results of our research with the individual horses at Theodore Roosevelt National Park to test of GonaCon’s effectiveness in suppressing growth rates of other free-ranging populations.”
Baker DL, Powers JG, Ransom JI, McCann BE, Oehler MW, Bruemmer JE, et al. (2018) Reimmunization increases contraceptive effectiveness of gonadotropin-releasing hormone vaccine (GonaCon-Equine) in free-ranging horses (Equus caballus): Limitations and side effects. PLoS ONE 13(7): e0201570. https://doi.org/10.1371/journal.pone.0201570