Animal birth control could soon be just a shot away: A new injection makes male and female mice infertile by tricking their muscles into producing hormone-blocking antibodies. If the approach works in dogs and cats, researchers say, it could be used to neuter and spay pets and to control reproduction in feral animal populations. A similar approach could one day spur the development of long-term birth control options for humans.

“This looks incredibly promising,” says William Swanson, director of animal research at the Cincinnati Zoo and Botanical Garden in Ohio. “We’re all very excited about this approach; that it’s going to be the one that really works.”

For decades, the go-to methods for controlling animal reproduction have been spay or neuter surgeries. But the surgeries, which require animals to be anesthetized, can be expensive—one reason so many dogs and cats remain unfixed and feral animal populations continue to grow. Nearly 2.7 million dogs and cats were euthanized in U.S. shelters last year. A cheaper, faster method of sterilization is considered a holy grail for animal population control.

To get there, researchers have already created vaccines that trigger an immune response in animals. This response produces antibodies that block gonadotropin-releasing hormone (GnRH), required by all mammals to turn on the pathways that spur egg or sperm development. The vaccines in this class—including deer contraceptive GonaCon—have been shown to effectively work as both male and female birth control in animals. But, like many human immunizations, the vaccines rely on an immune response that eventually dwindles away, forcing the use of booster shots every few years.

Biologist Bruce Hay of the California Institute of Technology in Pasadena and colleagues took a different approach to blocking GnRH. Rather than rely on animals’ immune systems to create antibodies, he and his colleagues engineered a piece of DNA that—when packaged inside inactive virus shells and injected into mice—turned their muscle cells into anti-GnRH antibody factories. Because muscle cells are some of the longest lasting in the body, they continue to churn out the antibodies for 10 or more years. Both male and female mice with high enough levels of the antibodies were rendered completely infertile when Hay’s team allowed them to mate 2 months later, the team reports online today in Current Biology.

“That 2-month delay is because of how long it takes the muscle to start producing enough antibody,” Hay explains. “Going forward, one goal is certainly to try other systems that wouldn’t have that time lag.”

Hay’s team also showed that the same approach could be used to make female mice generate antibodies to the zona pellucida, a layer of proteins that surrounds egg cells. In these cases, animals continue to produce eggs but sperm can’t fertilize them. The advantage to blocking these proteins, Hay says, is that the treatment doesn’t affect hormone levels, which are critical in regulating all types of behavior. “There might be some instances in wild animals where you just want to inhibit fertility but not disrupt an animal’s behavior,” he says. And if the approach was used to design long-term contraception for humans, researchers would similarly not want to drastically affect hormone levels.

For now, the question is how effective the drug will be in animals other than mice. “The challenge is always moving between species,” says Swanson, who is already planning to test out Hay’s approach in cats. If it works, he says, it could change the way communities deal with feral cat populations. “We have to figure out how to control these populations without being harmful to individual cats,” he says. “And this kind of lifelong contraception might be a safe, effective way to do that.”

Joyce Briggs, president of the Alliance for Contraception in Cats and Dogs, an organization aimed at speeding advances in nonsurgical contraception in dogs and cats, says that “a long-term contraceptive with a duration of 10 years could be a true game changer for dog and cat welfare, preventing unwanted litters for the many animals who will never have access to sterilization surgery, and [providing] a much less invasive and affordable option for pet owners."

Hay’s team is now testing their new approach to block other hormones and proteins in mice. “There are a lot of other molecules we can try to target now that we know this works.”

*Update: 5 October, 3:10 p.m.: The story has been updated with a comment from Joyce Briggs.