Experts call it the holy grail of pet contraception: a single shot or pill that would permanently sterilize cats and dogs without the need for expensive and time-consuming spay/neuter surgery. Scientists have struggled to develop such a product for more than 20 years, but every effort has failed—until now.

In a study published today in Nature Communications, researchers report a gene therapy approach that appears to prevent conception in female cats with no apparent side effects. A single shot keeps felines kitten-free for almost 2 years, and possibly much longer.

“This research is a huge leap—we’re really excited about it,” says Joyce Briggs, president of the Alliance for Contraception in Cats & Dogs, a nonprofit that has been advocating for a nonsurgical contraceptive since 2000. Briggs says if the approach can be scaled up—and works in dogs as well—it could make a “huge impact” in the cat and dog overpopulation crisis worldwide.

More than half of the approximately 1.5 billion dogs and cats on Earth are homeless. Many die on the streets, victims of cars or disease; others are killed in mass culling campaigns that seek to protect wildlife or prevent the spread of rabies. Nearly 1 million are euthanized each year in overcrowded shelters in the United States alone. Spay/neuter surgeries can help, but the approach isn’t feasible for large-scale sterilization campaigns.

In 2009, to accelerate efforts to find nonsurgical alternatives, a nonprofit called the Michelson Found Animals Foundation announced $50 million in funding and a $25 million prize. The program has since given out 41 grants, supporting everything from toxicants that target reproductive cells to RNA-based drugs that attempt to silence the genetic machinery that leads to conception.

“Very few of them have led to anything,” says Thomas Conlon, the foundation’s chief scientific officer. They either didn’t work or they weren’t safe.

That changed when an unusual pair of scientists came together. David Pépin, a reproductive biologist at Massachusetts General Hospital, spent his early career researching antimüllerian hormone (AMH), which is produced by follicles in the ovary that give rise to eggs. When, in one experiment, he amped up the expression of the hormone in female mice, their ovaries stopped forming follicles, sterilizing the animals.

“When I told people about it, they said, ‘So what? We already have [human] contraceptives,’” Pépin says. Then he saw the Michelson grants. “I said, ‘I already have what they’re looking for.’”

Meanwhile, William Swanson was trying to get cats to reproduce. A conservation biologist at the Cincinnati Zoo, he was looking for ways to improve conception in ocelots and other wild cats. His expertise on cat reproduction got him invited to review grants for the Michelson foundation, including Pépin’s proposal. “It looked very promising,” Swanson says. The two decided to join forces.

In the new study, Pépin, Swanson, and colleagues inserted the cat version of the AMH gene into a harmless virus widely used in gene therapy to ferry replacement genes into cells. The team then injected the virus into the thigh muscle of six young female domestic cats living in a colony at the Cincinnati Zoo.

The strategy worked. Other than having low levels of progesterone—a hormone produced after ovulation—the cats’ sex hormones remained normal. But the treated cats did not ovulate. And when they were placed in a room with a male for several hours a day over a 4-month period—an experiment repeated both 8 months and 20 months after the gene therapy—none became pregnant. Four of the females refused to mate; the other two mated but could not conceive. Meanwhile, three control females given a viral injection that didn’t contain the AMH gene became pregnant and gave birth to kittens.

“It’s heads and tails more effective than anything we’ve seen so far,” Conlon says.

“It’s fantastic work and a great advance,” agrees William Ja, a neuroscientist at the Herbert Wertheim UF Scripps Institute for Biomedical Innovation & Technology. Ja received early Michelson funding to explore using a toxin to destroy cells that give rise to sperm and eggs, but the project stalled when the grant ran out. He says the AMH approach is “quite promising,” but that additional studies will be required to determine whether it’s safe and effective in the long-term.

One open question is how long the extra AMH will stick around. The introduced gene doesn’t become part of the cat’s muscle cell DNA, so it’s possible that it will disappear over time as muscle cells regenerate. Pépin notes that AMH levels did decrease over the course of the study, but that they remained elevated in all of the treated cats, including one followed for 5 years.

Another question is how exactly the treatment prevents conception, says Rebecca Robker, a reproductive biologist at the University of Adelaide. Pépin and Swanson suspect the hormone stops ovarian follicles in the cats from developing normally, but they admit that the exact mechanism is still unclear. “It’s really exciting,” Robker says, but until the team pins down the details, the research remains “really preliminary.”

Julie Levy, director of the Shelter Medicine Program at the University of Florida, says the approach could be a boon for combating feral cat populations. But she questions whether cat and dog owners would choose gene therapy over spay/neuter, which may have the added benefit of protecting against mammary cancer and other diseases; it also stops the yowling and other unwanted behaviors associated with estrous. “It’s not fun to live in a house with a cat in heat,” she says.

Pépin and Swanson are continuing to monitor the treated cats, closely following their health and behaviors. But the Michelson foundation likes what it sees so far. The organization, which plans to help commercialize any viable product, will be meeting with the U.S. Food and Drug Administration next month to map out larger safety and efficacy studies.

Swanson says it will be at least 5 years before a commercial product is available. To be a viable solution for developing countries, a dose would have to be cheap.

For the product to truly be a holy grail—and to qualify for the $25 million Michelson prize—it would need to work in dogs as well. (It would also need to work in males, which seems beyond the reach of the current approach.)

But the money isn’t Swanson’s motivation. His main goal, he says, is to keep stray pets off the streets, and as many cats and dogs in loving homes as possible. “That’s the real prize.”