Sterilizing Cats, No Surgery Required
A single shot might someday replace spaying as a tool for cat-population control.
Last year, the team at Operation Catnip spent close to $1 million spaying and neutering more than 7,000 of the cats that prowl the streets and parks of Alachua County, Florida. It wasn’t even close to enough. Roughly 40,000 feral felines live in the community; to keep their numbers in check, curb disease transmission, and protect the county’s birds, the team probably would have needed to operate on approximately 10,000 more. “It’s an almost insurmountable number to reach by surgery,” Julie Levy, Operation Catnip’s founder and a shelter-medicine expert at the University of Florida, told me.
But surgery remains the only available option for permanent cat contraception, locking vets, techs, and volunteers into the days-long rigmarole of trapping, transporting, operating on, monitoring, and releasing the animals, one by one by one. Plus, the process can be grueling and risky for the cats, who must go under anesthesia and the knife. Having a quick and simple alternative “would be amazing, huge, transformative,” Levy told me. “We’re just craving the day we have something else.”
That day might be on its way soon. After years of tinkering, an American team of researchers has come up with a genetic treatment that, with one injection, can safely and sustainably halt ovulation in cats—a breakthrough that could replace onerous surgeries with the simple act of “going out into the community and giving an animal an injection, and then just letting them go,” says Valerie Benka, the director of programs at the Alliance for Contraception in Cats & Dogs. The therapy has only been examined in a series of small studies, and hasn’t yet entered clinical trials. And the scientists behind it can’t yet say how long, or how well, their invention might work. But if their early results pan out, this one-and-done injection could finally offer female cats safe, lifelong birth control without a single scalpel cut.
Alternatives to surgical sterilization for dogs and cats have been in the works since at least the 1970s, and for the most part, they’ve delivered mixed or disappointing results. Hormones that can be swallowed, injected, or implanted tend to be fussy or come with nasty side effects; vaccines that block ovulation and pregnancy in some livestock and wildlife have underperformed when adapted to pets. No single option has yet hit the perfect intersection of safe, effective, easy, and permanent—the necessary combination to truly displace surgery,
Meanwhile, the need for a decent alternative to surgical sterilization has only grown. An estimated 500 million free-roaming cats stalk the Earth, far overwhelming the world’s spotty supply of spay-and-neuter services. In some countries, wildlife managers have turned to culling; in many parts of the U.S., veterinary shortages, exacerbated by the pandemic, have left shelters “in crisis,” Levy told me, overcrowded to the point where they’re battling massive disease outbreaks and reluctantly raising euthanasia rates. Some adoption organizations, including PetSmart Charities, are now allowing some puppies and kittens to go to new homes without first being spayed or neutered—a change that is meant to move animals more quickly out of shelters, Levy told me, but also risks them not getting their surgeries before they reproduce.
The new injectable, developed by a team led by William Swanson and David Pépin, won’t arrive in time to address the current shortage. But of all the experimental methods in development for cats, “this is the first one that has gotten this far,” says Cheryl Asa, the director of research at the St. Louis Zoo. Unlike other contraceptives that rely on lab-made hormones, the new treatment tasks the cat’s body itself with manufacturing the birth control. Each shot delivers DNA to cat muscle cells, instructing them to pump out the feline version of anti-Müllerian hormone—a reproductive signal that mammals naturally produce—at levels high enough that they seem to block the ovaries from maturing and releasing eggs, Pépin, a reproductive biologist at Massachusetts General Hospital, told me. Because the contraceptive is the body’s own product, supply doesn’t run low.
So far, the researchers have released findings from two very small studies of the injection, which included just 12 lab-raised female cats, nine of which received the treatment. Their early results seem to suggest that the shot keeps cats from getting pregnant for at least a couple of years, according to Pépin, without any serious side effects. Three of the cats are now six years out from their injections, and still seem happy and healthy. Swanson, a wildlife veterinarian at the Cincinnati Zoo, would know: He adopted them. “I see them every day, and they’re doing fine,” he told me over the phone, just feet away from where the trio were sunbathing by a window.
Still, only a much-larger clinical trial will be able to prove that the team’s injection is safe and effective long-term. “The next question is, how long does it last?” says Pei-Chih Lee, a cat-reproduction biologist at the Smithsonian. Anti-Müllerian hormone is pretty understudied; it’s still unclear, for instance, exactly how it might alter cats’ other hormones, or even how long it takes for the contraceptive effect to kick in. For now, the team’s results have to be treated as “really preliminary,” says Daniela Chavez, a cat-reproduction biologist at Towson University.
Even if this particular injection does succeed, experts told me, it’s unlikely to render spaying totally obsolete, especially among pet cats. Unlike several other options being explored, the new shot seems to work only in females; it also might not keep cats from going into heat and filling alleyways with their yowls and sprays, or from developing the reproductive-health complications that surgical sterilization helps prevent. The main benefit of the treatment is about giving vets “more options,” Benka told me. Several other groups remain hard at work on other technologies that could further flesh out the pet-contraception toolkit—some of them trying the gene-therapy route, as Swanson and Pépin did, others attempting hormonal injections that can keep dogs and cats from reaching reproductive maturity.
All of that leaves Levy hopeful that shelter medicine will eventually put mass surgical sterilization in its rearview. Maybe, she told me, she’ll live to see the day when teams around the world can deliver contraception to cats with tech simple enough to carry around in a backpack; maybe, before she retires, she’ll describe surgical sterilization to the next generation of vets primarily in the past tense. “I want to be able to tell my students, ‘We used to cut animals open for fertility control!’” she told me. “And it will shock them, because there’s a better alternative”—one safer, easier, and more palatable for cat and human alike.