The “Pill” for Strays: Nonsurgical Sterilzation

New approaches to overpopulation
By Twig Mowatt, June 2011, Updated June 2021
dog Nonsurgical Sterilzation pill

When the animal welfare group, All Sato Rescue, Inc., held a low-cost spay/ neuter clinic in Pinones, P.R., it took 10 weeks of planning and fundraising, a dozen volunteers, $1,750 in donations, and about 60 secondhand towels and sheets. By the end of the day, a total of 38 animals had been sterilized: 28 female dogs, four male dogs and six female cats. That total represented more than eight hours in surgery on the part of Dr. Gwen Davis, one of only a few local vets trained in high-volume procedures, who drove three hours each way to participate.

Ensuring that 38 animals will not be contributing to the cycle of overpopulation, abandonment and abuse on this island, whose unfortunate record on animal welfare is well documented, is a huge achievement by any measure. Most animal advocates are painfully aware of the procreative powers of female companion animals: over their lifetimes, one unspayed dog and her offspring can produce more than 6,000 pups, and one unspayed cat and her offspring can produce more than 400,000 kittens, according to SPCA International.

Which means that this single event in Pinones is likely to have saved hundreds of thousands of animals from lives of misery on local streets and beaches. But the herculean effort required to achieve these 38 procedures doesn’t bode well for Puerto Rico’s estimated 150,000-plus stray dogs. Apply this same scenario to every country on the planet that has an animal overpopulation problem — which is most of them — and the task of humanely addressing this problem feels utterly overwhelming.

Non-Surgical Dog Sterilization

Zeuterin was commercially launched in the U.S. in 2014. The product, which was manufactured by Ark Sciences, is not currently being produced or distributed in any country. While there is presently no indication of returning to the market, the formulation retains approval by the U.S. Food & Drug Administration (FDA). 

What if there were a simple way to permanently sterilize every dog and cat — male and female, infant and adult — without the need for a highly skilled vet, anesthesia, and expensive medications and equipment? Without a recovery time and with a limited risk of infection and complications?


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That is, in fact, the holy grail being pursued by a smattering of researchers, scientists, veterinarians and animal welfare professionals. These highly motivated individuals are trying to come up with a new product or products that will provide permanent or even significantly long-term contraception, delivered as a single treatment that doesn’t require a veterinarian to administer.

“The idea of birth control for straydog populations is almost magical,” said Laura Simpson, founder of Harmony Fund, an international animal rescue charity that supports sterilization and rescue efforts around the world. “It’s a pulse-racing possibility to fundamentally transform the fortune of hundreds of millions of dogs.”

Making magic a reality is the goal of the Alliance for Contraception in Cats & Dogs (ACC&D), a small, Portland, Ore.–based nonprofit. Its president, Joyce Briggs, describes her organization’s role as a catalyst in introducing nonsurgical sterilization methods and supporting their distribution and promotion. To maintain objectivity, the group does not conduct the actual research. Rather, it keeps a finger on the pulse of who’s developing what and for what, how far along it is in development and how promising it is.

“The goal is not to eliminate cats and dogs, of course, but to prevent the births of unwanted cats and dogs — of which there are millions upon millions around the world,” says Briggs. “The reality is that many communities lack the resources to provide appropriately for the number of cats and dogs being born in them today. Our ideal would be to achieve a balance between numbers of animals and resources.”

The group has received a thumbsup from almost every major animal welfare group in the nation, if not the world. Its website is a virtual “Who’s Who” in support letters from everyone from the ASPCA and HSUS to WSPA. Likewise, its board of directors and scientific advisory board include leading experts in animal welfare, animal health, veterinary medicine, international dog and cat population control, regulatory processes and public health. And then there’s its partnership with billionaire businessman turned animal advocate, Dr. Gary Michelson, whose Found Animals Foundation has pledged a $25 million award to any scientist or company who can crack the code, and $50 million in grants to advance promising work.

“This gift has made a dramatic difference in terms of the number of laboratories involved and the range of approaches being undertaken,” says Stephen L. Zawistowski, PhD, CAAB, science advisor for the ASPCA and member of ACC&D’s board of directors. He notes that the focus had been squarely on a hormonal approach, something akin to the pill for women, but now, multiple biological mechanisms are being considered. “I think the most exciting thing is that we now have people concentrating on [nonsurgical sterilization], whereas over the past 20 years, people worked on it, but on the back bench. Now it’s on the front bench.”

Briggs is bent on keeping it there. Part of ACC&D’s mission is to reach beyond traditional animal-health circles, appealing to researchers in diverse disciplines who have fresh eyes and approaches to a problem they didn’t even know existed. Two recent ACC&D-sponsored think tanks, for instance, brought together a diverse group of top academics, senior scientists at biotech companies, cancer researchers and specialists in infectious diseases. “What we found in the think tanks is that people were fascinated by the intellectual curiosity of the whole thing,” says Briggs.

Challenging Biology

To brilliant minds seeking new challenges, this one’s a doozy. It means taking on tens of thousands of years of biological and evolutionary forces focused on a single objective: continuing the species. So effective and multitiered is the reproductive system that the moment one element is knocked out, another moves in to compensate.

“We’re trying to overcome the most potent force of nature — the essence of life,” says Julie Levy, DVM, director of Maddie’s Shelter Medicine Program at the University of Florida’s College of Veterinary Medicine. “Reproductive success drives evolution, so reproductive forces are the strongest biological factors [in] any species. That’s why no one has figured this out yet — it’s hard.”

Levy, who has been researching contraception in cats for the last eight years, now oversees clinical trials of promising products, including a vaccine formulation that was originally developed by the U.S. Department of Agriculture to humanely manage wildlife populations, such as wolves and deer. GonaCon™ worked in about 75 percent of cats for two-and-a-half years, but fell to 27 percent at five years. In dogs, however, it tended to cause infection at the injection site. Levy is trying to make it both more effective and safer.

Another product being used in the field is an injectable sterilant for male dogs called EsterilSol™, which has U.S. Food and Drug Administration approval (under the name Neutersol®) for dogs age three to 10 months. ACC&D has helped fund projects using EsterilSol in dogs from three months to adult in Guatemala, the Galapagos, the Dominican Republic and the Samoan Islands. Data reported back by grantees help increase knowledge on best practices in EsterilSol use. The price for EsterilSol can be as little as four dollars per dog (or even lower for small dogs), and only light sedation is needed to inject the drug. As many as 200 male dogs can be treated in just several hours. Moreover, maintaining an intact male dog (with its male parts and behaviors!) helps overcome cultural objections to the perceived emasculating effects of castration.

Vet Gwen Davis, who is also founder of the Puerto Rico Animal Welfare Society, used Neutersol on more than 300 stray male dogs. She found that about one in every 100 reacted with a bad inflammation at the injection site — a figure that she says might be acceptable in strays, but not in owned animals.

Then there’s the fertility-control product, ContraPest™, which was developed by the biotech company SenesTech to humanely control rodent infestations in rice-producing countries in Southeast Asia. As ChemSpay™, this product is being expanded for use in other species; ultimately, it will be used to address overpopulation of dogs and cats. The active ingredient, which is based on an industrial chemical, has been shown to cause permanent sterility in female rodents by depleting egg follicles in the ovaries without adverse effects to either the animal or the environment.

A dose of ChemSpay in the form of dog biscuits doled out over 15 days was given to 18 dogs by its co-developer and SenesTech’s chief scientific officer, Loretta P. Mayer, PhD. The dogs, who had been rescued from the Navajo reservation and subsequently adopted, were then spayed by traditional surgical procedures to analyze their ovarian egg-count levels. Each dog showed 73 percent depletion, which results in sterility in other species. However, Mayer is quick to point out that this sample does not provide conclusive proof. To get that, the dogs would have to have been artificially inseminated and then tested for subsequent pregnancies. In mice and rats, the dosing number has been reduced to three days, which is promising for other species as well.

“This isn’t a pill sitting on my desk: this is a technology based on scientific fact that we believe can be developed,” says Mayer. “We believe that anything that decreases eggs in the ovary of one mammal can also deplete them in another mammal, but that is an academic argument that hasn’t been tested yet.”

Mayer would dearly love to do that testing. In particular, she wants to see the technology further developed to be effective as a single exposure — injection, implant or biscuit. But finding the funding necessary to continue development and then get the product through the many U.S. regulatory hurdles is challenging. SenesTech has allied itself with the nonprofit rescue group 600 Million Stray Dogs Need You to bring attention to the issue and appeal to potential donors.

Wide-Angle Impact

In U.S. markets, an FDA-approved nonsurgical sterilant would likely have the most impact in shelter medicine, where it could address the still-shameful statistic of four million dogs and cats euthanized every year.

“We know from surveys that the reason some people in this country still don’t sterilize their pets is primarily cost, but also convenience, such as transportation to and from the clinic,” says Zawistowski. “We also know that even people who intend to get their pets sterilized can end up with what we call the ‘oops’ litter.” That’s “oops” as in, “Who knew a four-month-old kitten could get pregnant?” In the United States, this accounts for about threequarters of kitten births and about half the puppies.

Zawistowski envisions a return to the days when families slogged over to the local armory to stand in line for their polio-vaccine sugar cubes. Or perhaps teams of vet techs could fan out into rural areas, going door to door, offering health exams and services to companion animals. A permanent sterilant would be just one of many tools in their portable health kit. Of course, it’s in the international arena where the impact of a nonsurgical sterilant is the most critical. It has the potential to do for animal welfare what Facebook did for social interaction. That’s an idea that makes a rescuer’spulse start racing. For years, major cities around the world have resorted to mass poisonings, shootings and worse in their efforts to reduce the number of dogs on the streets. Not only are these methods barbaric, they are also ineffective.

“The field is so exciting because if an affordable, easy-to-administer product is developed, it will reach all corners of the earth and improve animal welfare in a way that we have never seen before — and really never thought would be possible,” says Levy.

If, rather than a single product that meets every need, there were several products with specific complementary targets, the world’s strays would still be better off by several orders of magnitude. An injectable sterilant for male puppies, for instance, and an implant that keeps female cats sterile for three years would be huge steps forward.

“It’s like you’re working toward a car that will get 100 miles per gallon and in the meantime, someone comes up with one that gets 60. That’s pretty great,” says Zawistowski. “I’d still get pretty excited about that.” Briggs and others think it will be about 10 years before an effective, all-purpose product is approved for use in the United States, with much of that time consumed by regulatory issues.

For many veteran animal rescuers, the idea of handing out dog-biscuit sterilants instead of appealing for donations, volunteers, operating space, a skilled vet, anesthesia and towels would be a lifetime dream fulfilled.

“After 25 years in animal welfare in Puerto Rico, I have probably spent thousands of hours and much of my personal income on getting as many dogs and cats sterilized as possible — because it’s the only way we can begin to address the horrible problems we have here,” says Edilia Vazquez, president of All Sato Rescue. “If we could stop that and put all our time and money into other programs, like education and enforcement of our animal cruelty laws, we would have a real chance.”

Twig Mowatt covered the drug war in Colombia for the New York Times and the Associated Press and now writes about animal issues. She works closely with dog rescue organizations in Puerto Rico and with GREY2K USA.