• Posted June 6, 2023

Injected Birth Control Could Be Game-Changer to Curb Stray Cat Populations

Millions of stray cats roam the world over, and surgical sterilization has long been the primary method of population control.

But a small new study shows promising results for a one-and-done contraceptive injection.

Researchers say this first-of-its-kind approach appears safe and effective.

"A non-surgical contraceptive that could result in lifetime sterility following a single injection would present many advantages over the current standard of care of surgical sterilization,"said study co-author Dr. William Swanson.

Swanson is director of animal research at the Cincinnati Zoo & Botanical Garden.

Currently, the male stray cat population is controlled via "the surgical removal of the gonads under general anesthesia,"he noted. "In the female, the procedure is either an ovariectomy or an ovariohysterectomy,"Swanson said, referring to removal of the ovaries and removal of the ovaries plus uterus, respectively.

But there are downsides to surgical sterilization, commonly called neutering and spaying.

For one, said Swanson, "the whole procedure requires specialized instruments, medical supplies, veterinary expertise, anesthetic and analgesic drugs, and postoperative care."

And while surgery can be an effective control tool when dealing with small, contained feline populations, he stressed that the approach is "not sufficient to control the reproduction of hundreds of millions of unowned free-roaming cats worldwide."

Of the estimated 600 million domestic cats worldwide, only 20% are owned pets; 8 in 10 live in the street, according to background notes with the study. They aren't cared for and tend to prey on wildlife, yet euthanasia in overpopulated shelters raises ethical questions, the authors noted.

To explore the potential of nonsurgical gene therapy as a solution, investigators focused on nine sexually mature female cats.

Six of the cats received a single intramuscular injection of a hormone called AMH (anti-Müllerian hormone) that prevents ovulation by curtailing the growth process of ovarian follicles.

Over a two-year follow-up period, none of the six that received the gene therapy shot became pregnant. The three that were untreated did become pregnant.

"Our study shows an effectiveness of 100% in preventing pregnancy,"said study co-author David Pépin, associate director of Massachusetts General Hospital's Pediatric Surgical Research Laboratories. He is also an associate professor at Harvard Medical School, in Boston.

"We were not surprised by the high degree of effectiveness,"added Pépin, "as we previously observed similar results in experiments in mice and rats."

No side effects were observed following the injection.

"More than 3 million cats enter shelters in the United States each year, many of which are kittens, and shelters often struggle to find homes for all of the animals in their care,"said Dr. Lauren Overman, senior director of shelter medicine for the American Society for the Prevention of Cruelty to Animals (ASPCA). "Spaying or neutering can help address this challenge, and this research on new advancements in non-surgical feline contraceptives has the potential to increase feline sterilization across the board, especially for community cats and communities that lack the resources for traditional surgical spay and neuters, resulting in fewer animals homeless or suffering.

Swanson said he and his colleagues continue to track the treated cats to see how they fare. (All are intended to be adopted into private homes.) Additional clinical studies are also planned.

But don't expect the gene therapy shot to be employed in the field anytime soon, he warned.

"It is difficult to estimate the timeline,"said Swanson, "but it will be years away before it is available."

The report was published online June 6 in Nature Communications.

More information

The University of Florida has more on the challenges of cat population control.

SOURCES: David Pépin, PhD, associate director, Pediatric Surgical Research Laboratories, Massachusetts General Hospital, and associate professor, Harvard Medical School, Boston; William Swanson, director, animal conservation research, Family Center for Conservation and Research of Endangered Wildlife, Cincinnati Zoo & Botanical Garden; Lauren Overman, senior director of shelter medicine, American Society for the Prevention of Cruelty to Animals; Nature Communications, June 6, 2023, online