Gene therapy can provide birth control for cats

For all the cats that share our homes as companions, there is a vast shadow world of strays – a sprawling and fast-growing multitude.

Their lives are ravaged by the threat of infectious diseases, predators and speeding cars. And they are large predators themselves, preying on millions of birds and small mammals each year.

In the United States, volunteers are particularly active in capturing the cats, taking them to clinics to be surgically sterilized, and then returning them to their colonies. But controlling stray cat populations is costly and logistically cumbersome. Many communities, especially in countries outside the United States and Europe, lack the veterinary and economic resources to coordinate such efforts.

“Coming up with an alternative to surgery has been a goal for many people for decades, and there’s just nothing else that’s been proven effective,” said William Swanson, director of animal research at the Cincinnati Zoo & Botanical Garden.

Such a method could finally be on the horizon. In a study published Tuesday in the journal Nature Communications, a single shot of a gene therapy prevented pregnancy in cats for at least two years. The study was extremely small: six female cats who received the gene therapy injection were compared to three who did not.

Limiting the study size to just a few cats allowed the researchers to follow them all extensively, analyzing 15,220 freeze-dried poop samples for estrogen and progesterone levels and examining 1,200 hours of video of mating behavior, said Dr. Swanson.

The birth control shot delivers a gene that enters muscle cells, allowing them to pump a substance called anti-Müller hormone, or AMH, which interferes with the development of egg follicles in the ovaries.

Researchers warned that much more research would be needed to test the preliminary findings. And if larger studies confirm that the treatment — the first gene therapy developed specifically for animals — is safe and effective over a cat’s life, controlling cat populations won’t require the surgical expertise of veterinarians, said Dr. Swanson.

David Pépin, a reproductive biologist at Massachusetts General Hospital in Boston, originally studied AMH as a potential therapy for ovarian cancer, but decided to investigate its effect on the ovaries. When he injected the hormone into mice, their ovaries shrank to newborn size, suggesting that AMH may have contraceptive properties.

Dr. Pépin is investigating the possible use of AMH in humans, not as gene therapy but as a pill or injection that must be taken continuously. Most contraceptives today prevent ovulation, but AMH is said to work earlier and block follicle maturation.

He thinks it could be helpful for women who cannot take birth control pills containing progesterone or estrogen for medical reasons or help women undergoing cancer treatments to preserve their fertility. “It’s a hormone that we haven’t had the opportunity to play with before and that potentially has many different uses in women’s health,” he said.

As a gene therapy that could be permanent, using AMH in humans is unlikely. “But it’s actually the perfect tool for controlling cat overpopulation,” he said. Four of the cats in the study showed no behavior indicating they were ready to mate, and two allowed male cats to mate with them but not ovulate.

Dr. Pepin and Dr. Swanson, an expert in feline reproduction (and a member of the scientific advisory board of the Michelson Found Animals Foundation, which funded the work), plans a larger study that could support a filing with the Food and Drug Administration to consider the therapy properly marketed for use in cats.

They are also testing the therapy in kittens, which can be treated as young as eight weeks old, and in dogs, which also have huge stray populations, especially in other countries.

“This is really exciting and I hope it works out,” said Julie Levy, a veterinarian at the University of Florida College of Veterinary Medicine in Gainesville, who was not involved in the study. “Wouldn’t it be great if we could send a technician into the field to inject cats and then set them loose?”

The study is an example of the Michelson Foundation’s practice of “throwing a lot of money at the problem” to find nonsurgical birth control for stray cats and dogs, said Dr. Levy, who works with cats in outdoor colonies and shelters both in the United States and abroad.

But she cautioned that there is still a lot to learn from a larger study, such as how long the injection lasts, whether it is as safe as it seems, and what proportion of cats it will actually protect against pregnancy, “because it probably won “. not be 100 percent.

Others note that it may not be that simple. If the injection is effective, long-lasting and less expensive than spaying and neutering, it could be very valuable, said Autumn Davidson, a veterinarian at Cornell’s College of Veterinary Medicine in Ithaca, NY. But getting the shot requires capturing animals, and queens skilled at evading humans’ traps can still make population control a struggle.