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

The study is an example of the Michelson foundation’s practice of “throwing a lot of big money at the problem” to find nonsurgical contraception 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 was still much to learn from a larger study, such as how long the shot lasts, whether it is as safe as it seems, and what proportion of cats it will actually protect from pregnancy, “because it probably won’t be 100 percent.”

Others note that it might not be quite so easy. If the shot is effective, long-lasting and cheaper than spay and neuter surgery, it could be very valuable, said Autumn Davidson, a veterinarian at Cornell’s College of Veterinary Medicine in Ithaca, N.Y. But to receive the injection, animals have to be captured, and queens who are adept at evading people’s traps might still make population control a struggle.