When I was a veterinary student years ago, I sometimes found a big orange cat sunning himself on my front porch at the end of my workday. He always ambled away as soon as he saw me—not a panicked dash, because he was used to humans—but he certainly was not going to let me get close enough to pet him. No one owned this cat, although my next-door neighbors sometimes fed him, and he clearly lived in the neighborhood; he and my dogs knew one another well, interacting loudly across the fence.

He was a feral cat, otherwise known as a community cat. He probably lived in a small colony, and indeed I sometimes saw several other individuals around. This orange cat seemed to have a pretty good life, which is often true for cats living in colonies that get some food and other care from people. His life, however, was liable to be shorter than that of an owned cat; during my tenure in that neighborhood, I took two badly injured community cats to the local veterinary hospital for euthanasia. (As the area's veterinary student, I found that one of my de facto responsibilities was dealing with felines that had been hit by cars or attacked by other animals.)

He was also at risk of disease, such as feline leukemia or feline AIDS, because he had probably never been vaccinated. He was not the only one at risk: his colony may maintain a reservoir of these feline diseases that can then be transmitted into the population of owned cats that are allowed outdoors to interact with their feral cousins. Moreover, community cats are often sexually intact and certainly contribute to the overwhelming number of kittens that my neighborhood saw every spring and summer. And, of course, bird lovers complain of the depredations by feral cats on the local wildlife.

It used to be that people trapped community cats and brought them to animal shelters. The shelters had few options other than euthanizing the cats, which were unable to adapt to life as pets. This approach did not provide effective population control, however, because when a colony of cats was depleted, new cats would move in to take their place.

It turns out that maintaining a healthy colony keeps new cats out. In the past decade or so volunteers and animal shelters have been implementing trap-neuter-vaccinate-return (TNVR) programs. Teams trap cats and bring them to veterinary clinics, where they are vaccinated and spayed or neutered and then returned to their colonies, which are managed by caretakers. In this way, colonies are kept small but healthy. Although the remaining cats may still attack small animals, the alternative—trapping and euthanizing them—does not reduce the population of predators either.

Maintaining TNVR programs takes a lot of money, however. Veterinary surgeons are not cheap, and even when they volunteer to help, they still need access to a surgical suite. Cats have to be trapped on a specific day when a TNVR clinic is scheduled (not an easy task itself), transported to the clinic and then transported back. The expense slows the process. And in the end, it is not possible to spay and neuter cats fast enough to keep up with the population.

A simple medical intervention would be much more efficient than surgery. The ideal contraceptive would be inexpensive to make and easy to administer; a single treatment would have a long-term or even permanent effect; it would have a wide margin of safety for both cats and the environment (you would not want a dead cat to be full of some toxic substance that would endanger other animals); and it would act rapidly. The drug should prevent litters and reduce the nuisance behaviors associated with breeding because—let's face it—cats having sex are extremely noisy. Ideally many cats would receive and respond to the drug, although studies suggest that it needs to affect only 70 to 80 percent of female cats to achieve population reduction.

One promising approach is an immunocontraceptive, or birth-control vaccine, that works by evoking an immune response. Eliciting immune activity may sound like a strange way to prevent pregnancies, but vaccines do tend to work long term and are actually a logical choice for this situation.

One approach that has been tested in cats targets gonadotropin-releasing hormone (GnRH), the master switch of the reproductive system. Through minion hormones, GnRH controls production of testosterone, estrogen and sperm, as well as ovulation. Hence, eliciting an immune response against it should reduce both pregnancy and mating behaviors.

The idea is enticing: volunteers could trap cats, then dose them with the contraceptive and the usual array of antidisease vaccines right in the trap and release them, never having had to bring them to a veterinary clinic. A commercial GnRH-blocking vaccine, GonaCon, is approved in cervids and has been successfully used in white-tailed deer. Initial encouraging results in cats in the laboratory suggested vaccination might prevent pregnancy in them for a year or more. Yet this vaccine did not do well in a field study, in which many of the vaccinated females became pregnant.

The Alliance for Contraception in Cats and Dogs (ACC&D) continues to sponsor new research on immunocontraception, and at least one lab is now developing a vaccine that targets a different part of the reproductive system. Alternatively, future studies could uncover the problems with GonaCon in felines because it works in other species. A one-shot immunocontraceptive is coming for cats soon, and it might be a game changer.

Real life is never simple, of course. When I was a veterinary student, I discussed contraceptive vaccination with experienced veterinary assistants at an animal shelter. They loved the idea but pointed out that in Massachusetts, where I lived at the time, once you are providing any medical care for a cat, such as contraception, you have to make sure they are vaccinated for rabies.

Unfortunately, rabies vaccination legally requires the presence of a veterinarian, and once you have to bring in a vet, the expense starts going up again. We batted around some ideas—maybe you could distribute an oral form of a vaccine in bait form and get around the rabies vaccination requirement by dint of never actually touching the cat. But it seems that policy changes might be necessary in some states before immunocontraception could be widely used in community cats.

These contraceptives for feral cats are not ready for prime time yet, but studies are under way to move this important work forward. My suspicion is that the biggest problem is simply willingness on the part of society to commit the resources necessary to develop a workable solution. Cat rescuers and animal shelters, all of whom have notorious funding problems, cannot possibly represent attractive markets to drug companies. This situation leaves paying for the necessary research and advocacy for required policy changes up to private nonprofit organizations. You can stay up-to-date with news as the story unfolds at the ACC&D [see “More to Explore,” below].

Community cats may be our responsibility, as domesticated animals gone feral, or they may not be, but either way they affect us and the animals with which we live. Efficient and humane management of their populations benefits both us and them.