Sometimes when I come home from work there is a big orange cat sunning himself on my front porch. He ambles away as soon as he sees me – not a panicked dash, because he’s used to humans, but there is certainly no way I’d ever get close enough to pet him. No one owns this cat, although my next door neighbors sometimes feed him, and he clearly lives in the neighborhood; he and my dogs know each other well from high-volume interactions across the fence.

He is a feral cat, sometimes known as a community cat. He probably lives in a small colony, and indeed there are several other individuals that I have seen around, although this is the only one who likes to sleep on my front step.

This orange cat seems to have a pretty good life, which is often true for cats living in colonies that are managed. This colony, as I’ve said, is at least getting fed regularly. However, his life is liable to be shorter than that of an owned cat, and in fact I have already taken two badly-injured community cats to the local veterinary hospital for euthanasia. (As the neighborhood veterinary student I find that one of my responsibilities is dealing with cats who have been hit by cars or attacked by other animals.)

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

Time was, people trapped community cats and euthanized them as a means of population control. This didn’t work as well as you might think, 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 last decade or so, volunteers and animal shelters have been implementing trap/neuter/return (TNR) programs. Cats are trapped and brought to veterinary clinics, where they are vaccinated and spayed or neutered. They are returned to their colonies, which are managed by caretakers. In this way, colonies are kept small but healthy.

However, TNR programs are maintained at great expense. Veterinary surgeons are not cheap, and even with volunteer veterinarians, a surgical suite also has to be acquired. Cats have to be trapped on a specific day when a TNR clinic is scheduled, not an easy task itself, transported to the clinic and then transported back. The expense slows down the process, and it’s not clear that we can spay and neuter fast enough to keep up with the population.

A simple medical intervention would be much more efficient than surgery. The ideal chemical 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 wouldn’t want a dead cat to be full of some toxin that would endanger other animals); and it would have a rapid onset of action. Ideally, it would not just prevent litters, but would also reduce the nuisance behaviors associated with breeding, because cats having sex are extremely noisy. The contraceptive should be widely effective, although studies suggest that it only needs to affect 70-80% of female cats in order to achieve population reduction.

There are some possibilities already being studied. Both are vaccines – it’s an amusing idea to vaccinate against pregnancy, but of course vaccines do have a long term effect, so they're logical choices for this situation. One vaccination target is the zona pellucida. This is the coating around the egg which allows in one, and only one, sperm; vaccinated animals produce antibodies which attack the ZP and therefore inactivate the egg. The nice thing about ZP vaccination is that is is highly species-specific – the ZP is, in fact, part of the mechanism that keeps species from being able to interbreed with each other. Unfortunately, the ZP vaccine which is currently available was not developed specifically against cat ZP, and does not work well in cats. A cat-specific ZP has also been tested but, surprisingly, is not highly effective either. Perhaps more research will sort the problem out, but for now this is not a viable alternative. Additionally, as you might guess from the mechanism, ZP vaccination doesn’t affect mating behaviors even when it works; it only affects conception. So cats will still yowl during sex after ZP vaccination.

A more promising alternative is vaccination against GnRH, the master hormone of the sex hormones. Through minion hormones, GnRH controls production of sperm and ovulation of eggs. Unlike the ZP vaccine, the GnRH vaccine reduces both pregnancy and mating behaviors. Its effectiveness is somewhat unpredictable, so some vaccinated animals keep right on getting pregnant. Its length of effectiveness is also somewhat variable, but can last up to several years in some studies. Although your housecat might live into its late teens, several years of birth control are probably sufficient in shorter-lived community cats.

A commerical GnRH vaccine, GonaCon, is approved in cervids and has been successfully used in white-tailed deer. It has been tested in cats in laboratory settings, but not in the field. The idea is enticing: volunteers could trap cats, then vaccinate them with GonaCon and the usual array of anti-disease vaccines right in the trap, then release them, never having had to bring them in to a veterinary clinic. Efficiency would be hugely increased. Hopefully initial trials would show that GonaCon is effective at population reduction in cats, something that hasn’t yet been proven.

Real life is never so simple, of course. I talked about contraceptive vaccination with a few vet techs at an animal shelter recently. They loved the idea, but pointed out that in our state, once you are providing any medical care for a cat, such as contraception, you have to make sure they are vaccinated for rabies, which legally requires the presence of a veterinarian. Once you have to bring in a vet, of course, 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. In this case, the ZP vaccine might be better, as it is more species-specific and presumably could be eaten by other species without effect, but of course the ZP vaccine requires more work before it will be effective in cats.

So chemical contraceptives for community cats aren’t quite ready for prime time, but there are some promising candidates. 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, both with notorious money problems, can’t possibly represent attractive markets to drug companies. Who will fund the necessary research and the advocacy for policy changes that are necessary? You can stay up to date with news as the story unfolds at the Alliance for Contraception in Cats and Dogs ( You can even donate to them. 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 we live with. Efficient and humane management of their populations benefits both them and us.


* Julie K. Levy. Contraceptive Vaccines for the Humane Control of Community Cat Populations. American Journal of Reproductive Immunology, Special Issue: Special Issue on Contraceptive Vaccines, Volume 66, Issue 1, July 2011

* Julie K. Levy, JohnA. Friary, et al. Long-term fertility control in female cats with GonaCon(TM), a GnRH immunocontraceptive Original Research Article Theriogenology, Volume 76, Issue 8, November 2011, Pages 1517-1525.

* Alliance for Contraception in Cats and Dogs,