Oral Contraceptives for Cats
A Cats & Kittens Special Report

Last March 160 people, among them animal shelter and humane society members, veterinarians, technicians and cat rescuers from around the United States, gathered at the Worcester, Massachusetts, Holiday Inn to attend a daylong summit meeting dedicated to discussing solutions to this country's feral-cat problem. If statistics from the National Council on Pet Population Study and Policy and the U.S. Census Bureau can be credited, 54,000 kittens were born while the people attending the conference were discussing what to do about the 40 to 60 million feral cats that live paw to mouth on the margins of our pet-loving society.

Salmonella to the Rescue

Current answers to the feral-cat problem include education, legislation, sterilization and euthanization. These strategies are a start, but unless there were 54,000 fertile cats removed from the feral-cat population — or rendered sterile and returned to it — that day last March and every day thereafter, those who would solve this problem using current solutions are fighting a rear guard action. Reinforcements may be on the way, however, in the form of a strain of salmonella that does not produce disease.

Michelle Meister-Weisbarth, 32, a third-year student at Virginia-Maryland Regional College of Veterinary Medicine (VMRCVM), has genetically engineered a strain of Salmonella, one that does not produce disease, for use as an oral contraceptive vaccine with female cats. Her creation is an immunocontraceptive vaccine, i.e., one that prompts a cat's immune system to produce antibodies that prevent sperm from fertilizing her eggs.

"Immunocontraceptive vaccines have been around for a while," says Meister-Weisbarth, "but no one had married the idea of our feral cat problem with the vaccine. The key is to make the vaccine species-specific so you can put it in food pellets, drop them as bait, and not worry about blocking fertilization in any other animal."

After the vaccine-carrying pellets have been eaten, the vaccine passes through the digestive system, attaches itself to lymphoid tissue, and is absorbed by the body. Once that happens, the vaccine replicates enough to produce a antigen protein, then the Salmonella dies off. The protein induces antibodies to the sperm receptor, and those antibodies attach to the female cat's eggs, blocking the receptor sites so the head of a sperm cannot attach.

In the Meantime

According to current plans, vaccine-carrying food pellets will be scattered in places that feral-cat colonies are known to frequent. "Using bait to deliver vaccines isn't a new idea," says S.M. Boyle, Ph.D., a professor of biomedical sciences and pathology at VMRCVM. "A similar program in Europe using a virus to deliver a rabies vaccine has virtually wiped out rabies from foxes. We hope to do the same thing to help reduce the feral-cat population."

Studies in animals other than cats show that the contraceptive vaccine is dose dependent: A small dose prevents a female from reproducing for a year or so; a large dose renders her permanently sterile. Nevertheless, researchers do not expect the vaccine to replace hysterectomies in house cats. The vaccine doesn't prevent female animals from going into heat, nor does it eliminate the often tiresome behavior occasioned by that process. Besides, hysterectomies provide long-term health benefits such as protection against ovarian cancer and mammary tumors.

One of Meister-Weisbarth's main concerns as testing continues on the vaccine is its effect on the behavior of the feral cats. "If the vaccine, for example, changes the behavior of the male leader of the cat colony, he might be replaced as the leader, and that could change the whole stability of the colony. We want the behavior to stay the same as it is -- but with no more babies.

"It can be dangerous for humane societies and animal-control groups to try to trap and spay feral animals that may be wild or even rabid," she continues. "This vaccine will serve the feral cat population by making it easy and cost effective to reduce the birthrate of the females."

As promising as this vaccine may sound to persons combating the feral-cat problem, it won't be available to humane societies for at least two to five years at the earliest -- by which time an additional 118 to 296 million kittens will have been born.

"It's nowhere close to being on the market," Meister-Weisbarth says. "We'll do lab tests over the next couple of years, but the Food and Drug Administration won't put it in the field until we're sure it doesn't have any adverse impact on the environment, including the animals."