Dr. Stephen Zawistowski, Ph.D.,
ASPCA Sr. V.P. Animal Sciences and Science Advisor
The Magic Pill
By Stephen L. Zawistowski, Ph.D.
Back in the ’60s, Grace Slick and Jefferson Airplane sang, “One pill makes you larger/and one pill makes you small…” In that psychedelic era, there was always a pill or potion to provide the right vision of the world. At the same time, the Pill was giving American women a safe and easy way to prevent pregnancy. Since that time, animal welfare proponents have waited anxiously for the magic pill that would control the reproduction of dogs and cats.
Surgical sterilization of dogs and cats has been a part of the arsenal used to prevent unwanted breeding for decades. Innovations in methods and procedures have led to mobile sterilization vans and, more recently, to growing acceptance of the sterilization of puppies and kittens. There is no question that spaying and neutering has had an enormous impact on reducing the numbers of unwanted puppies, kittens, dogs and cats at animal shelters in many areas of the country. On the other hand, targeting difficult-to-reach communities and feral populations of cats and dogs would be easier if a simpler, nonsurgical method was available. The dream has always been to render a dog or cat sterile with a single pill or injection that could be administered by a nonveterinarian.
There has been considerable work in the area of nonsurgical sterilization of animals over the past several decades. For instance, many zoos use contraceptive methods to control the production of unwanted young (“Zoos 2000,” Animal Watch, Summer 2000). This is often accomplished through the use of a hormonal implant that permits reversible control of fertility in females. However, according to Dr. Patty Olson, a board-certified veterinary theriogenologist, “Steroidal implants can predispose females to uterine disorders, such as pyometria, or mammary tumors. Due to these concerns, greater attention is now being paid to the use of such implants as contraceptives for women, as well.”Furthermore, animals in zoos are under constant professional care and scrutiny, allowing for potential side effects to be evaluated on a regular basis.
The use of steroidal hormones to control fertility in dogs or cats through either implants or oral delivery mechanisms requires similar vigilance in monitoring the animals for side effects and efficacy. As a result, many veterinarians will not recommend these forms of contraception to their clients. Such methods are clearly impractical for controlling breeding in free-living dogs or cats, or when an animal’s caretaker is unable or uninterested in making the needed commitment. It’s unlikely that we will see a product anytime soon that could be added to pet foods, distributed in baits left for free-living dogs and cats or distributed easily and cheaply from mobile units or clinics.
The most exciting area of nonsurgical sterilization research in animals over the past decade has been immunocontraception. The Spay USA conference held at Bentley College, MA, in July 2000 included a special session that highlighted this technology.
Immunocontraception is based on developing vaccines to stimulate the production of antibodies that interfere with the fertilization process. One common method uses an extract of the egg coating (zona pellucida) from the egg cells of pigs. This extract is injected into females, where it stimulates the production of antibodies to the egg coating proteins. These antibodies then bind to the eggs of the female and interfere with the process of egg fertilization by the sperm. Immunocontraceptive treatments have proven effective in reducing the fertility of mammalian target species that include horses, white-tailed deer and a number of others. Early versions required an initial vaccination and subsequent booster shots to maintain sterility. Continued research has shown promise for single-dose, permanent sterilization of dogs and cats. As with nearly all such interventions, it will be important to evaluate the presence and significance of any potential side effects. If ongoing research shows that immunocontraception is effective and safe for dogs and cats, it could usher in a new era in pet population control.
In the meantime, the old standby of spaying and neutering remains safe and effective. What’s more, the side effects are all positive: elimination of ovarian or testicular cancers, decreased chance of mammary tumors and reduced aggression, marking and roaming.
Dr. “Z” is senior vice president, ASPCA Animal Sciences, and Animal Watch science advisor.
© ASPCA 2000
ASPCA Animal Watch – Winter 2000
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