Questions and Answers about Immunocontraception

What is immunocontraception?

Immunocontraception is a birth control method that uses the body's immune response to prevent pregnancy.

Why is The HSUS sponsoring research in immunocontraception?

The HSUS believes that immunocontraception may offer a humane, nonlethal solution to conflicts between people and wildlife in urban and suburban areas as well as a solution to local problems of animal overabundance. Immunocontraception can also help reduce the overproduction of captive animals in zoos and other facilities. In the future, it might play a role in controlling dog and cat overpopulation.

What are the current objectives of immunocontraceptive research?

The HSUS is working to develop a porcine zona pellucida (PZP) immunocontraceptive vaccine that will meet U.S. Environmental Agency standards for safety and effectiveness and serve as a practical tool for the humane control of wildlife populations. We are also developing field techniques, examining the effects of immunocontraception on wildlife population growth and exploring the potential for the use of immunocontraceptives on companion animals.

What is PZP?

PZP (porcine zona pellucida) is a protein that occurs naturally in pig ovaries. The HSUS and others are conducting research to develop a synthetic form of PZP. The PZP being tested by The HSUS is produced at the Science and Conservation Center, ZooMontana, Billings, Montana.

How does PZP prevent pregnancy?

Zona pellucida (ZP) proteins surround the unfertilized eggs of all mammals. Sperm must attach to ZP before an egg can be fertilized. When pig ZP (PZP) is injected into a female animal, that animal's body produces antibodies to it. These antibodies attach to the female's ZP proteins, preventing sperm from attaching and blocking fertilization.

How is PZP administered?

PZP is administered by hand injection or via a dart fired from a dart rifle, CO2 pistol or blowgun. Darting is preferred whenever possible, because it avoids the need to capture and handle the animal. Darting is generally carried out on foot or from a vehicle, but African elephants are darted most efficiently and most safely from helicopters.

How long does PZP last?

In earlier research, two injections were given in the initial year, followed by annual boosters. However, one-shot PZP vaccines that last two or more years have been tested successfully on wild horses, deer and other species by The HSUS and other investigators. One technology used successfully by The HSUS and its collaborators involves packaging PZP in timed-release pellets, which stimulate annual boosters. Spay-Vac^®, a different PZP vaccine produced by Immuno Vaccine Technologies in Nova Scotia, has also demonstrated one-shot, long-term effectiveness in studies involving a variety of species, including deer. One-shot procedures are rapidly becoming standard. 

Is PZP the only immunocontraceptive being tested?

Other immunocontraceptives are being developed by other researchers. The most notable of these is  GonaCon^TM, which is being developed by the USDA/APHIS National Wildlife Research Center in Fort Collins, Colo. GonaCon^TM is a vaccine that shuts down the reporductive processes of both males and females. It has been successfully tested on captive animals of many species, including deer, elk and pigs, but has received only limited field testing.

Is PZP experimental?

PZP is experimental in the sense that it has not been formally registered with EPA. It is also experimental in the sense that all current uses of PZP are built around scientific studies. However, the PZP vaccine and its effects are very well known. PZP was developed as a contraceptive more than three decades ago, and has been extensively tested on many species in the lab and in the field. HSUS will begin filing applications for EPA registration soon.

Has PZP been shown to reduce wildlife populations?

Much more work must be done to determine where and to what extent, and how fast PZP can reduce wildlife populations. At this writing, however, PZP has been shown to reduce wild horse population size at Assateague Island National Seashore, Md., and has reduced deer population size at Fire Island National Seashore, N.Y., and the National Institute of Standards and Technology, Md. PZP vaccinations are also stabilizing populations of African elephants at crowded private wildlife preserves in South Africa.

What other wildlife species are being treated with PZP?

Tests on captive animals of about 100 species in about 100 zoos and aquaria worldwide indicate that PZP works very well on a wide variety of hoofstock, including antelope, deer, wild cattle, sheep, zebra, giraffes, bears and sea lions. PZP has also been given to free-roaming tule elk at Point Reyes National Seashore, Calif., and water buffalo on the island of Guam. SpayVac^®, an alternative form of PZP manufactured in Canada, has shown excellent effectiveness with grey seals and fallow deer, as well as white-tailed deer.