What is immunocontraception?

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

Why is the Humane Society of the United States sponsoring research in immunocontraception?

The Humane Society of the United States (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 the HSUS's immunocontraception program?

The HSUS is working to develop the porcine zona pellucida (PZP) immunocontraceptive vaccine into a safe, effective, and practical tool for the humane control of wildlife populations. To make this goal a reality, the HSUS and its partners are working on improving field techniques, demonstrating the effectiveness of immunocontraception in the field, refining the vaccine manufacturing process, and developing training standards. 

As part of this effort, the HSUS obtained U.S. Environmental Protection Agency registration under the name “ZonaStat-H” for use of PZP on wild horses and burros in January 2012, and will be working to extend the registration to include deer and other animals. The HSUS is also 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 synthetic, longer-acting formulations of PZP. Zonastat-H is produced at the Science and Conservation Center in Billings, Montana.

How does PZP prevent pregnancy?

Zona pellucida (ZP) proteins surround the unfertilized eggs of all mammals. Sperm must attach to the sperm-receptor sites along the ZP before an egg can be fertilized. When pig ZP (PZP) is injected into a female animal, her body produces antibodies to it. These antibodies attach to the sperm-receptor sites on the ZP 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, but darting from helicopters is often the safest and most efficient way to dart African elephants.

How long does PZP last?

For Zonastat-H, two injections are 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 simulate annual boosters.

Is PZP the only immunocontraceptive being tested?

Other immunocontraceptives are being developed by other researchers. The most notable of these is GonaConTM, which was developed by the USDA/APHIS National Wildlife Research Center in Fort Collins, Colo. and registered for use as a contraceptive on white-tailed deer by the EPA in September of 2009. GonaConTM is a vaccine that shuts down the reproductive processes of both males and females. It has been successfully tested on captive animals of many species, including deer, elk and pigs, and field tested on deer, wild horses and other animals. In 2009, scientists from Colorado State University began a research program to evaluate the effectiveness of GonaConTM for controlling reproduction in horses at Theodore Roosevelt National Park in North Dakota. The research is ongoing and preliminary results are encouraging.

Is PZP experimental?

With the achievement of EPA registration for PZP, supported by a long record of success on captive animals and in the field, many uses of PZP on wild horses and burros can no longer be considered experimental. Uses on deer and other species, as well as tests of novel preparations and delivery methods for PZP are still experimental, although PZP was developed as a contraceptive more than three decades ago, and its effects are very well known.

Has PZP been shown to reduce wildlife populations?

More work must be done to determine where, to what extent and how fast PZP can reduce wildlife populations. At this writing, however, PZP has reduced wild horse populations at Assateague Island National Seashore in Maryland and Rachel Carson Refuge in North Carolina, white-tailed deer populations at Fire Island National Seashore in New York, the National Institute of Standards and Technology in Maryland, Fripp Island, South Carolina, and American bison populations on Santa Catalina Island, California. PZP vaccinations are also stabilizing populations of African elephants at provincial parks and 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 and giraffes as well as 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.