• ‚Äč
    • Share to Facebook
    • Twitter
    • Email
    • Print

Is PZP Safe? Immunocontraceptive Vaccines and Their Regulation

Porcine zona pellucida is reviewed to determine effects of drug

  • In 2012, EPA formally registered the PZP vaccine for use as a contraceptive in wild horses and burros. Kayla Grams/The HSUS

  • PZP has significantly reduced white-tailed deer populations at Fripp Island, South Carolina. Allen Rutberg/Tufts

The zona pellucida (PZP) immunocontraceptive vaccine has been studied for more than 30 years, and its effects are well known. Thousands of PZP vaccinations have been delivered to wild horses and deer by The HSUS' research team and its many university and government collaborators. Hundreds of zoo animals have received the vaccine, too.

As a naturally occurring protein, PZP is biodegradable and does not pass through the food chain. In deer and wild horses, the best studied species, the contraceptive effects of PZP are reversible even after several consecutive years of treatment (in horses, up to at least five years). PZP vaccination has no effect on ongoing pregnancies, and the offspring of treated females survive and reproduce as well as the offspring of untreated females. 

The side effects of PZP are very limited—and not all of them are bad. A small proportion (less than 2 percent) of deer and horses that receive PZP by dart can suffer from abscesses at the injection site.These are small (an inch or less across), and heal within a month. Female white-tailed deer, which typically mate in November and December, can go through repeated breeding cycles (as late as March) when treated with PZP. However, there is no evidence that this causes any harm to the PZP-treated female deer or to the male deer who might pursue them, and it does not put them at higher risk of collisions with vehicles. PZP does not extend breeding cycles in wild horses.

Both deer and wild horses treated with PZP actually show comparable or better body condition than females who continue to have offspring. In wild horses, at least, this improvement in condition actually leads to longer lifespans. On Assateague Island, a number of  PZP-treated mares have approached or passed 30 years old, which is about two times the expected lifespan of wild horse mares.

Until about 2006, all animal contraceptives fell under the jurisdiction of the federal Food and Drug Administration. The HSUS and its collaborators studied PZP under the auspices of an Investigational New Animal Drug exemption from the FDA. (The INAD is the FDA's mechanism for authorizing and guiding research directed at moving new drugs through the approval process). However, jurisdiction over most wildlife contraception has passed to the Environmental Protection Agency (EPA), which regulates wildlife contraceptives as "pesticides" under the Federal Insecticide, Fungicide, and Rodenticide Act. Although the legalistic designation of some of our favorite animals as "pests" and a life-extending contraceptive as a “pesticide” are unfortunate and uncomfortable, this transfer of authority provides wildlife contraceptives with a more suitable and appropriate federal review process.   

In 2012, EPA formally registered the PZP vaccine for use as a contraceptive in wild horses and burros. The HSUS is currently working to amend the registration to expand its use to white-tailed deer and other wildlife, which will significantly expand the vaccine’s potential to offer a humane solution to conflicts with wildlife.