In an effort to promote and advance humane, sustainable approaches to resolving conflicts between humans and wildlife, for years, the Humane Society of the United States (HSUS) has collaborated with researchers, NGOs and academic institutions, as well as federal, state and local agencies to help develop the porcine zona pellucida (PZP) immunocontraceptive vaccine for use in managing wildlife populations, beginning with the famous wild ponies on Assateague Island National Seashore. 

The PZP vaccine has been used for over two decades in dozens of wild and captive species of wildlife and is proven to be safe, effective and humane. Animals vaccinated with PZP produce antibodies that bind to the membranes surrounding their eggs, blocking fertilization. While continual improvements in the practical delivery of the vaccine and associated costs have been made, using  PZP to control fertility in overabundant populations could be even more appealing   to government agencies and communities if costs associated with producing the vaccine were further reduced and the duration of time the vaccine was effective was further increased.

In 2016, Purdue University, in partnership with the HSUS, began a research project with the goal of producing a PZP vaccine that will provide effective multi-year contraception to help manage populations of wild horses, wild burros, urban deer, African elephants and many other species around the world. By eliminating the need for cruel and costly roundups and/or lethal  control, we will help agencies and communities save taxpayer dollars while advancing a more humane and innovative approach to managing human-wildlife conflicts in our communities, parks and public lands.    

The goal of the project is to develop a contraceptive vaccine for wildlife that demonstrates a replicable, multi-year efficacy in several species of mammals (including equids, ungulates, pachyderms and other species). 

The objectives are to: 

Increase the initial immune response 

Adjuvants are substances that enhance the effectiveness of a vaccine by stimulating an effective immune response against the vaccination. Further research into optimal dosages of PZP and optimal combinations of adjuvants would produce a more potent and long-acting immune response in vaccinated animals. 

Increase the efficacy, decrease the cost 

The current process of producing PZP from pig ovaries is laborious and inefficient so researchers will develop an effective, synthetic formulation of the PZP vaccine  that is easier and cheaper to produce without the need to use pig ovaries.  

Develop a longer-acting vaccine 

The current vaccine is effective for two or three years. In many areas, geography makes repeated immunizations of free-ranging wildlife populations challenging and impractical. Using the current technology, a female deer (who lives an average of seven to 10 years) may need to be retreated three times in her life, but a female horse lives longer (15 to 20 years) and may need to be treated more than five times.

A longer-lasting formulation would mean fewer vaccinations over the course of an animal’s life, an important cost benefit when darting individual animals. 

The ideal immunocontraception vaccine must:

  • Be highly efficacious
  • Last for multiple years
  • Not permanently affect fertility
  • Not interfere with an individual animal’s basic behavior and/or group social structure
  • Not affect existing pregnancies
  • Be nontoxic to either the animal or the person delivering the vaccine
  • Not pass through the food chain
  • Be capable of being delivered remotely
  • Be cost effective

While the current PZP immunocontraceptives meet almost all of these criteria, this next generation of PZP Project will generate a longer-lasting immune response, reduce the cost of producing it and make it easier to deliver remotely, providing a humane, nonlethal tool that wildlife managers can incorporate into their wildlife conflict mitigation plans.