A non-hormonal contraceptive, porcine zona pellucida (PZP), has given birth to an alliance between the Bureau of Land Management (BLM) and the Humane Society of the United States (HSUS). The two organizations agreed to work together on using this drug to control the wild horse population at a November meeting held in Santa Fe, N.M.

The HSUS holds an Investigational New Drug Permit from the Federal Drug Administration (FDA) for PZP. This new partnership allows the BLM to use the drug in non-commercial, experimental settings without FDA approval.

According to the USDA, PZP is an intramuscular immunocontraceptive agent that uses the immune system rather than hormones to prevent egg fertilization. It is derived from pig zona pellucida, a strong membrane that forms around an ovum as it develops in the ovary.

When injected, PZP causes the mare’s immune system to make antibodies against it; these then bind to the mare’s zona pellucida and prevent sperm from penetrating the membrane. Treated mares will continue to cycle and ovulate normally, but are unable to conceive. The drug will not affect mares already in foal. A study conducted in a BLM herd in 2000 showed that PZP was 90% effective in preventing pregnancies in horses