This summer, Habitat for Horses, a non-profit equine rescue center in Hitchcock, Texas, began giving mares the porcine zona pellucida (PZP) vaccine, an animal form of birth control. Since HR 857, a bill that would eliminate the slaughter of horses in and from the United States for human consumption, was put into sub-committee, horse rescue groups are now turning to alternative ways to help reduce the number of horses bound for slaughter. Habitat for Horses is the first to utilize the PZP shot. “We are trying to reduce the number of grade slaughter-bound horses,” said Habitat for Horses spokeswoman Connie Charmichel. “We hope that other rescue organizations follow suit. The birth control is cheaper, being only $20 a shot, whereas spaying the mare is more intensive and taxing for both horse and human and requires recuperation time, this does not.”

According to USDA, the PZP vaccine is an intramuscular immuno-contraceptive agent that is derived from zona pellucida (the strong membrane that forms around an ovum as it develops in the ovary) from pigs. When injected, it causes production of antibodies that bind to the host’s zona pellucida and prevent fertilization of the egg because the sperm cannot penetrate it. Females that are given PZP will cycle and ovulate normally, but will not conceive. If an animal has conceived and is given the shot, it does not cause abortion of the fetus. It is proven to be 90% effective against pregnancy (in deer) but it is not as effective as spaying.

PZP has been used on deer, coyote, elephant, and zoo populations. Following suit, the Bureau of Land Management has used the PZP vaccine in more than 1,500 wild horse mares in various formulations since 1992, in an effort to reduce the wild horse populations out West and some of the wild herds along the Atlantic Coast. In a field trial using the two-year PZP vaccine on 96 wild mares, the vaccine resulted in efficacy of a minimum of 94% in the year it was given, 82.4% in year two, and 68% in year three