About 130 mares in Nevada were treated this winter with a contraceptive vaccine in the first phase of a national wild horse fertility control pilot project for the Bureau of Land Management. First developed and tested in 1984 by Irwin K.M. Liu, professor of reproduction at the UC Davis School of Veterinary Medicine, the vaccine lasts one breeding season, or about one year, with a 95 percent success rate and no side effects when boosted every 2-3 years. The key ingredient in the vaccine is a ubiquitous protein called ZP3 found on the surface of all animals' eggs to which sperm attach prior to fertilization. Injected into the horse, the ZP3 antigen is foreign enough to trigger antibodies in the mare that those antibodies to bind to the mare's own ZP3 sites -- called a "cross reaction." The vaccine is also being used to control deer populations, as well as captive populations of related species in zoos. Liu, whose primary research interest is promoting fertility in horses, eventually hopes to test the vaccination in elephants in Asian and African countries.